JP2004282847A - Switching power unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching power unit which can avoid the breakdown of a three-terminal commutating element SW2 in a light load state. <P>SOLUTION: A synchronous rectifying circuit 31 comprises a three-terminal rectifying element SW1 and a thre-terminal commutating element SW2. The three-terminal rectifying element SW1 and the three-terminal commutating terminal SW2 synchronously rectify the voltage induced in a second coil N2, by the synchronous control signal supplied from a transformer T to a control electrode. A decision means 33 monitors the voltage signal which has a time duration corresponding to the intermittence of switching output, on the side of an output circuit 3, and decides it as being a light load, when the time duration corresponding to the ON period of the three-terminal commutation element becomes a certain value or smaller, and switches off the three-terminal commutation element SW2. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a synchronous rectification type switching power supply device.
[0002]
[Prior art]
2. Description of the Related Art In recent years, there has been known a synchronous rectification type switching power supply device in which the efficiency is improved by using a synchronous rectification circuit. In this type of switching power supply, a synchronous rectifier circuit is configured using a field effect transistor (FET) having a lower voltage drop than a rectifier diode, thereby reducing a loss generated in an output circuit.
[0003]
Specifically, the output circuit of the synchronous rectification type switching power supply includes a rectification FET and a commutation FET, and the commutation FET is connected in parallel to a series circuit of a secondary coil of the transformer and the rectification FET. It is connected.
[0004]
In the self-excited synchronous rectification type switching power supply, the rectifying FET and the commutating FET are configured such that the commutating FET is turned off when the rectifying FET is turned on, and the commutating FET is turned on when the rectifying FET is turned off. It is controlled based on a synchronization control signal from the device.
[0005]
However, in the self-excited synchronous rectification type switching power supply device, when the load becomes light, the synchronization of the rectification FET and the commutation FET becomes unstable, and the rectification FET and the commutation FET may be simultaneously turned on. .
[0006]
As described above, since the commutation FET adopts a configuration in which the commutation FET and the second coil are connected in parallel to a series circuit, when the commutation FET and the commutation FET are simultaneously turned on, the commutation FET is turned off. A through current flows through the diversion FET, the rectifying FET and the second coil, and a large surge voltage is generated in the second coil by the through current. Due to the circuit configuration of the synchronous rectifier circuit, the surge voltage is applied to both ends of the commutation FET. Therefore, if the surge voltage exceeds the drain-source breakdown voltage of the commutation FET, the commutation FET may be damaged.
[0007]
There is no technical document that clearly discloses a configuration for preventing the damage of the commutation FET at the time of such a light load. For example, Patent Literature 1 discloses a synchronous rectification type DC-DC converter that can detect a light load state by providing a current detection resistor at an output terminal.
[0008]
However, Patent Literature 1 does not clearly disclose a configuration for preventing breakage of a commutation FET under a light load. Moreover, in the technology described in Patent Document 1, since a light load state is detected using a resistor provided at the output terminal, a large loss in resistance occurs not only at a light load but also at a normal load. This leads to a reduction in efficiency.
[0009]
Further, in Patent Document 1, since the load current is detected, the detected current level becomes small when the load is light. For this reason, it becomes difficult to separate the current signal from the noise, and a problem arises in that the reliability of detection of the light load state is reduced.
[0010]
[Patent Document 1]
JP-A-2000-023456 (Page 2-3, FIG. 1)
[0011]
[Problems to be solved by the invention]
An object of the present invention is to provide a high-efficiency switching power supply device using a synchronous rectifier circuit.
[0012]
Another object of the present invention is to provide a switching power supply device capable of avoiding breakage of a three-terminal commutation element in a light load state.
[0013]
Still another object of the present invention is to provide a switching power supply device capable of reliably detecting a light load state without causing a large loss.
[0014]
[Means for Solving the Problems]
In order to solve the above problems, a synchronous rectification type switching power supply according to the present invention includes a switch circuit, a transformer, and an output circuit. The switch circuit switches a DC voltage to generate a periodically intermittent switching output. The transformer includes a first coil and a second coil, and converts a switching output supplied from the switch circuit to the first coil to a second output by electromagnetic coupling between the first coil and the second coil. To the coil. The output circuit includes a synchronous rectification circuit and a determination unit. The synchronous rectifier circuit includes a three-terminal rectifying element and a three-terminal commutating element, and the three-terminal rectifying element and the three-terminal commutating element are configured to receive a second control signal supplied from the transformer to the control electrode. Synchronously rectifies the voltage induced in the coil.
