JP6247176B2 - Switching power supply - Google Patents

Description

  The present invention relates to a switching power supply device that functions as an isolated converter that converts an input voltage into a desired voltage and supplies it to an electronic device.

(Single Ended Forward Converter)
Conventionally, a single-ended forward converter is known as an isolated converter capable of obtaining an output voltage by converting an input voltage into a predetermined voltage (Patent Document 1).

  As shown in FIG. 17, in the single-ended forward converter 100, the primary winding 104 a of the main transformer 104 and the main switching element 106 are connected to the input power source 102. Connected to the secondary winding 104b of the main transformer 104 is a rectifier circuit 107 including a forward-side synchronous rectifier element 108 and a flywheel-side synchronous rectifier element 110, and a smoothing circuit 111 including an output choke coil 112 and an output capacitor 114. The load is connected to the output capacitor 114 to obtain output power.

  The control circuit 116 is a PWM control circuit that generates a fixed switching frequency, and controls on / off of the main switching element 106.

  The synchronous rectifying element 108 on the forward side and the synchronous rectifying element 110 on the flywheel side are synchronous rectifying elements including a switching element 126, a diode 128, and a Zener diode 130 that operate in response to a voltage generated in the tertiary winding 104 c of the main transformer 104. On / off is controlled by a gate discharge circuit 120 including a drive circuit 122, a switching element 122, and a signal transmission transformer 124.

  The forward side synchronous rectifying element 108 is turned on / off in synchronization with the main switching element 106 being turned on / off. The flywheel side synchronous rectification element 110 is turned on when the main switching element 106 is off, and is turned off when the main switching element 106 is on. The flywheel side synchronous rectifier element 110 is turned off and the main switching element 106 is turned on (corresponding to the forward side synchronous rectifier element 108 being turned on) by a delay circuit 118 with a dead time.

  The power supplied from the input power supply 102 is supplied to the main transformer 104 as intermittent power by turning on and off the main switching element 106. The primary winding 104a and the secondary winding 104b of the main transformer 104 are used in forward coupling, and power when the main switching element 106 is on is transmitted from the primary winding 104a to the secondary winding 104b. Is done.

  The rectifier circuit 107 and the smoothing circuit 111 generate output power Vo by rectifying and smoothing the power transmitted by the forward coupling of the main transformer 104. The output voltage Vo is an input voltage Vin, a turn ratio (N2 / N1) based on the turn number N1 of the primary winding 104a and the turn number N2 of the secondary winding 104b in the main transformer 104, and the on-duty duty of the main switching element 106. It is determined.

Vo = (N2 / N1) · Vin · duty
Since the single-ended forward converter 100 operates at a fixed switching frequency and the output voltage can be controlled by the on-duty of the main switching element 106, it can be operated by a simple control circuit (PWM control circuit). Further, since the main transformer 104 is used as a forward transformer, the conversion efficiency of the transformer is high.

(LLC resonant converter)
Conventionally, there is an LLC resonant converter as another method of an isolated converter that can obtain an output voltage by converting an input voltage into a predetermined voltage (Patent Documents 2 and 3).

  FIG. 18 shows a conventional LLC resonant converter (Patent Document 2). A series resonance circuit of a resonance capacitor C1 and a resonance inductor L1 is connected to the primary winding N1 of the main transformer T1 provided in the LLC resonance converter. The switching elements Q1 and Q2 are half-bridge circuits that are symmetrically and alternately turned on and off with a duty of about 50%, so that the resonant capacitor C1, the resonant inductor L1, and the main transformer T1 are in a resonant state. The switching frequency of Q1 and Q2 is controlled.

  A center tap full-wave rectifier circuit including rectifier elements D3 and D4 and a smoothing circuit including an output capacitor Co are connected to the secondary winding N2 of the main transformer T1, and a load is connected to the output capacitor Co. It is the composition which becomes.

  A control circuit 202 for driving the switching elements Q1 and Q2 is a frequency modulation control circuit (PFM control circuit), and a VCO (voltage controlled oscillator) corresponding to an error voltage between an output voltage from the differential amplifier 203 and a reference voltage. By controlling the switching frequency, the switching frequency is controlled so that the output voltage becomes a predetermined value. At this time, the switching frequency is controlled so that the resonant capacitor C1, the resonant inductor L1, and the main transformer T1 are always in a resonant state.

  Unlike the single-ended forward converter shown in FIG. 17, the LLC resonant converter does not increase switching loss or generate a surge voltage because the switching elements Q1 and Q2 are driven by a soft switching operation. Therefore, a highly efficient and low noise switching power supply can be obtained.

JP2012-100100A Japanese Patent Application Laid-Open No. 08-066025 JP 2014-060850 A

  However, in the conventional single-ended forward converter shown in FIG. 17, the main switching element is used for hard switching, and an increase in switching loss and generation of a surge voltage are observed. I have a problem that said to occur.

  On the other hand, in the conventional LLC resonant converter shown in FIG. 18, since the switching element is driven by a soft switching operation, there is no increase in switching loss or generation of a surge voltage, and a high-efficiency, low-noise switching power supply is provided. Can be obtained.

  However, the conventional LLC resonance converter needs to control the switching frequency so that the resonance state is maintained even when the conditions such as the input voltage and the load current change, and a complicated and expensive control circuit is required. .

  In addition, when the input voltage, output voltage, load current, or the like changes, if the control for changing the switching frequency cannot catch up, so-called “resonance deviation” occurs. When "resonance loss" occurs, the switching element cannot perform a soft switching operation, causing a phenomenon such as a hard switching state or a through current flowing, giving the switching element stress due to a large surge voltage, In the worst case, the switching power supply device is caused to malfunction.

In addition, the frequency modulation method of the LLC resonant converter has a problem that it affects the measurement system for devices that handle minute signals such as a measuring instrument. For example, when the sampling rate of the AD converter is close to the switching frequency, there is a problem that the conversion error of the AD converter increases.

  It is an object of the present invention to provide a switching power supply device that realizes a soft switching operation and enables high efficiency, low noise, and low cost for an isolated converter circuit.

(Switching power supply)
The present invention
As a primary circuit, one end of a main switching element is connected to one end of an input power supply, one end of a commutation element is connected to the other end of the main switching element, and a primary circuit of a choke coil and a series circuit of a buffer capacitor the one end of the connection, synchronous buck chopper circuit and the other end to the other end of the input power of the commutation element to the other end of the series circuit is connected in the primary winding and the buffer capacitor of the choke coil is constituted,
As the secondary circuit, the series circuit of the output capacitor and the rectifying device in the secondary winding connection of the choke coil, a switching power supply circuit is configured from both ends to obtain an output voltage of the output capacitor,
The inductance of the choke coil is set to such a value that the current flowing through the choke coil flows in both the positive direction and the negative direction within one switching cycle when the output current is below a predetermined rated value (maximum output current). And
Switching having the function of complementarily turning on and off the main switching element and the commutation element with a dead time based on the PWM signal that is the on-duty of the main switching element for controlling the output voltage to a predetermined voltage A control circuit is provided .

(Prevents surge from excessive load current)
The present invention
As a primary circuit, one end of a main switching element is connected to one end of an input power supply, one end of a commutation element is connected to the other end of the main switching element, and a primary circuit of a choke coil and a series circuit of a buffer capacitor the one end of the connection, synchronous buck chopper circuit and the other end to the other end of the input power of the commutation element to the other end of the series circuit is connected in the primary winding and the buffer capacitor of the choke coil is constituted,
As the secondary circuit, the series circuit of the output capacitor and the rectifying device in the secondary winding connection of the choke coil, a switching power supply circuit is configured from both ends to obtain an output voltage of the output capacitor,
The inductance of the choke coil is set to such a value that the current flowing through the choke coil flows in both the positive direction and the negative direction within one switching cycle when the output current is below a predetermined rated value (maximum output current). And
Based on a PWM signal that is an on-duty of the main switching element for controlling the output voltage to a predetermined voltage, the main switching element and the commutation element are controlled to be complementarily turned on and off with a dead time. A switching control circuit;
A polarity detection circuit that detects the polarity of the choke coil and outputs a polarity detection signal;
When the polarity detection circuit outputs a polarity detection signal indicating a state in which the primary winding of the choke coil is passing a current due to energy discharge toward the buffer capacitor, the main switching element output by the switching control circuit is turned on. and it controls to continue off pending signal, the main switching element on hold control circuit to suppress the generation of surge voltage commutation element,
It is provided with.

Here, the switching power supply device further Ru includes a current limiting circuit for limiting the maximum output current is controlled to so that provided the off period of the main switching element in one cycle of the switching period.

(Regenerative operation)
The present invention
As a primary circuit, one end of a main switching element is connected to one end of an input power supply, one end of a commutation element is connected to the other end of the main switching element, and a primary circuit of a choke coil and a series circuit of a buffer capacitor the one end of the connection, synchronous buck chopper circuit and the other end to the other end of the input power of the commutation element to the other end of the series circuit is connected in the primary winding and the buffer capacitor of the choke coil is constituted,
As the secondary circuit, the series circuit of the rectifying element and an output capacitor having a two-way rectification function in the secondary winding of the choke coil is connected, the circuit to obtain an output voltage from both ends of the output capacitor is constituted, from the output side a switching power supply apparatus having a function of regenerating power to the input side,
The inductance of the choke coil is such that when the regenerative current flowing from the output side to the input side is less than a predetermined rated value (maximum output current), the current flowing through the choke coil is both positive and negative within one switching cycle. It ’s set to a flowing value,
Based on the PWM signal that is the on-duty of the main switching element for controlling the output voltage to a predetermined voltage, the main switching element and the commutation element are complementarily turned on and off with a dead time, and switching A main switching control circuit for controlling to provide an off period of the commutation element in one cycle;
A polarity detection circuit that detects the polarity of the choke coil and outputs a polarity detection signal;
When the polarity detection circuit outputs a polarity detection signal indicating that the current of the primary winding of the choke coil is flowing current toward the input power supply, the commutation element output by the switching control circuit is turned on. and it controls to continue off pending signal, and commutation device on hold control circuit to suppress the occurrence of a surge voltage of the main switching element,
It is provided with.

(Basic effect)
In the present invention, as a primary circuit, one end of a main switching element is connected to one end of an input power supply, one end of a commutation element is connected to the other end of the main switching element, and a series circuit of a choke coil and a buffer capacitor A synchronous rectification step-down chopper circuit in which one end is connected and the other end of the commutation element and the other end of the input power supply are connected to the other end of the series circuit of the choke coil and the buffer capacitor is configured as a secondary side circuit. In a switching power supply device in which a secondary winding is provided and a series circuit of a rectifier element and an output capacitor is connected to the secondary winding to obtain output power from both ends of the output capacitor, the inductance of the choke coil is When the output current is below the specified rated value (maximum output current), the current flowing through the choke coil is zero within one switching cycle. The switching control is set to a value that flows in both the positive direction and the negative direction, and has a function of complementarily turning on and off the main switching element and the commutation element at a predetermined switching frequency by providing a dead time. Since the circuit is provided, the primary side synchronous rectification step-down chopper circuit uses a capacitor connected to the choke coil (primary winding) as a buffer capacitor, and the choke is turned on when the main switching element is on. When storing the excitation energy in the coil, the charge energy is stored in the buffer capacitor. When the commutation element is on, the excitation energy and the charge energy of the buffer capacitor are transferred from the secondary winding of the output choke coil to the load side. Transmit to. At this time, the choke coil operates as a forward transformer simultaneously with the flyback transformer, and the primary winding and the secondary winding are coupled. Therefore, the voltage of the buffer capacitor and the output capacitor is the voltage of the choke coil (transformer). It becomes proportional relationship turns ratio of the primary and secondary windings, to allow operation of the isolated converters.

  In addition, when the output current is in the range from zero to the rated value (rated current), the inductance of the output choke coil is set so that the commutation element can be turned off while the choke coil current is in the negative direction. By driving the main switching element and the commutation element with dead time, it is possible to pull out the parasitic capacitance of the main switching element, so that soft switching operation is possible, and a switching power supply device with high efficiency and low noise can be obtained. Can be made.

  Further, even if the input voltage, the output voltage, the output current, and the like change, the operation is performed at a fixed frequency, and the output voltage can be controlled by the on-duty of the main switching element.

 In addition, since the surge voltage, which was a problem with single-ended forward converters, does not occur, semiconductor devices such as MOS-FETs with low breakdown voltage (semiconductor devices with low on-resistance) can be used, contributing to higher efficiency and switching. Noise can also be reduced.

  In addition, since the frequency modulation control required in the conventional LLC resonant converter is unnecessary, the control circuit can be configured at a low cost and operates at a fixed frequency. No adverse effects such as measurement errors.

  Also, by setting the inductance of the output choke coil to provide a soft switching operation with a dead time for the main switching element and the commutation element, even if the input voltage, output voltage, load current, etc. change, the LLC resonant converter Thus, no excessive stress is applied to the switching element.

(Effect of surge generation against excessive load current)
In the present invention, as a primary circuit, one end of a main switching element is connected to one end of an input power supply, one end of a commutation element is connected to the other end of the main switching element, and a series circuit of a choke coil and a buffer capacitor A synchronous rectification step-down chopper circuit in which one end is connected and the other end of the commutation element and the other end of the input power supply are connected to the other end of the series circuit of the choke coil and the buffer capacitor is configured as a secondary side circuit. In a switching power supply device in which a secondary winding is provided and a series circuit of a rectifier element and an output capacitor is connected to the secondary winding to obtain output power from both ends of the output capacitor, the inductance of the choke coil is When the output current is below the specified rated value (maximum output current), the current flowing through the choke coil is zero within one switching cycle. Switching that controls the main switching element and commutation element to be turned on and off in a complementary manner with a dead time at a predetermined switching frequency. The control circuit, the polarity detection circuit that detects the polarity of the choke coil and outputs a polarity detection signal, and when the polarity detection circuit outputs the polarity detection signal, the ON signal of the main switching element that the switching control circuit outputs A main switching element on hold control circuit for controlling to hold and to keep off; and
Since the main switching element is turned on when no current flows through the parasitic diode of the commutation element even when a current exceeding the maximum output current value flows, the surge voltage is reduced. It does not occur.

  In addition, since no surge voltage is generated, a semiconductor element such as a MOS-FET used as a main switching element and a commutation element can be a semiconductor element having a low withstand voltage and a low conduction resistance. A power supply device can be realized.

  In addition, since the switching power supply device is provided with a current limiting circuit that limits the maximum output current by providing an off period of the main switching element within one switching period, the current peak value of the choke coil is determined by the on time of the main switching element. Therefore, it does not become more than one cycle of switching, and it also has an overcurrent protection function that is a function of suppressing an excessive current flow.

(Effects of regenerative operation)
In the present invention, as a primary circuit, one end of a main switching element is connected to one end of an input power supply, one end of a commutation element is connected to the other end of the main switching element, and a series circuit of a choke coil and a buffer capacitor A synchronous rectification step-down chopper circuit in which one end is connected and the other end of the commutation element and the other end of the input power supply are connected to the other end of the series circuit of the choke coil and the buffer capacitor is configured as a secondary side circuit. In a switching power supply device in which a secondary winding is provided and a series circuit of a rectifying element and an output capacitor is connected to the secondary winding to obtain output power from both ends of the output capacitor, the inductance of the choke coil is When the current is below the specified rated value (maximum output current), the current flowing through the choke coil becomes zero within one switching cycle. The main switching element and the commutation element are complementarily turned on and off at a predetermined switching frequency by providing a dead time, and the switching 1 A switching control circuit that controls the commutation element to be turned off within the cycle, a polarity detection circuit that detects the polarity of the choke coil and outputs a polarity detection signal, and the polarity detection circuit outputs the polarity detection signal. A commutation element on-hold control circuit for holding the on-signal of the commutation element output by the switching control circuit and controlling the on-off to continue so that power is regenerated from the output side to the input side. It is possible to realize a low-noise and high-efficiency switching power supply device having a function to

  This regenerative operation is performed when an output voltage setting higher than the output voltage setting value output by the switching power supply is applied to the output side of the switching power supply or when a large-capacity capacitor is attached to the output side. Occurs when the value is suddenly lowered.

  Moreover, the switching power supply device of the present invention makes it possible to make a bidirectional switching power supply device by utilizing a regenerative operation.

  Further, in the switching power supply device of the present invention, if the current of the output choke coil that becomes the regenerative current changes across zero even during the regenerative operation, the recovery current is supplied to the parasitic diodes of the main switching element and the commutation element. Since no current flows, no surge voltage is generated. Moreover, since the parasitic capacitance of the main switching element and the commutation element is extracted, a soft switching operation can be realized. As a result, a high-efficiency, low-noise bidirectional switching power supply device can be realized.

  Even when a large regenerative current flows, the commutation element is not turned on while the current is flowing through the parasitic diode of the main switching element, so that a surge voltage is not generated. In addition, since there is no surge voltage generation, it is possible to use semiconductor elements such as MOS-FETs with low breakdown voltage and low conduction resistance for main switching elements and commutation elements, switching with low noise, high efficiency and regenerative function. A power supply can be realized.

  In addition, since the peak value of the regenerative current of the output choke coil is limited by the on-time of the commutation element so that it does not exceed one switching cycle, the regenerative overcurrent is a function that suppresses the flow of an excessive regenerative current. It also has a current protection function.

Circuit block diagram showing an embodiment of a switching power supply device functioning as an isolated converter FIG. 1 is an explanatory diagram showing operation waveforms when the output current is zero. Explanatory drawing which divided | segmented and showed the circuit operation | movement of 1 period of switching in case an output current is zero in the period AF FIG. 1 is an explanatory diagram showing operation waveforms when the output current is large. An explanatory diagram showing the circuit operation of one switching cycle when the output current is large divided into periods A to F The circuit block diagram which showed embodiment of the switching power supply device provided with the surge voltage prevention function with respect to an excessive output current Explanatory drawing showing operation waveforms when there is no surge voltage prevention function against excessive output current FIG. 1 is an explanatory diagram showing circuit operations during periods G and H in which the output current exceeds the rated current in the embodiment of FIG. 1 in which a function for preventing surge voltage against excessive output current is not provided. Explanatory drawing which showed the operation waveform of embodiment of FIG. 6 which provided the surge voltage prevention function with respect to the excessive output current Explanatory drawing which showed the circuit operation | movement of the period I holding on of the main switching element of FIG. Circuit block diagram showing an embodiment of a switching power supply device having a surge voltage prevention function and an overcurrent protection function against an excessive output current Explanatory diagram showing the operation waveforms when the regenerative current is below the rated current and when the rated current is exceeded when there is no surge voltage prevention function against excessive regenerative current Explanatory drawing which showed circuit operation | movement of the period J and K of FIG. 12, and the period L and M Circuit block diagram showing an embodiment of a switching power supply device having a surge voltage prevention function and a regenerative overcurrent protection function against an excessive regenerative current Explanatory drawing which showed the operation | movement waveform of embodiment of FIG. Explanatory drawing which showed the circuit operation | movement of the period N holding on of the commutation element of FIG. Circuit block diagram showing a conventional single-ended forward converter A circuit block diagram showing a conventional LLC resonant converter

[Configuration of switching power supply unit]
FIG. 1 is a circuit block diagram showing an embodiment of a switching power supply device that functions as an insulating converter.

(Power circuit)
As shown in FIG. 1, the switching power supply according to the present embodiment is provided with a synchronous rectification step-down chopper circuit 10 as a power circuit on the primary side. In the synchronous rectification step-down chopper circuit 10, one end of a main switching element 14 using a MOS-FET is connected to one end of an input power supply 12, and one end of a commutation element 16 using a MOS-FET is connected to the other end of the main switching element 14. Are connected, and one end of a series circuit of the choke coil 18 and the buffer capacitor 20 is connected. The other end of the commutation element 16 and the other end of the input power supply 12 are connected to the other end of the series circuit of the choke coil 18 and the buffer capacitor 20. Is connected.