[0015]
The judging means monitors a voltage signal having a time width corresponding to the intermittent switching output on the output circuit side, and when the time width corresponding to the ON period of the three-terminal rectifying element becomes equal to or less than a certain value, the light load is reduced. Then, the three-terminal commutation element is turned off.
[0016]
Here, the light load indicates a case where the light load connected to the output terminal is small, and includes a no-load state. A specific example of the light load is a load that is lower than a point (critical value) at which the load current flowing through the secondary choke coil becomes intermittent (discontinuous).
[0017]
The above-described switching power supply according to the present invention is a synchronous rectification type switching power supply, and includes a switch circuit and a transformer. The switch circuit switches a DC voltage to generate a periodically intermittent switching output. The transformer includes a first coil and a second coil, and converts a switching output supplied from the switch circuit to the first coil to a second output by electromagnetic coupling between the first coil and the second coil. To the coil.
[0018]
The switching power supply according to the present invention further includes an output circuit. The output circuit includes a synchronous rectifier circuit, and the synchronous rectifier circuit includes a three-terminal rectifying element and a three-terminal commutating element. In a synchronous rectifier circuit, by using a field effect transistor (FET) as a three-terminal rectifying element and a three-terminal commutating element, a voltage drop is reduced in an ON state as compared with a rectifier diode, and sufficiently reduced in an OFF state. The resistance value can be increased, the loss at the time of rectification can be reduced, and higher efficiency can be achieved as compared with a switching power supply device using a rectifier diode.
[0019]
The three-terminal commutation element is generally connected in parallel to a series circuit of the three-terminal rectification element and the second coil, and one end is connected to an output terminal via a choke coil. In a normal load state, the three-terminal rectifying element and the three-terminal commutating element can synchronously rectify the voltage induced in the second coil by the synchronous control signal supplied from the transformer to the control electrode.
[0020]
Further, the switching power supply according to the present invention includes a determination unit. The judging means monitors a voltage signal having a time width corresponding to the intermittent switching output on the output circuit side, and judges that the load is light when the time width corresponding to the ON period of the three-terminal rectifying element becomes equal to or less than a certain value. Then, the three-terminal commutation element is turned off. Therefore, at the time of light load, the through current does not occur, so that the above-described problem of damage to the three-terminal commutation element can be avoided.
[0021]
The light load can be detected from a voltage generated in a second coil provided in the transformer or a voltage generated in a choke coil usually connected to one end of the three-terminal commutation element.
[0022]
The switching power supply device according to the present invention is different from the conventional one in that the method detects the voltage generated in the second coil instead of the method of detecting the current flowing through the resistor provided at the output terminal. Level does not decrease. Therefore, it is possible to reliably determine whether or not the load is light. Further, in the switching power supply device according to the present invention, since no resistor is provided at the output terminal, the loss can be reduced accordingly.
[0023]
Other features of the present invention and the operation and effect thereof will be described in more detail with reference to the accompanying drawings and embodiments. However, the attached drawings are merely examples.
[0024]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is an electric circuit diagram showing one embodiment of the switching power supply device according to the present invention. 1, the switching power supply is a synchronous rectification forward type switching power supply, and includes a switch circuit 7, a transformer T, and an output circuit 3.
[0025]
The switch circuit 7 converts the input AC voltage Vin (AC) into a DC voltage, switches this DC voltage, and generates a periodically intermittent switching output.
[0026]
The transformer T includes a first coil N1, a second coil N2, and a third coil N3. The switching output supplied from the switch circuit 7 to the first coil N1 is supplied to the second coil N2 and the third coil N3 by electromagnetic coupling of the first coil N1, the second coil N2, and the third coil N3. Transmitted to
[0027]
The output circuit 3 includes a synchronous rectification circuit 31 and a determination unit 33. The synchronous rectifier circuit 31 includes a three-terminal rectifying element SW1 and a three-terminal commutating element SW2. As the three-terminal rectifying element SW1 and the three-terminal commutating element SW2, a semiconductor element having a lower voltage drop than a rectifying diode can be used. In this embodiment, the three-terminal rectifying element SW1 and the three-terminal commutating element SW2 are field effect transistors (FETs).
[0028]
The three-terminal rectifying element SW1 is connected in series with the second coil N2. The three-terminal commutation element SW2 is connected in parallel to a series circuit of the three-terminal rectification element SW1 and the second coil N2, and one end is connected to the output terminal 51 via the choke coil L1. The other end of the three-terminal commutation element SW2 is connected to the output terminal 52.