  As a power circuit on the secondary side of the switching power supply device, a secondary winding 22 is provided in the choke coil 18, and a series circuit of a rectifying element 26 using a diode and an output capacitor 24 is connected to the secondary winding 22. The circuit configuration obtains the output power of the load 28 from both ends of 24.

  The choke coil 18 has a period for operating as an inductor and a period for operating as a transformer. When operating as a transformer, the choke coil 18 serves as a primary winding of the transformer. In the following description, when operating as a transformer, the choke coil 18 is referred to as a choke coil (primary winding) 18.

  Further, the main switching element 14 and the commutation element 16 generate parasitic diodes 60 and 64 and parasitic capacitances 62 and 66 in parallel with each source and drain due to the semiconductor element structure of the MOS-FET. Yes.

  In the present embodiment, the main switching element 14 is disposed on the high side, but may be disposed on the low side. Further, the choke coil 18 and the buffer capacitor 20 may be interchanged as long as they are in series. Similarly, the rectifying element 26 and the output capacitor 24 may be interchanged as long as they are in series.

  The primary side synchronous rectification step-down chopper circuit 10 stores excitation energy in the choke coil 18 and simultaneously stores charge energy in the buffer capacitor 20 when the main switching element 14 is on. When the commutation element 16 is on (the main switching element 14 is off), the excitation energy of the choke coil 18 and the charge energy of the buffer capacitor 20 are transferred from the secondary winding 22 of the choke coil 18 to the load 28 side. Is transmitted to.

  When the commutation element 16 is on, the choke coil 18 operates as a flyback transformer to release the excitation energy stored when the main switching element 14 is on, and at the same time, the energy of the buffer capacitor 20 is secondary. Operates as a forward transformer that transmits to the side.

  That is, the switching power supply device stores energy in the choke coil 18 and the buffer capacitor 20 during a period in which the main switching element 14 is on and the commutation element 16 is off, and the commutation element 16 is on and the main switching element 14 is turned on. The operation of transmitting the energy stored in the choke coil 18 and the buffer capacitor 20 to the output capacitor 24 during the period when the signal is off is performed.

  The choke coil 18 operating as a transformer is in a state in which the primary winding and the secondary winding 22 by the choke coil 18 are coupled, so that the buffer capacitor 20 and the output capacitor 24 are coupled. Therefore, the voltage of each capacitor is a value proportional to the turn ratio (N2 / N1) of the number of turns N1 of the choke coil (primary winding) 18 operating as a transformer and the number of turns N2 of the secondary winding 22. It becomes.

  Since the power circuit on the primary side of the switching power supply device constitutes the synchronous rectification step-down chopper circuit 10 for the buffer capacitor 20, the voltage VCb of the buffer capacitor 20 is generally used to obtain the output voltage of the synchronous rectification step-down chopper circuit. It can be obtained using a typical calculation formula, and is determined by the input voltage Vin and the on-duty duty of the main switching element 14, and is expressed as shown in Formula (1).

VCb: voltage Vin of the buffer capacitor 20: input voltage duty: on-duty of the main switching element 14 The buffer capacitor 20 and the output capacitor 24 are the number of turns N1 and 2 of the choke coil (primary winding) 18 operating as a transformer. Since it becomes a value proportional to the turn ratio (N2 / N1) of the turn N2 of the next winding 22, the output voltage Vo of the switching power supply device is determined by Expression (2).

Vo: output voltage of switching power supply device VCo: output capacitor voltage N1: number of turns of choke coil (primary winding) 18 N2: number of turns of secondary winding 22 (switching control circuit)
The switching control circuit 30 includes a switching frequency generation circuit 32, a triangular wave generation circuit 34, a PWM circuit (pulse width modulation circuit) 36, a first dead time generation circuit 38, a second dead time generation circuit 40, and a commutation control inverter 42. It is configured.

  The switching frequency generation circuit 32 includes an oscillation circuit 31 and outputs a clock signal E1 having a predetermined switching frequency fsw.

  The triangular wave generation circuit 34 includes a switching element 44 using a MOS-FET, a charging / discharging circuit in which a resistor 46 and a capacitor 48 are connected in series, and the switching element when the pulse signal E1 of the oscillation circuit 31 falls to the L level. 44 is turned off, the capacitor 48 is charged via the resistor 46 to generate a linearly increasing signal voltage, and then the switching element 44 is turned on at the rise of the pulse signal E1 of the oscillation circuit 31 to the H level. The capacitor 46 is discharged and reset, thereby generating a triangular wave signal E2 that repeats at the oscillation period of the oscillation circuit 31.

  The PWM circuit 36 receives a duty control signal E3 that is a difference voltage between the output voltage and a predetermined reference voltage and a triangular wave signal E2 from the triangular wave generation circuit 34, and has an on duty according to the signal level of the duty control signal E3. A signal E4 is output to control on / off of the main switching element 14.

  The PWM signal E4 from the PWM circuit 36 is input to the commutation control inverter 42 via the first dead time generation circuit 38, and the commutation control signal E7 inverted by the commutation control inverter 42 is used to generate the main switching element 14. The on / off control of the commutation element 16 is performed in a complementary manner to the on / off control.

  The first dead time generation circuit 38 is a delay circuit in which a resistor 54 and a capacitor 56 are connected in series, and outputs a delay signal E6 obtained by delaying the PWM signal E4 for a fixed time to the commutation control inverter 42. By outputting a commutation control signal E7 that becomes H level after a delay that has become equal to or lower than the threshold level of the commutation control inverter 42, a predetermined first time is between the main switching element 14 and the commutation element 16 being on. Dead time is provided.

  The second dead time generation circuit 40 includes an oscillation circuit 50 and a diode 52, synchronizes the frequencies of the oscillation circuit 31 and the oscillation circuit 50, and outputs a pulse from the oscillation circuit 50 immediately before the pulse signal E1 of the oscillation circuit 31 is generated. A predetermined second dead time is generated by outputting the signal E5, and a second dead time is provided between the commutation element 16 being turned off and the main switching element 14 being turned on.

(Inductance of choke coil)
In the switching power supply according to the present embodiment, the current flowing through the choke coil 18 flows in both the positive direction and the negative direction across zero in one switching period, regardless of the output current, and the main switching element 14 is turned on. At the timing, the inductance Lp of the choke coil (primary winding) 18 is set as shown in Expression (3) so that the current of the choke coil 18 flows toward the input power supply 12. Details of this will be made clear later.

Lp: inductance of choke coil (primary winding) 18 fsw: switching frequency IoMAX: maximum output current (rated value) of switching power supply
By setting the inductance Lp and driving the main switching element 14 and the commutation element 16 with the first and second dead times, the main switching element 14 and the commutation element 16 can be soft-switched. A highly efficient and low noise switching power supply can be realized. Further, since the switching operation can be performed at a fixed frequency and the output voltage can be controlled by the on-duty of the main switching element 14, the switching control circuit 30 can also be realized inexpensively and easily.

[Operation of switching power supply unit]
(Operation when output current is zero)
FIG. 2 is an explanatory diagram showing an operation waveform when the output current becomes zero with respect to FIG. 1, and FIG. 3 is an explanatory diagram showing a circuit operation of one switching cycle when the output current is zero divided into periods A to F. It is.

  Here, FIG. 2 shows the pulse signal E1 of the oscillation circuit 31, the pulse signal E5 of the oscillation circuit 50, the triangular wave signal E2 and the duty control signal E3 input to the PWM circuit 36, and the PWM circuit. PWM signal E4 output from 36, delay signal E6 of first dead time generation circuit 38, gate-source voltage VGS1 of main switching element 14, gate-source voltage VGS2 of commutation element 16, drain of main switching element 14 A source-to-source voltage VDS1, a drain-source voltage VDS2 of the commutation element 16, a voltage VKA across the rectifying element 26, a current IKA of the rectifying element 26, and a choke coil current IL of the choke coil 18 are shown. FIG. 3 shows the circuit operation of one cycle at the switching times t1 to t2 in FIG. 2 divided into six periods A to F.

  In the switching power supply according to the present embodiment, when no output current flows due to no load, no current flows through the secondary winding 22, so that the power circuit on the primary side has both ends of the buffer capacitor 20. Can be considered as a synchronous rectification step-down chopper circuit 10 that is regarded as an output.

    The switching power supply device of this embodiment is controlled by a separately excited type by a switching frequency generation circuit 32. The voltage VCb of the buffer capacitor 20 in the synchronous rectification step-down chopper circuit 10 in which the switching frequency fsw is fixed is determined by the input voltage Vin and the on-duty duty of the main switching element 14, and is the same as the above expression (1) and the following expression (4 )

    When the output current Io is zero, the average value of the current ICb flowing through the choke coil 18 becomes zero. Further, the buffer capacitor 20 and the output capacitor 24 are in a relationship of a turns ratio times the number of turns N1 of the choke coil (primary winding) 18 and the number of turns N2 of the secondary winding 22.

    When the switching power supply device is operating with no load, the main switching element 14 is turned off as shown at the end of the period A while the current of the choke coil 18 is in the positive direction. After the main switching element 14 is turned off, the charge B stored in the parasitic capacitance 66 of the commutation element 16 is obtained by having a period B that is a first dead time in which both the main switching element 14 and the commutation element 16 are turned off. To be recovered.

  At the timing when the first commutation element 16 in the period C is turned on, the charge of the parasitic capacitance 66 is extracted, and thus the commutation element 16 can perform a soft switching operation.

  At the timing when the last commutation element 16 in the period D is turned off, the current of the choke coil 18 is in the negative direction.

  After the commutation element 16 is turned off, the charge E stored in the parasitic capacitance 62 of the main switching element 14 is obtained by having a period E that is a second dead time in which both the main switching element 14 and the commutation element 16 are turned off. To be recovered. At the timing when the first main switching element 14 in the period F is turned on, the charge of the parasitic capacitance 62 is extracted, so that the main switching element 14 can perform a soft switching operation.

  The operation during this period A to F will be described in more detail as follows.

(Operation during period A)
When the output current Io is zero, as shown in the period A in FIG. 2, the main switching element 14 is turned on because VGS1 in FIG. 2 (f) is at the H level, and VGS2 in FIG. 2 (g). Is at the L level, the commutation element 16 is turned off, and the choke coil current IL in FIG. 2 (l) increases in the positive direction.

  At this time, as indicated by an arrow in FIG. 3 (period A), current flows through the path from the positive side of the input power source 12 to the main switching element 14, choke coil 18, buffer capacitor 20, and negative side of the input power source 12, and choke Energy is stored in the coil 18 and the buffer capacitor 20.

  When the triangular wave signal E2 shown in FIG. 2C crosses the level of the duty control signal E3 during the period A, the PMW signal E4 shown in FIG. 2D falls from the H level to the L level, and FIG. VGS1 changes from H level to L level, and as shown at the end of period A, main switching element 14 is turned off.

(Operation during period B)
After the main switching element 14 is turned off, there is a period B that is a first dead time in which both the main switching element 14 and the commutation element 16 are turned off. At this time, the choke coil 18 operates so as to keep the choke coil current IL flowing, so that the side indicated by the dot of the choke coil 18 has a positive polarity and the opposite side has a negative polarity. Therefore, as indicated by an arrow in FIG. 3 (period B), the choke is on the path from the plus side indicated by the dot of the choke coil 18 to the buffer capacitor 20, the parasitic capacitance 66 of the commutation element 16, and the minus side of the choke coil 18. A current flows due to the energy release of the coil 18. For this reason, the electric charge stored in the parasitic capacitance 66 of the commutation element 16 in the period B of the first dead time is extracted.

(Operation during period C)
At the timing when the first dead time of period B passes and VGS2 in FIG. 2G rises from L level to H level and commutation element 16 turns on, the charge of parasitic capacitance 66 of commutation element 16 is in period B. Since it is extracted, the commutation element 16 can be turned on by the soft switching operation at the beginning of the period C.

  That is, since the parasitic capacitance 66 is in a discharge state, the drain-source voltage VDS2 of the commutation element 16 is zero volts, and a soft switching operation in which the commutation element 16 is turned on can be performed in this state.

  When the commutation element 16 is turned on, as shown in FIG. 3 (period C), the choke passes along the path from the plus side indicated by the dots of the choke coil 18 to the minus side of the buffer capacitor 20, the commutation element 16 and the choke coil 18. A choke coil current IL flows due to the energy release of the coil 18, and the choke coil current IL decreases linearly.

  At this time, the choke coil (primary winding) 18 and the secondary winding 22 operate as a forward transformer simultaneously with the flyback transformer, and are induced in the secondary winding as shown in FIGS. The voltage rectifier 26 is turned on by the voltage, and the voltage VKA at both ends becomes substantially zero. However, since there is no load, no load current flows, and the current IKA of the rectifier 26 is zero.

(Operation during period D)
In a period C, when the current of the choke coil 18 decreases and exceeds zero in a state where the main switching element 14 is turned off and the commutation element 16 is turned on, a period D in which the current direction becomes a negative direction is entered.

  At this time, as shown in FIG. 3 (period D), the current is stored so that energy is stored in the choke coil 18 along the path from the plus side of the buffer capacitor 20 to the minus side of the choke coil 18, the commutation element 16, and the buffer capacitor 20. As shown in FIG. 2 (l), the choke coil current IL increases linearly in the negative direction, but no load current flows on the secondary side.

  When approaching the end of period D, VGS2 in FIG. 2 (g) changes from H level to L level by pulse signal E5 of oscillation circuit 50 shown in FIG. 2 (b), and as shown at the end of period D, the commutation element 16 turns off.

(Operation during period E)
After the commutation element 16 is turned off, there is a period E that is a second dead time in which both the main switching element 14 and the commutation element 16 are turned off. At this time, the choke coil 18 operates so as to continue to flow the choke coil current IL, so that the dot-free side of the choke coil 18 has a positive polarity and the side indicated by a dot has a negative polarity.

  Therefore, as indicated by an arrow in FIG. 3 (period E), from the plus side of the choke coil 18, the parasitic capacitance 62 of the main switching element 14, the input power source 12, the buffer capacitor 20, and the minus side indicated by the dots of the choke coil 18 In this path, a current due to the energy release of the choke coil 18 flows. For this reason, the electric charge stored in the parasitic capacitance 62 of the main switching element 14 in the period E of the second dead time is recovered.

(Operation during period F)
At the timing when VGS1 in FIG. 2F rises from the L level to the H level and the main switching element 14 is turned on after the second dead time of the period E has passed, the charge of the parasitic capacitance 62 of the main switching element 14 is in the period E. Since it is recovered, the main switching element 14 can be turned on by the soft switching operation at the beginning of the period F.

  That is, since the parasitic capacitance 62 is in a discharge state, the drain-source voltage VDS1 of the main switching element 14 is zero volts, and a soft switching operation for turning on the main switching element 14 can be performed in this state.

  When the main switching element 14 is turned on, as shown in FIG. 4 (period F), from the plus side of the choke coil 18, the main switching element 14, the input power supply 12, the buffer capacitor 20, and the minus side indicated by dots of the choke coil 18 In this path, a current due to the energy emission of the choke coil 18 flows, and the choke coil current IL changes linearly from the minus direction toward zero as shown in the period F of FIG.

(Operation when output current is large)
FIG. 4 is an explanatory diagram showing operation waveforms when the output current is large with respect to FIG. 1, and is divided into FIGS. 4 (a) to (l) as in FIGS. 2 (a) to (l). FIG. 5 is an explanatory diagram showing the circuit operation of one switching cycle when the output current is large divided into periods A to F. FIG.

  When the output current of the switching power supply device is large, the output current flows to the secondary side in the period C and the period D, which is different from the case where the output current is zero.

  In the period C and the period D, a current obtained by superimposing a current flowing in the secondary side on a current flowing in the choke coil (primary winding) 18 flows on the primary side. When the output current is zero, the average value of the primary side current is zero, but when the output current is large, the average value of the primary side current moves to the plus side as the current increases. To do.

  The operation in the period C and the period D when the output current is large will be described in detail as follows.

(Operation during period C)
At the timing when VGS2 in FIG. 4G rises from the L level to the H level and the commutation element 16 is turned on after the first dead time of the period B passes, the charge of the parasitic capacitance 66 of the commutation element 16 is in the period B. Since it is extracted, the commutation element 16 can be turned on by the soft switching operation at the beginning of the period C.

  That is, since the parasitic capacitance 66 is in a discharge state, the drain-source voltage VDS2 of the commutation element 16 is zero volts, and a soft switching operation in which the commutation element 16 is turned on can be performed in this state.

  When the commutation element 16 is turned on, as shown in FIG. 5 (period C), the choke is routed from the plus side indicated by the dot of the choke coil 18 to the minus side of the buffer capacitor 20, the commutation element 16 and the choke coil 18. A choke coil current IL due to the energy release of the coil 18 flows.

  At this time, the choke coil (primary winding) 18 and the secondary winding 22 operate as a forward transformer simultaneously with the flyback transformer, and the excitation energy of the choke coil 18 and the charge energy of the buffer capacitor 20 are the secondary winding of the choke coil 18. The rectifying element 26 and the output capacitor 24 rectify and smooth the electric power transmitted by the choke coil (primary winding) 18 and the flyback operation and the forward operation of the secondary winding 22 from the line 22 to the load 28 side. As a result, output power is generated, and a load current flows through the load 28.

  Further, as shown in FIGS. 4 (k) and 4 (l), the current IL flowing through the choke coil 18 is equal to the current IKA flowing through the secondary side rectifying element 26 to the current flowing through the choke coil (primary winding) 18. The choke coil current IL decreases linearly after being stepwise reduced.

(Operation during period D)
In a period C, when the current of the choke coil 18 decreases and exceeds zero in a state where the main switching element 14 is turned off and the commutation element 16 is turned on, a period D in which the current direction becomes a negative direction is entered.

  At this time, as shown in FIG. 5 (period D), the current is stored so that energy is stored in the choke coil 18 along the path from the plus side of the buffer capacitor 20 to the minus side of the choke coil 18, the commutation element 16, and the buffer capacitor 20. (Current obtained by superimposing the secondary side current on the primary side choke coil current) flows, and the choke coil current IL increases linearly in the negative direction, as shown in FIG.

  Further, the choke coil (primary winding) 18 and the secondary winding 22 continue to operate as a forward transformer simultaneously with the flyback transformer, and the excitation energy of the choke coil 18 and the charge energy of the buffer capacitor 20 are 2 of the choke coil 18. The rectifying element 26 and the output capacitor 24 rectify and transmit the power transmitted by the flyback operation and the forward operation of the choke coil (primary winding) 18 and the secondary winding 22 from the secondary winding 22 to the load 28 side. By smoothing, output power is generated, and a load current flows through the load 28.

  When approaching the end of period D, VGS2 of FIG. 4G changes from H level to L level by pulse signal E5 of oscillation circuit 50 shown in FIG. 16 turns off.

  In the switching power supply device of the present embodiment, even when the output current is large, the main switching element 14 is used when the current of the choke coil (primary winding) 18 is in the positive direction, as in the case where the output current is zero. Is turned off, and then the first dead time of period B is provided for turning on the commutation element 16, whereby the charge stored in the parasitic capacitance 66 of the commutation element 16 is extracted.

  Further, the commutation element 16 is turned off when the current of the choke coil (primary winding) 18 is in the negative direction, and then a second dead time of period E is provided for the main switching element 14 being turned on. As a result, the electric charge stored in the parasitic capacitance 62 of the main switching element 14 is extracted.

  For this reason, even when the load current is large, the inductance Lp of the choke coil (primary winding) is set as will be described later so that the operation has the first dead time and the second dead time. Similarly to the case where the output current shown in FIG. 3 is zero, the main switching element 14 and the commutation element 16 can be soft-switched.