[0029]
The three-terminal commutation element SW2 is connected such that the direction of the internal diode is such that when the power is supplied to the transformer T, the current flowing through the second coil N2 is blocked. When power is supplied to the transformer T, the three-terminal rectifying element SW1 is connected such that the direction of its internal diode does not block the current flowing through the second coil N2. When power is supplied to the transformer T, it corresponds to “continuation” in the intermittent operation of the switch circuit 7.
[0030]
In a normal load state, the three-terminal rectifying element SW1 and the three-terminal commutating element SW2 synchronously rectify the voltage induced in the second coil N2 by the synchronous control signal supplied from the transformer T to the control electrode. .
[0031]
Specifically, when power is supplied to the transformer T, the three-terminal commutation element SW2 is turned off, and power is output via the three-terminal rectification element SW1 and the choke coil L1. On the other hand, when power is not supplied to the transformer T, the three-terminal commutation element SW2 is turned on, and power is output via the three-terminal commutation element SW2 and the choke coil L1.
[0032]
On the other hand, in the light load state, the judging means 33 monitors a voltage signal having a time width corresponding to the intermittent switching output on the output circuit 3 side, and determines that the time width corresponding to the ON period of the three-terminal rectifying element SW1. When the load falls below a certain value, it is determined that the load is light, and the three-terminal commutation element SW2 is turned off.
[0033]
The switching power supply according to the above-described embodiment includes the switch circuit 7, the transformer T, and the output circuit 3. The switch circuit 7 switches a DC voltage to generate a periodically intermittent switching output. The transformer T includes a first coil N1 and a second coil N2, and outputs a switching output supplied from the switch circuit 7 to the first coil N1 to the first coil N1 and the second coil N2. Can be transmitted to the second coil N2.
[0034]
The output circuit 3 includes a synchronous rectifier circuit 31, and the synchronous rectifier circuit 31 includes a three-terminal rectifying element SW1 and a three-terminal commutating element SW2.
[0035]
In the synchronous rectifier circuit 31, by using a field effect transistor (FET) as the three-terminal rectifying element SW1 and the three-terminal commutating element SW2, the voltage drop is reduced in the ON state as compared with the rectifier diode, and the OFF state is achieved. In this case, the resistance value can be sufficiently increased, the loss at the time of rectification can be reduced, and higher efficiency can be achieved as compared with a switching power supply device using a rectifier diode.
[0036]
In the switching power supply device according to the present invention, as shown in the illustrated embodiment, the output circuit 3 includes a judging unit 33. When the judging unit 33 detects a light load, the output circuit 3 is irrespective of the state of the three-terminal rectifying element SW1. Instead, since the three-terminal commutation element SW2 is turned off, it is possible to prevent a through current from occurring at a light load, and to avoid the above-described problem of damage to the three-terminal commutation element.
[0037]
In the switching power supply according to the present embodiment, a light load is detected on the output circuit 3 side from a voltage signal having a time width corresponding to the intermittent switching output. Specifically, the switching output intermittent period is detected from a voltage signal having a time width corresponding to the switching output intermittent operation, and the power supplied to the transformer T is monitored based on this period. Then, by monitoring the power supplied to the transformer T, it is determined whether the load is a light load or not. The voltage signal having a time width corresponding to the switching output intermittently includes, for example, a voltage generated in the second coil N2.
[0038]
Further, the switching power supply device according to the present embodiment differs from the conventional one in that the method detects the voltage generated in the second coil N2 instead of the method of detecting the current flowing through the resistor provided at the output terminal. Also, the level of the detection signal does not decrease. Therefore, it is possible to reliably determine whether or not the load is light. Further, in the switching power supply device according to the present embodiment, since no resistor is provided at the output terminal, the loss can be reduced accordingly.
[0039]
FIG. 2 is an electric circuit diagram showing the switching power supply device shown in FIG. 1 in more detail, and FIG. 3 is another electric circuit diagram showing the switching power supply device shown in FIG. 1 in more detail.
[0040]
In FIG. 2, the switch circuit 7 includes a rectifier circuit 1 and a power factor correction type power conversion circuit 2. The rectifier circuit 1 is a circuit that rectifies an AC power supply AC to generate a rectified output, and is configured by a diode bridge or the like. The AC power supply AC is, for example, a commercial AC power supply.