(Inductance Lp of choke coil)
The switching power supply according to the present embodiment stores excitation energy in the choke coil 18 and simultaneously stores charge energy in the buffer capacitor 20 when the main switching element 14 is on (the commutation element 16 is off). When the commutation element 16 is on (the main switching element 14 is off), the excitation energy of the choke coil 18 and the charge energy of the buffer capacitor 20 are output from the secondary winding 22 of the choke coil 18 to the load side. It is transmitted to the capacitor 24.

  When the commutation element 16 is on, the choke coil 18 operates as a flyback transformer that releases the excitation energy stored when the main switching element 14 is on, and at the same time transmits the energy of the buffer capacitor 20 to the secondary side. Operates as a forward transformer.

  When the commutation element 16 is turned on in this way, the choke coil 18 operates as a forward transformer, and the buffer capacitor 20 and the output capacitor 24 are coupled. The current Io is equal to the energy output to the buffer capacitor 20 by the primary side synchronous rectification step-down chopper circuit 10. Here, the excitation energy of the choke coil 18 transmitted to the secondary side may be considered to be equivalent to the energy once stored in the buffer capacitor 20 and then transmitted to the output capacitor 24 on the secondary side. it can.

  Therefore, the current output from the switching power supply device to the secondary side can be determined by determining the current of the choke coil 18 considering the synchronous rectification step-down chopper circuit 10 in which both ends of the buffer capacitor 20 are regarded as outputs.

  The virtual maximum output current (predetermined rated value) of the synchronous rectification step-down chopper circuit 10 in which both ends of the buffer capacitor 20 are regarded as outputs is defined as ICbMAX. Further, it is assumed that the current amplitude (current change amount) of the choke coil 18 when operating as the synchronous rectification step-down chopper circuit 10 is ΔIL.

  As a soft switching condition, when the main switching element 14 is turned on, the current IL of the choke coil 18 must always be in the negative direction. Thus, the current amplitude ΔIL of the choke coil 18 must be set to cross zero.

  From this, the minimum value of the current amplitude ΔIL of the choke coil 18 is twice the imaginary maximum output current ICbMAX of the synchronous rectification step-down chopper circuit 10, and has the relationship of Expression (5). This is because the average value of the current amplitude ΔIL is a virtual output current ICb of the synchronous rectification step-down chopper circuit 10, and the current amplitude ΔIL is a value larger than twice the virtual maximum output current ICbMAX of the synchronous rectification step-down chopper circuit 10. Otherwise, the current will not return to zero.

  The current amplitude ΔIL of the choke coil 18 is determined by the voltage VL applied to the choke coil 18, the application time Ton, and the inductance Lp, and is given by the following equation.

  The voltage VL applied to the choke coil 18 is the difference between the input voltage Vin and the voltage VCb of the buffer capacitor 20, and is given by the following equation.

  The time Ton during which the voltage VL is applied to the choke coil 18 is the on-time of the main switching element 14, is obtained from the switching frequency fsw and the duty duty, and is given by the following equation.

  The duty duty of the main switching element 14 is obtained from the input voltage Vin and the buffer capacitor voltage VCb using a general calculation formula for calculating the duty of the synchronous rectification step-down chopper circuit, and becomes the following formula.

  Next, consider the relationship between the output current Io output to the secondary side as a switching power supply and the current of the choke coil 18 of the primary circuit.

  The switching power supply device of this embodiment transmits the energy stored in the buffer capacitor 20 to the output capacitor 24 during the period when the commutation element 16 is on. Therefore, the energy Pout output to the load is the same as the energy PCb supplied to the buffer capacitor 20.

  Since energy is a product of voltage and current, considering the case where the output current Io is the maximum output current IoMAX, the energy supplied to the buffer capacitor 20 is the voltage VCb of the buffer capacitor 20 and the choke coil (primary winding). 18 is the product of the maximum value ICbMAX of the current flowing through 18. Further, the energy output from the switching power supply device is the product of the output voltage Vo and the maximum output current IoMAX, and the relationship of Expression (10) is obtained.

  Since the voltage VCb of the buffer capacitor 20 and the output voltage Vo have a relationship of the turns ratio of the number of turns N1 of the choke coil (primary winding) 18 and the number of turns N2 of the secondary winding 22, the relationship of Expression (11) Is obtained.

  Therefore, the following equation (12) is obtained from the equations (10) and (11).

  Therefore, when formula (12) is arranged, the relationship between the maximum output current IoMAX of the switching power supply and the maximum value ICbMAX of the average current of the choke coil 18 of the primary side synchronous rectification step-down chopper circuit 10 is expressed by the following formula (13). Is obtained as follows.

  By obtaining the inductance Lp of the choke coil 18 from the above relational expression, the following expression (14) equivalent to the expression (3) is obtained.

Lp: inductance of choke coil (primary winding) 18: input voltage VCb: voltage of buffer capacitor 20 (output voltage when considered as synchronous rectification step-down chopper circuit 10)
ICbMAX: Maximum value of the average current of the choke coil 18 (maximum output current when considered as the synchronous rectification step-down chopper circuit 10)
fsw: switching frequency As a result, by setting the inductance Lp of the choke coil (primary winding) 18 as shown in the equation (3), the main switching element in the range of the output current Io of the switching power supply device from zero to the rated current 14 and the commutation element 16 can be soft-switched.

(Usefulness of the switching power supply device of FIG. 1)
The switching power supply device shown in FIG. 1 can realize a soft switching operation of a semiconductor element such as a MOS-FET provided as a main switching element 14 and a commutation element 16 with a simple configuration. Even if the output current changes, it can be operated at a fixed frequency.

  In addition, since the surge voltage that has been a problem of the conventional single-ended forward converter shown in FIG. 17 is not generated, a semiconductor element having a low withstand voltage (a semiconductor element having a low on-resistance) can be used. It can contribute, and can also reduce switching noise.

  Further, since the frequency modulation control required in the LLC resonant converter shown in FIG. 18 is unnecessary, the switching control circuit 30 can be configured at a low cost, and operates at a fixed frequency, so that the measuring instrument or the like Even if it is used for measurement, it does not adversely affect measurement errors.

  Furthermore, if the inductance Lp of the choke coil (primary winding) 18 is set so as to satisfy the expression (3), even if the input voltage, output voltage, load current, etc. change, as in the LLC resonant converter, An excessive stress is not applied to the switching element.

[Embodiment for preventing surge voltage against excessive output current]
(Configuration of switching power supply)
FIG. 6 is a circuit block diagram showing an embodiment of a switching power supply device having a surge voltage preventing function against an excessive output current. The switching power supply device of FIG. 6 is characterized in that no surge voltage is generated even when a current exceeding the maximum output current IoMAX flows, in addition to the features of the switching power supply device of FIG.

  As shown in FIG. 6, in the switching power supply device of the present embodiment, the synchronous rectification step-down chopper circuit 10 provided as the primary side power circuit includes a main switching element 14 using a MOS-FET at one end of the input power supply 12. One end is connected, the other end of the main switching element 14 is connected to one end of a commutation element 16 using a MOS-FET, and one end of a series circuit of a choke coil 18 and a buffer capacitor 20 is connected to the choke coil 18. The other end of the commutation element 16 and the other end of the input power source 12 are connected to the other end of the series circuit of the buffer capacitor 20.

  Further, as a secondary side power circuit, a secondary circuit 22 is provided in the choke coil 18 corresponding to the primary coil, and a series circuit of a rectifier element 26 using a diode in the secondary coil 22 and an output capacitor 24 is provided. A circuit configuration is obtained in which the output power of the load 28 is obtained from both ends of the output capacitor 24. Note that parasitic diodes and parasitic capacitances generated in parallel between the source and drain of the main switching element 14 and the commutation element 16 are omitted.

  The switching control circuit 30 includes a switching frequency generation circuit 32, a triangular wave generation circuit 34, a PWM circuit 36, a first dead time generation circuit 38, a second dead time generation circuit 40, and a commutation control inverter 42.

  These configurations are the same as those in the embodiment of FIG. 1, but in this embodiment, a polarity detection circuit 70 and a main switching element on hold control circuit 74 are added.

  The polarity detection circuit 70 includes a polarity detection coil 72 coupled to the choke coil 18, detects the polarity of the voltage generated in the choke coil 18, and outputs a polarity detection signal E8. That is, the polarity detection circuit 70 outputs a polarity detection signal E8 that becomes L level when a positive voltage is generated on the side of the choke coil 18 indicated by the dot, and the polarity detection signal E8 on the side indicated by the dot of the choke coil 18 When a positive voltage is generated on the opposite side, a polarity detection signal E8 that is H level is output.

  The main switching element on-hold control circuit 74 outputs a polarity detection signal E8 that generates a positive voltage on the choke coil 18 on the side indicated by a dot by the polarity detection circuit 70 and outputs an L level. The PWM signal E4 that is at H level output from the output 30 is fixed at L level, and the main switching element 14 is controlled to be held off and kept off.

(Operation without surge voltage prevention function)
As for the embodiment of FIG. 1 in which the polarity detection circuit 70 and the main switching element on hold control circuit 74 are not provided as in the embodiment of FIG. 6, the operation when the output current Io flows above the maximum output current IoMAX is shown in FIG. 7 and FIG.

  FIG. 7 is an explanatory diagram showing operation waveforms when there is no surge voltage prevention function for an excessive output current, and the signals of each part are shown separately in FIGS. 7 (a) to (j). FIG. 8 is an explanatory diagram showing circuit operations in the period G and the period H in which the output current of FIG. 1 exceeds the rated current.

(Operation during period A)
As shown in FIG. 7C, when the load 28 demands a large current and the output current Io exceeds the maximum output current IoMAX, the signal voltage of the duty control signal E3 rises, and the triangular wave signal E2 becomes a duty cycle. Since the period until the control signal E3 is crossed becomes longer, the period until the PMW signal in FIG. 7D falls from the H level to the L level becomes longer.