[0041]
The power factor improvement type power conversion circuit 2 is a circuit to which a dither circuit is applied, and includes first and second switching elements SW3 and SW4, an inductor L2, a capacitor C1, and a diode D1.
[0042]
The transformer T includes a first coil N1, a second coil N2, and a third coil N3. The first coil N1 is a coil for supplying the power generated by the power factor improving type power conversion circuit 2 to the second coil N2 and the third coil N3.
[0043]
In the power factor improving power conversion circuit 2, one end of the inductor L2 is connected to the positive output terminal of the rectifier circuit 1, and the other end is connected to one end of the diode D1 and one end of the first switching element SW3. This diode D1 is oriented to prevent a reverse voltage across inductor L2.
[0044]
The other end of the diode D1 is connected to one end of the capacitor C1 and one end of the first coil N1. The other end of the first coil N1 is connected to one end of a second switching element SW4. The negative output terminal of the rectifier circuit 1 is connected to the other end of the first switching element SW3, the other end of the capacitor C1, and the other end of the second switching element SW4. As the first and second switching elements SW3 and SW4, for example, a semiconductor element such as a bipolar transistor or a field effect transistor (FET) can be used.
[0045]
The control circuit 4 detects the voltages of the output terminals 51 and 52, generates pulse width control signals S31 and S41 according to the detected voltages, and outputs the pulse width control signals S31 and S41 to the first and second switching elements SW3. , SW4.
[0046]
3, the output circuit 3 includes a synchronous rectification circuit 31, an output smoothing circuit 32, and a determination unit 33. The synchronous rectifier circuit 31 includes a three-terminal rectifying element SW1, a three-terminal commutating element SW2, and a diode D2.
[0047]
In the synchronous rectifier circuit 31, the three-terminal commutation element SW2 is connected in parallel to a series circuit of the three-terminal rectification element SW1 and the second coil N2, and one end is connected to the output terminal 51 via the choke coil L1. ing. The other end of the three-terminal commutation element SW2 is connected to the output terminal 52.
[0048]
The diode D2 is connected in parallel with the three-terminal commutation element SW2 such that when the power is supplied to the transformer T, the diode D2 has a direction of blocking a current flowing through the second coil N2. The capacitor C2 is connected in parallel with the output terminals 51 and 52. The output smoothing circuit 32 includes a choke coil L1 and an output smoothing capacitor C2, and is configured as a choke input type.
[0049]
The third coil N3 is connected in series with the second coil N2, and the terminal not connected to the second coil N2 is connected to the gate of the three-terminal commutation element SW2. The third coil N3 is for supplying a voltage (referred to as a flyback voltage) generated in the third coil N3 to the gate of the three-terminal commutation element SW2 when power is not supplied to the transformer T. is there.
[0050]
The determination unit 33 includes an integration circuit 34 and a switch unit 35. The integration circuit 34 includes an operational amplifier IC41, a capacitor C41, a diode D41, and resistors R41, R42, R43, R44. The diode D41 is connected between the second coil N2 and the inverting input terminal of the operational amplifier IC41 in a direction that does not block the current flowing in the second coil N2 when power is supplied to the transformer T. . The capacitor C41 is connected between the inverting input terminal of the operational amplifier IC41 and the output terminal 52.
[0051]
The switch means 35 includes a field effect transistor (FET) SW5, a diode D42, and a resistor R45. The diode D42 is connected between the gate of the three-terminal commutation element SW2 and the drain of the field-effect transistor SW5 in the direction of extracting the gate charge of the three-terminal commutation element SW2. The gate of the field effect transistor SW5 is connected to the output terminal of the operational amplifier IC41.
[0052]
Referring again to FIG. 2, in the switching power supply device having such a configuration, the first and second switching elements SW3 and SW4 are synchronized at a frequency higher than the frequency of the AC power supply AC based on the pulse width control signals S31 and S41. Then, it is controlled to the ON / OFF state. Specifically, the second switching element SW4 is turned on during the ON period of the first switching element SW3, and turned off during the off period of the first switching element SW3.
[0053]
Thereby, the power factor improvement type power conversion circuit 2 stores energy in the inductor L2 during the ON period of the first switching element SW3, charges the capacitor C1 via the diode D1, and supplies power to the first coil N1. Supply. The electric power supplied to the first coil N1 is supplied to the second coil N2. On the other hand, during the OFF period of the first switching element SW3, the energy stored in the inductor L2 is released to charge the capacitor C1, and does not supply power to the first coil N1.
[0054]
Further, in FIG. 3, the integrating circuit 34 integrates the voltage generated in the second coil N2 when the power is supplied to the transformer T using the capacitor C41. The time constant of the integration is adjusted by the resistors R41 and R44. The integral value is equal to or higher than the reference voltage Vth when the load is normal and becomes lower than the reference voltage Vth when the load becomes light. The reference voltage Vth is a reference voltage supplied to the non-inverting input terminal of the operational amplifier IC41. For example, a stable voltage output from the output terminal 51 can be used.
[0055]
The operational amplifier IC41 performs the function of a comparator, and outputs a low-level signal Low from the output terminal of the operational amplifier IC41 when the integrated value is higher than the reference voltage Vth. On the other hand, when the integrated value becomes lower than the reference voltage Vth, the high level signal High is output from the output terminal of the operational amplifier IC41.
[0056]
That is, the integration circuit 34 determines whether the load is a light load or not by detecting the voltage generated in the second coil N2, and outputs the high level signal High if the load is light. If the load is not light, a low level signal Low is output.
[0057]
The switch means 35 turns on the field effect transistor SW5 when the voltage signal of the high level signal High is output from the output terminal of the operational amplifier IC41, pulls out the gate charge of the three-terminal commutation element SW2, and removes the three-terminal commutation. The diversion element SW2 is turned off. On the other hand, when the voltage signal of the low-level signal Low is output from the output terminal of the operational amplifier IC41, the field effect transistor SW5 is turned off, and the operation of the three-terminal commutation element SW2 is not affected.
[0058]
In the normal load state, the field effect transistor SW5 of the switch means 35 is in the OFF state, so that the three-terminal commutation element SW2 is not affected by the operation of the switch means 35.
[0059]
Therefore, in the normal load state, the output circuit 3 is controlled in synchronization with the three-terminal rectifying element SW1 and the three-terminal commutating element SW2. That is, the three-terminal rectifying element SW1 is controlled by the forward voltage S1 supplied from the second coil N2 so as to be in phase with the ON / OFF timing of the first and second switching elements SW3 and SW4. The three-terminal commutation element SW2 is controlled by the flyback voltage S2 supplied from the third coil N3 so that the ON / OFF timing of the first and second switching elements SW3 and SW4 is reversed.
[0060]
In the ON state of the three-terminal rectifying element SW1, the output circuit 3 stores energy in the choke coil L1 and charges the capacitor C2. At this time, the three-terminal commutation element SW2 is turned off by the voltage supplied from the third coil N3, and the current flowing through the second coil N2 does not flow between the drain and the source of the three-terminal commutation element SW2.
[0061]
When the three-terminal rectifying element SW1 is in the OFF state, the output circuit 3 stops flowing current to the second coil N2. Then, the three-terminal commutation element SW2 is turned on, discharges the energy stored in the choke coil L1, and charges the capacitor C2.
[0062]
On the other hand, in the light load state, since the field effect transistor SW5 of the switch means 35 is turned on, the three-terminal commutation element SW2 is turned off. At this time, the three-terminal rectifying element SW1 is controlled to be in phase with the ON / OFF timing of the first and second switching elements SW3 and SW4.
[0063]
FIGS. 4 and 5 are operation waveform diagrams of the switching power supply device shown in FIGS. FIG. 4 shows an operation in a normal load state, and FIG. 5 shows an operation in a light load state. Hereinafter, the operation of the switching power supply device shown in FIGS. 2 and 3 will be described in detail with reference to FIGS.
[0064]
As shown in FIG. 4A, in a normal load state, the load current Io output from the output terminals 51 and 52 is continuous. Therefore, the three-terminal rectifying element SW1 and the three-terminal commutation element SW2 are stably controlled synchronously using the slight ripple component included in the current Io.
[0065]
FIG. 4B is a waveform diagram illustrating the drain-source voltage Vds of the three-terminal commutation element SW2. A period t1 during which the voltage Vds is output indicates a period during which the three-terminal rectifying element SW1 is in the ON state. A period t2 in which the voltage Vds becomes zero indicates a period in which the three-terminal rectifying element SW1 is in the OFF state.
[0066]
In this embodiment, since the constant voltage control is performed using PWM (Pulse Width Modulation), the period (t1 + t2) increases when the load becomes heavy, and the period (t1 + t2) decreases when the load becomes light. The voltage waveform of the second coil N2 is the same as the voltage Vds in the period t1, and has a reverse voltage waveform in the period t2.