  When a large current flows in the load 28 in this way, the on-duty of the main switching element 14 increases, and the choke coil current IL increases as shown in the period A in FIG. 7 (j).

(Operation during period B)
After the main switching element 14 is turned off, the charge B stored in the parasitic capacitance 66 of the commutation element 16 is obtained by having a period B that is a first dead time in which both the main switching element 14 and the commutation element 16 are turned off. Pulled out.

(Operation during period C)
At the timing when the first dead time of period B passes and VGS2 in FIG. 7G rises from L level to H level and commutation element 16 is turned on, the charge of parasitic capacitance 66 of commutation element 16 is in period B. Since it is drawn out, the commutation element 16 can be turned on by a soft switching operation. In the period C in which the commutation element 16 is on, the choke coil 18 releases energy, and the choke coil current IL decreases.

  In this case, the choke coil current IL decreases with the main switching element 14 turned off and the commutation element 16 turned on, but the output current Io is larger than the maximum output current IoMAX. As a result, the choke coil current IL does not drop below zero.

  When approaching the end of period C in this state, VGS2 of FIG. 7G changes from H level to L level by pulse signal E5 of oscillation circuit 50 shown in FIG. 7B, and as shown at the end of period C, as shown in FIG. The commutation element 16 is turned off.

(Operation during period G)
After the commutation element 16 is turned off, there is a period G that becomes a second dead time in which both the main switching element 14 and the commutation element 16 are turned off. At this time, since the output current Io is larger than the maximum output current IoMAX, the choke coil current IL does not decrease below zero.

  For this reason, as indicated by an arrow in FIG. 8 (period G), the choke is routed from the plus side indicated by the dot of the choke coil 18 to the buffer capacitor 20, the parasitic diode 64 of the commutation element 16, and the minus side of the choke coil 18. A current due to the energy release of the coil 18 flows.

(Operation during period H)
After the second dead time of period G has passed, VGS1 in FIG. 7F rises from the L level to the H level, and the main switching element 14 is turned on. However, immediately after the main switching element 14 is turned on, the recovery operation of the parasitic diode 64 of the commutation element 16 causes the positive side of the input power supply 12, the main switching element 14, the commutation circuit, as indicated by an arrow in FIG. 8 (period H). A large through current flows through the parasitic diode 64 of the current element 16 and the path on the negative side of the input power supply 12, energy is stored in the parasitic inductance due to the wiring, and the energy stored in this parasitic inductance is transferred to the commutation element 16 as shown in FIG. A surge voltage Vs is generated as indicated by VSD2 in (i).

  Thus, in the case of the embodiment of FIG. 1 in which the polarity detection circuit 70 and the main switching element on hold control circuit 74 are not added as shown in FIG. 6, when the load requests a large current, the current of the choke coil 18 Occurs, and a surge voltage is generated in the commutation element 16.

(Operation with surge voltage prevention function)
The operation when the output current Io flows to the maximum output current IoMAX or more when the polarity detection circuit 70 and the main switching element on hold control circuit 74 are added as in the embodiment of FIG. explain.

  FIG. 9 is an explanatory diagram showing operation waveforms when a surge voltage prevention function for an excessive output current is provided, and the signals of each part are shown separately in FIGS. 9 (a) to 9 (k). The polarity detection signal E8 of FIG. 9F is added. FIG. 10 is an explanatory diagram showing a circuit operation in a period I in which the main switching element of FIG. 9 is suspended.

(Operation during period A)
As shown in FIG. 9C, when the load 28 requests a large current and the output current Io exceeds the maximum output current IoMAX, the signal voltage of the duty control signal E3 rises, and the triangular wave signal E2 is fed back. Since the period until it crosses the control signal E3 becomes longer, the period until the PMW signal E4 in FIG. 9D falls from the H level to the L level becomes longer.

  When a large current flows in the load 28 in this way, the on-duty of the main switching element 14 increases, and the choke coil current IL increases as shown in the period A in FIG. 9 (k).

  At this time, since the polarity detection signal E8 output from the polarity detection circuit 70 is at the H level, the main switching element on hold circuit 74 does not hold the main switching element 14 on, and the PWM from the PWM circuit 36 does not hold. The main switching element 14 is turned off in synchronization with the signal E4.

(Operation during period B)
After the main switching element 14 is turned off, the charge B stored in the parasitic capacitance 66 of the commutation element 16 is obtained by having a period B that is a first dead time in which both the main switching element 14 and the commutation element 16 are turned off. Pulled out.

(Operation during period C)
At the timing when VGS2 in FIG. 9H rises from the L level to the H level and the commutation element 16 is turned on after the first dead time of the period B has passed, the parasitic capacitance charge of the commutation element 16 is pulled in the period B. Since it is removed, the commutation element 16 can be turned on by a soft switching operation. In the period C in which the commutation element 16 is on, the choke coil 18 releases energy, and the choke coil current IL decreases.

  In this state, the pulse signal E5 of the oscillation circuit 50 shown in FIG. 9B changes the VGS2 in FIG. 9H from the H level to the L level, and the commutation element 16 is turned off as shown at the end of the period C. .

(Operation during period G)
After the commutation element 16 is turned off, there is a period G that becomes a second dead time in which both the main switching element 14 and the commutation element 16 are turned off. In the period G, current due to the energy discharge of the choke coil 18 flows through the path from the plus side indicated by the dots of the choke coil 18 to the buffer capacitor 20, the parasitic diode 64 of the commutation element 16, and the minus side of the choke coil 18, The coil current IL continues to decrease, and at the same time, the coil current IL is transmitted from the secondary winding 22 of the choke coil 18 to the load 28 side, and the load current flows.

(Operation during period I)
In the period I, the PWM signal E4 in FIG. 9D becomes H level to try to turn on the main switching element 14, but at this time, the choke coil 18 is in a state of releasing energy, The polarity detection signal E8 from the polarity detection circuit 70 maintains the L level state.

  Thus, when the polarity detection signal E8 is in the L level state, the AND circuit 76 of the main switching element on hold control circuit 74 is in the L level even if the PWM signal E4 becomes H level to turn on the main switching element 14. The main switching element ON hold control circuit 74 holds the main switching element 14 on and keeps it off while being prohibited by the level polarity detection signal E8 and fixed to the L level output.

  Therefore, as in the period G, when a current flows through the parasitic diode 64 of the commutation element 16, the choke coil 18 continues to release energy toward the buffer capacitor 20 and the load 28, and the choke coil current IL continues to decrease. .

  When the choke coil 18 finishes releasing the energy, the polarity detection signal from the polarity detection circuit 70 becomes H level, and the inhibition state of the AND circuit 76 provided in the main switching element hold control circuit 74 is released. At this time, the PWM signal E4 is Since it is at the H level, the main switching element 14 is turned on.

  Further, at the end of the period I, the main switching element 14 is turned on after the choke coil current IL becomes zero. Therefore, the main switching element 14 is turned on after the current flowing through the parasitic diode 64 of the commutation element 16 becomes zero. For this reason, the recovery operation does not occur in the parasitic diode 64 of the commutation element 16, so that a through current as shown in FIG. 8 (period H) does not flow, and no surge voltage is generated in the commutation element 16.

(Usefulness of the switching power supply device of FIG. 6)
The switching power supply device shown in FIG. 6 has a function of the polarity detection circuit 70 and the main switching element on-hold control circuit 74, and even when an output current Io exceeding the maximum output current IoMAX is attempted to flow, Since the main switching element 14 is not turned on while the current is flowing through the diode, a surge voltage is not generated in the commutation element 16, and a semiconductor element such as a MOS-FET used as the commutation element 16 In addition, an element having a low withstand voltage and a low conduction resistance can be used, and a switching power supply device with low noise and high efficiency can be realized.

[Switching power supply with overcurrent protection function]
FIG. 11 is a circuit block diagram showing an embodiment of a switching power supply device having a surge voltage prevention function and an overcurrent protection function against an excessive output current.

  As shown in FIG. 11, the switching power supply according to this embodiment includes a maximum duty limiting circuit 78 that functions as an output current limiting circuit in addition to the switching power supply shown in FIG. 6.

  The maximum duty limit circuit 78 includes an inverter 80 and an AND circuit 82. The inverter 80 inverts the pulse signal E1 from the oscillation circuit 31 and inputs the inverted signal to one of the AND circuits 82. The PWM circuit 36 is connected to the other of the AND circuits 82. The PWM signal E4 is input, and control is performed so as to provide a period in which the main switching element 14 is always turned off within one switching period.

  In other words, the maximum duty limiting circuit 78 controls the main switching element 14 to be turned off when the pulse signal E1 from the oscillation circuit 31 rises from the L level to the H level, thereby causing an excessive output current to flow. Suppress. As a result, the ON period of the main switching element 14 does not exceed one switching period.

  More specifically, in the operation waveform of FIG. 9, the state in which the duty control signal E3 intersects the triangular wave signal E2 has been described. However, the duty control signal E3 further increases as the output current Io increases. As a result, the triangular wave signal E2 cannot be crossed, the main switching element 14 cannot be turned off, and the choke coil current IL continues to rise and an excessive current flows.

  Therefore, as shown in FIG. 11, by providing a maximum duty limiting circuit 78, it becomes possible to determine the upper limit of the ON period of the main switching element 14, and the current rise of the output current is the maximum of the main switching element 14. Since it is limited by the duty, it is possible to suppress an excessive current from flowing through the switching power supply device.

[Switching power supply with regenerative function]
The switching power supply device shown in FIG. 1 can realize a function of regenerating power from the output side to the input side by changing the rectifying element 26 using a diode to a MOS-FET or the like having a bidirectional rectifying function. it can.

  Regenerative operation is performed when a voltage higher than the voltage output by the switching power supply (output voltage setting value) is applied to the output side of the switching power supply or when a large-capacity capacitor is attached to the output side. Occurs when the output voltage set value is suddenly lowered. Moreover, this switching power supply device can make a bidirectional switching power supply device by actively utilizing the regenerative phenomenon.

  In a switching power supply having a regenerative function, even during a regenerative operation, if the regenerative current is equal to or less than a predetermined maximum regenerative current IoMAX, no recovery current flows through the parasitic diodes of the main switching element 14 and the commutation element 16. Therefore, no surge voltage is generated. Further, since the parasitic capacitance is extracted, a soft switching operation can be realized. Thereby, a bidirectional switching power supply device with high efficiency and low noise can be produced.

(Regenerative operation of switching power supply)
A soft switching operation of the switching power supply device having a regenerative function by changing the rectifying element 26 using the diode of FIG. 1 to a MOS-FET will be described with reference to FIGS.

  FIG. 12 is an explanatory diagram showing operation waveforms when the regenerative current is less than or equal to the maximum regenerative current IoMAX and when the regenerative current exceeds the maximum regenerative current IoMAX, and the signals of each part are shown in FIGS. 12 (a) to 12 (j). ing. FIG. 13 is an explanatory diagram showing circuit operations during periods J, K, L, and M in which the regenerative current of FIG. 12 exceeds the rated current.

(Operation when the regenerative current is below the maximum regenerative current IoMAX)
When an external voltage is applied to the output of the switching power supply device having a regenerative function, a regenerative current flows. The switching power supply regenerates current so that the output voltage Vo becomes its set value. In other words, the voltage applied to the output terminal of the switching power supply device causes a voltage drop due to the flow of the regenerative current, and the voltage Vo applied to the output terminal of the switching power supply becomes the set value. When the regenerative current is flowing, the average value of the choke coil current IL of the choke coil 18 is negative.

  In addition, since it is controlled by a separate excitation method (fixed frequency method), the on-timing of the main switching element 14 is generated by the switching frequency generation circuit 32 including the oscillation circuit 31.

  The operation when the regenerative current is less than or equal to the maximum regenerative current IoMAX will be described with respect to periods A, B, J, K, and E in FIGS.

(Operation during period A)
When the PWM signal E4 is at the H level, the main switching element 14 is turned on, while the commutation element 16 is turned off, and the current of the choke coil 18 is directed in the positive direction regardless of the magnitude of the regenerative current. . In this state, when the triangular wave signal E2 shown in FIG. 12C crosses the duty control signal E3, the PMW signal in FIG. 12D falls from H level to L level, and VGS1 in FIG. By changing from the level to the L level, the main switching element 14 is turned off as shown at the end of the period A.

(Operation during period B)
After the main switching element 14 is turned off, the charge B stored in the parasitic capacitance 66 of the commutation element 16 is obtained by having a period B that is a first dead time in which both the main switching element 14 and the commutation element 16 are turned off. to recover.

(Operation during period J)
At the timing when the first dead time of period B passes and VGS2 in FIG. 12G rises from L level to H level and commutation element 16 is turned on, the charge of the parasitic capacitance of commutation element 16 is recovered in period B. Therefore, the commutation element 16 can be turned on by a soft switching operation. In this case, as shown in FIG. 13 (period J), regenerative current flows on the secondary side and the primary side, and regenerative energy is accumulated in the choke coil 18 and the buffer capacitor 20.

(Operation during period K)
In the period J, when the main switching element 14 is turned off and the commutation element 16 is turned on, the choke coil current IL of the choke coil 18 decreases and exceeds zero. to go into. In this case, as shown in FIG. 13 (period K), regenerative current flows on the secondary side and the primary side, and accumulation of regenerative energy in the choke coil 18 is continued. In this period K, the buffer capacitor 20 releases the regenerative energy accumulated in the period J toward the choke coil 18.

  When approaching the end of period K, VGS2 in FIG. 12G changes from H level to L level by the pulse signal E5 of the oscillation circuit 50 shown in FIG. 16 turns off.

(Operation during period E)
After the commutation element 16 is turned off, there is a period E that is a second dead time in which both the main switching element 14 and the commutation element 16 are turned off. At this time, the choke coil 18 operates so as to continue to flow the choke coil current IL, so that the dot-free side of the choke coil 18 has a positive polarity and the side indicated by a dot has a negative polarity.

  Therefore, the current due to the energy emission of the choke coil 18 is a path from the plus side of the choke coil 18 to the negative side indicated by the parasitic capacitance 62 of the main switching element 14, the input power supply 12, the buffer capacitor 20 and the choke coil 18. Flowing. For this reason, the electric charge stored in the parasitic capacitance 62 of the main switching element 14 in the period E of the second dead time is recovered.

  At the timing when VGS1 in FIG. 12F rises from the L level to the H level and the main switching element 14 is turned on after the second dead time of the period E passes, the charge of the parasitic capacitance 62 of the main switching element 14 is in the period E. Since it is recovered, the main switching element 14 can be turned on by a soft switching operation.

(Usefulness of regenerative operation)
As described above, in the switching power supply device having the regenerative function of the present embodiment, even during the regenerative operation, if the regenerative current is less than or equal to the maximum regenerative current IoMAX, that is, the choke coil current IL of the choke coil 18 varies across zero. Then, since no recovery current flows through the parasitic diodes of the main switching element 14 and the commutation element 16, no surge voltage is generated.

  Further, since the parasitic capacitance of the main switching element 14 and the commutation element 16 is extracted by setting the dead time, a soft switching operation can be realized. As a result, a switching power supply device and a bidirectional switching power supply device having a regenerative function with high efficiency, low noise, and low cost can be realized.

[Switching power supply with surge voltage protection against excessive regenerative current]
(Operation when there is no surge voltage prevention function)
A switching power supply having a regenerative function generates a surge voltage when a regenerative current exceeding the maximum regenerative current IoMAX is caused to flow by a regenerative operation. The operation in this case will be described for the periods F, L, and M in FIGS.

(Operation during period F)
If an external voltage is applied to the load side of the switching power supply device having a regeneration function and the regenerative current is flowing, and the applied voltage is increased in the direction of increasing the regenerative current, as shown in FIG. Since the signal voltage of the duty control signal E3 decreases and the period until the triangular wave signal E2 crosses the duty control signal E3 is shortened, this shortens the period until the PWM signal E4 changes from the H level to the L level. . When the external voltage is thus applied to the load side, the switching control circuit 30 operates so as to narrow the on-duty of the main switching element 14.

(Operation during period L)
After the main switching element 14 is turned off, there is a period L that is a first dead time in which both the main switching element 14 and the commutation element 16 are turned off. At this time, the choke coil 18 operates so as to continue to flow the choke coil current IL, so that the dot-free side of the choke coil 18 has a positive polarity and the side indicated by a dot has a negative polarity.

  Accordingly, as indicated by an arrow in FIG. 13 (period L), from the plus side of the choke coil 18, the parasitic capacitance 62 of the main switching element 14, the input power source 12, the buffer capacitor 20, and the minus side indicated by the dots of the choke coil 18 In this path, a current due to the energy release of the choke coil 18 flows.

(Operation during period M)
After the first dead time of period L has passed, VGS2 in FIG. 12 (g) rises from L level to H level, and commutation element 16 is turned on. However, immediately after the commutation element 16 is turned on, the parasitic operation of the main switching element 14 on the positive side of the input power supply 12 is performed by the recovery operation of the parasitic diode 60 of the main switching element 14 as indicated by an arrow in FIG. 13 (period M). A large through current flows through the diode 60, the commutation element 16, and the path on the negative side of the input power supply 12, energy is stored in the parasitic inductance due to the wiring, and the energy stored in the parasitic inductance is stored in the main switching element 14 as shown in FIG. A surge voltage Vs is generated as indicated by V DS 1 in (h).

  Therefore, FIG. 14 shows an embodiment of a switching power supply device that has no function of generating a surge voltage even when the regenerative current becomes equal to or greater than the maximum regenerative current IoMAX and has a function of suppressing the flow of an excessive regenerative current.

(Configuration of switching power supply)
FIG. 14 is a circuit block diagram showing an embodiment of a switching power supply device having a surge voltage preventing function and a regenerative overcurrent protection function against an excessive regenerative current.

  As shown in FIG. 14, a synchronous rectification step-down chopper circuit 10 provided as a primary side power circuit includes a main switching element 14 using a MOS-FET, a commutation element 16 using a MOS-FET, a choke coil 18, A buffer capacitor 20 is provided, and a secondary side power circuit is provided with a secondary winding 22 provided in the choke coil 18, a rectifying element 26a, and an output capacitor 24. A switching control circuit 30 is provided with a switching frequency generation circuit 32, a triangular wave generation. A circuit 34, a PWM circuit 36, a first dead time generation circuit 38, a second dead time generation circuit 40, and a commutation control inverter 42 are provided.

These configurations are the same as those in the embodiment of FIG. 1, but in this embodiment, a MOS that enables bidirectional rectification of the rectifying element 26a provided in the secondary power circuit in order to realize the regeneration function. -FET, and polarity detection circuit 84 and commutation element on hold control circuit 88 are added.

  The polarity detection circuit 84 includes a polarity detection coil 86 coupled to the choke coil 18, detects the polarity of the voltage generated in the choke coil 18, and outputs a polarity detection signal E9. That is, the polarity detection circuit 84 outputs a polarity detection signal E9 that becomes H level when a positive voltage is generated in the direction in which the dot is present in the choke coil 18, and a positive voltage is applied in the direction in which there is no dot. When it occurs, the polarity detection signal E9 which becomes L level is output. This is the reverse of the case of the polarity detection circuit 70 of FIG.

  The commutation element on-hold control circuit 88 outputs the polarity detection signal E9 that is at the L level when the polarity detection circuit 84 generates a positive voltage in the direction where there is no dot in the choke coil 18 and outputs the polarity detection signal E9. Even if the synchronous rectification control signal E7 output by the signal 30 becomes H level, the commutation element 16 is controlled to be kept on.

  FIG. 15 is an explanatory diagram showing operation waveforms when a surge voltage prevention function against an excessive regenerative current is provided, and the signals of each part are shown separately in FIGS. 15 (a) to 15 (k). The polarity detection signal E9 of FIG. 15 (f) is added.

  Since the operations in the periods A, B, J, K, and E are the same as those in FIG. 12, the operations in the subsequent periods F, L, and N will be described.

(Operation during period F)
If an external voltage is applied to the load side of the switching power supply device of FIG. 14 and the regenerative current is applied and the applied voltage is increased in the direction of increasing the regenerative current, the duty cycle is increased as shown in FIG. Since the signal voltage of the control signal E3 decreases and the period until the triangular wave signal E2 crosses the duty control signal E3 is shortened, this shortens the period until the PWM signal E4 changes from the H level to the L level. When the external voltage is thus applied to the load side, the switching control circuit 30 operates so as to narrow the on-duty of the main switching element 14.

(Operation during period L)
The period L is a first dead time in which both the main switching element 14 and the commutation element 16 are turned off, and the choke coil 18 operates to keep the choke coil current IL flowing. The polarity is positive, and the side indicated by a dot is negative polarity.

  Therefore, the current due to the energy emission of the choke coil 18 is a path from the plus side of the choke coil 18 to the negative side indicated by the parasitic capacitance 62 of the main switching element 14, the input power supply 12, the buffer capacitor 20 and the choke coil 18. The choke coil current IL continues to rise.

(Operation during period N)
In the period N, the commutation control signal E7 that becomes H level by outputting the delay signal E6 from the first dead time generation circuit 38 of the switching control circuit 30 to the commutation control inverter 42 is output. Since the choke coil 18 is releasing energy, the polarity detection signal E9 of the polarity detection circuit 84 is maintained at the L level.

  For this reason, even if the switching control circuit 30 outputs the commutation control signal E7 that becomes the H level, the AND circuit 90 of the commutation element on-hold control circuit 88 uses the polarity detection signal E9 that becomes the L level from the polarity detection circuit 84. In the prohibited state, the output is maintained at the L level, and the commutation element 16 is kept on.

  Accordingly, the period N is the same as the period L, as shown in FIG. 16 (period N), from the plus side of the choke coil 18 to the parasitic diode 62 of the main switching element 14, the input power supply 12, the buffer capacitor 20, and the choke coil 18. In the path on the negative side indicated by dots, a current due to the energy release of the choke coil 18 flows, and the choke coil current IL continues to change toward zero.

  In this state, when the choke coil 18 finishes releasing the energy, the polarity detection signal E9 of the polarity detection circuit 84 becomes H level, and the inhibition state of the AND circuit 90 of the commutation element on hold control circuit 88 is released. The commutation control signal E7 from the switching control circuit 30 at the level is output to the commutation element 16, and the commutation element 16 is turned on.

  Therefore, at the end of the period N, the commutation element 16 is turned on after the choke coil current IL of the choke coil 18 becomes zero, that is, after the current flowing through the parasitic diode of the main switching element 14 becomes zero. . Accordingly, the recovery operation of the parasitic diode of the main switching element 14 does not occur, the through current as shown in FIG. 13 (period M) does not flow, and the surge voltage is not generated in the main switching element 14.

(Regenerative current overcurrent protection function)
The switching power supply device of FIG. 14 is the on hold control of the commutation element 16 in FIG. 15 (period N) by the polarity detection circuit 84 and the commutation element on hold control circuit 88, and in the following periods J and K, Even if the ON period of the commutation element 16 becomes long, the commutation element 16 is controlled to be turned off by the pulse signal E5 from the oscillation circuit 50 provided in the second dead time generation circuit 40. Therefore, the switching power supply device It is possible to realize an overcurrent prevention function that suppresses the flow of an excessive regenerative current. That is, the switching power supply device can realize an overcurrent prevention function that suppresses the flow of an excessive regenerative current by controlling the commutation element 16 to be turned off within one cycle of switching for performing the regenerative operation. .

(Usefulness of the switching power supply device of FIG. 14)
The switching power supply having the regenerative function shown in FIG. 14 adds the polarity detection circuit 84 and the commutation element on hold control circuit 88, so that even when a large regenerative current flows, a current flows through the parasitic diode of the main switching element 14. Since the commutation element 16 is not turned on while the current flows, a surge voltage is not generated in the main switching element 14 and the semiconductor element such as a MOS-FET used for the main switching element 14 has a low breakdown voltage. An element with low conduction resistance can be used, and a switching power supply device having a low noise and high efficiency regeneration function can be realized.

  In addition, since the peak value of the regenerative current is limited by the on-time of the commutation element 16 and does not exceed one switching cycle, it also has a regenerative overcurrent protection function that is a function that suppresses the flow of an excessive regenerative current. It will be.

[Modification of the present invention]
The present invention includes appropriate modifications without impairing the objects and advantages thereof. For example, a switching power supply device having the main switching element on hold circuit 74 and the commutation element on hold circuit 88 at the same time may be used, or a polarity detection circuit in which the polarity detection coil 72 and the polarity detection coil 86 are shared by one coil is configured. The main switching element on-hold circuit 74 and the commutation element on-hold circuit 88 may be controlled by one polarity detection circuit. Further, at the same time that the polarity detection coil is used for the polarity detection circuit, an auxiliary power supply circuit may be configured to generate drive power for the control circuit. Furthermore, there is no limitation by the numerical values shown in the above embodiment.

10: synchronous rectification step-down chopper circuit 12: input power supply 14: main switching element 16: commutation element 18: choke coil 20: buffer capacitor 22: secondary winding 24: output capacitor 26: rectification element 28: load 30: switching control Circuits 31 and 50: Oscillation circuit 32: Switching frequency generation circuit 34: Triangular wave generation circuit 36: PWM circuit 38: First dead time generation circuit 40: Second dead time generation circuit 42: Inverters 60, 64 for commutation control Diodes 62 and 66: Parasitic capacitances 70 and 84: Polarity detection circuit 74: Main switching element on hold control circuit 78: Maximum duty limit circuit 88: Commutation element on hold control circuit

Claims (4)

As a primary circuit, one end of a main switching element is connected to one end of an input power supply, one end of a commutation element is connected to the other end of the main switching element, and a primary winding of a choke coil and a buffer capacitor are connected in series. one end of the circuit is connected, synchronous buck chopper circuit and the other end to the other end of the input power source of the commutation device in the other end of the series circuit is connected to the primary winding and the buffer capacitor of the choke coil It is composed,
As the secondary circuit, the series circuit of the output capacitor and the rectifying device in the secondary winding of the choke coil is connected, the circuit to obtain an output voltage from both ends of the output capacitor a is the switching power supply configuration,
The inductance of the choke coil is set to such a value that the current flowing through the choke coil flows in both the positive direction and the negative direction within one cycle of switching when the output current is equal to or less than a predetermined rated value.
Based on a PWM signal that is an on-duty of the main switching element for controlling the output voltage to a predetermined voltage, the main switching element and the commutation element are complementarily turned on and off with a dead time. switching power supply apparatus characterized by comprising a switching control circuit having a function.
As a primary circuit, one end of a main switching element is connected to one end of an input power supply, one end of a commutation element is connected to the other end of the main switching element, and a primary winding of a choke coil and a buffer capacitor are connected in series. one end of the circuit is connected, synchronous buck chopper circuit and the other end to the other end of the input power source of the commutation device in the other end of the series circuit is connected to the primary winding and the buffer capacitor of the choke coil It is composed,
As the secondary circuit, the series circuit of the output capacitor and the rectifying device in the secondary winding of the choke coil is connected, the circuit to obtain an output voltage from both ends of the output capacitor a is the switching power supply configuration,
The inductance of the choke coil is set to such a value that the current flowing through the choke coil flows in both the positive direction and the negative direction within one cycle of switching when the output current is equal to or less than a predetermined rated value.
Based on a PWM signal that is an on-duty of the main switching element for controlling the output voltage to a predetermined voltage, the main switching element and the commutation element are complementarily turned on and off with a dead time. A switching control circuit for controlling
A polarity detection circuit for detecting the polarity of the choke coil and outputting a polarity detection signal;
The switching control circuit outputs the polarity detection signal when the polarity detection circuit outputs the polarity detection signal indicating that the primary winding of the choke coil is flowing a current due to energy discharge toward the buffer capacitor. and it controls to continue off pending oN signal of the main switching element, and the main switching element on hold control circuit to suppress the occurrence of a surge voltage of the commutation device,
Switching power supply apparatus characterized by comprising a.
In the switching power supply apparatus according to claim 2, further that it has a current limit circuit for an off period of the main switching element is controlled to so that provided in the one period of the switching cycle, limiting the maximum output current A switching power supply device.
As a primary circuit, one end of a main switching element is connected to one end of an input power supply, one end of a commutation element is connected to the other end of the main switching element, and a primary winding of a choke coil and a buffer capacitor are connected in series. one end of the circuit is connected, synchronous buck chopper circuit and the other end to the other end of the input power source of the commutation device in the other end of the series circuit is connected to the primary winding and the buffer capacitor of the choke coil It is composed,
As the secondary circuit, the series circuit of the rectifying element and an output capacitor having a two-way rectification function is connected to the secondary winding of the choke coil, the circuit for obtaining an output voltage from both ends of the output capacitor is configured, the output a switching power supply apparatus having a function of regenerating power to the input side from the side,
The inductance of the choke coil is such that when the regenerative current flowing from the output side to the input side is less than a predetermined rated value, the current flowing through the choke coil flows in both the positive direction and the negative direction within one switching cycle. Is set to the value
Based on a PWM signal that is an on-duty of the main switching element for controlling the output voltage to a predetermined voltage, the main switching element and the commutation element are complementarily turned on and off with a dead time. And a switching control circuit for controlling to provide an off period of the commutation element within one cycle of the switching;
A polarity detection circuit for detecting the polarity of the choke coil and outputting a polarity detection signal;
When the polarity detection circuit is outputting the polarity detection signal indicating that the primary winding of the choke coil is passing a current due to energy discharge toward the input power supply, the switching control circuit outputs the polarity detection signal. and it controls to continue off pending oN signal of the commutation device, and commutation device on hold control circuit to suppress the occurrence of a surge voltage of the main switching element,
Switching power supply apparatus characterized by comprising a.

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