[0067]
FIG. 4C is a waveform diagram showing a liback voltage supplied from the third coil N3 to the gate of the three-terminal commutation element SW2, and during a period t3, supplied to the gate of the three-terminal commutation element SW2. This is a period during which the voltage is positive. The period t3 appears only in the period t2.
[0068]
FIG. 4D is a waveform diagram showing the ON / OFF state of the three-terminal commutation element SW2. In a normal load state, the three-terminal commutation element SW2 is stably controlled in synchronization, and is turned on in the period t3.
[0069]
FIG. 4E is a waveform diagram showing the drain current Id of the three-terminal commutation element SW2. The current Id exists only during the period t3 when the three-terminal commutation element SW2 is in the ON state.
[0070]
FIG. 4F is a waveform diagram showing the voltage of the choke coil L1. The voltage VL1 of the choke coil L1 has a waveform similar to the voltage waveform of the second coil N2.
[0071]
FIG. 4G is a waveform diagram showing the voltage of the capacitor C41. Under a normal load, the period t1 during which the three-terminal rectifying element SW1 is in the ON state is equal to or greater than a certain value. Therefore, as shown in FIG. It becomes higher, and the output of the determination means 33 becomes a low level signal Low.
[0072]
Next, the operation of the switching power supply according to the present embodiment in a light load state will be described with reference to FIG.
[0073]
In a light load state, the switching power supply according to the present embodiment performs control during a period in which the load current Io output from the output terminals 51 and 52 becomes intermittent and the current becomes zero, as shown in FIG. Becomes impossible. This state is called a choke discontinuous state.
[0074]
When the choke is in the discontinuous state, the flyback waveform generated in the third coil N3 is disturbed, and becomes a free vibration waveform, as shown in FIG. In this free oscillation waveform, a part of a period t3 in which the voltage supplied from the third coil N3 to the gate of the three-terminal commutation element SW2 is positive, and a part of the period t1 in which the three-terminal rectification element SW1 is in the ON state. But they overlap.
[0075]
As described above, when the period t3 and the period t1 overlap, if the three-terminal commutation element SW2 is controlled based on the voltage supplied from the third coil N3, the three-terminal rectification elements SW1 and 3 In a period in which the terminal commutation element SW2 is simultaneously turned on, a through current is generated.
[0076]
At this time, in the switching power supply according to the present embodiment, as shown in FIG. 5 (g), the integrated value obtained by the judging means 33 becomes lower than the reference voltage Vth, and the output of the judging means 33 becomes high level signal High. Therefore, as shown in FIG. 5D, the three-terminal commutation element SW2 is turned off based on the voltage signal of the high-level signal High. For this reason, generation of a through current can be suppressed, and damage to the three-terminal commutation element SW2 can be avoided.
[0077]
Further, in the present embodiment, since the three-terminal commutation element SW2 is constituted by an FET, even when the three-terminal commutation element SW2 is OFF, the internal diode of the three-terminal commutation element SW2 is used. A current can flow from the source to the drain of the three-terminal commutation element SW2. In this case, the loss is larger than when the three-terminal commutation element SW2 is turned on. However, in a light load state, only a weak current flows, so that the loss at this time hardly causes a problem.
[0078]
In addition, as shown in FIGS. 1 to 3, in this embodiment, since the diode D2 is provided in parallel with the three-terminal commutation element SW2, when the three-terminal commutation element SW2 is off, It is also possible to pass a current using the diode D2.
[0079]
Further, in the present embodiment, the voltage waveform of the second coil N2 in the period t1 is used as the voltage signal having the time width corresponding to the intermittent switching output, but another voltage signal may be used.
[0080]
Further, in the present embodiment, the description has been made by using the PWM control. However, it is obvious that the design can be appropriately made based on the technical idea of the present invention even in the case of using FM (Frequency Modulation). .
[0081]
Further, in the present embodiment, the N-channel FET has been described as the three-terminal rectifying element SW1 and the three-terminal commutating element SW2, but one or both of the three-terminal rectifying element SW1 and the three-terminal commutating element SW2 However, it is obvious that a P-channel FET can be appropriately designed based on the technical idea of the present invention.
[0082]
FIG. 6 is an operation waveform diagram of the switching power supply device of the comparative example, showing an operation in a light load state. Hereinafter, the operation of the switching power supply device of the comparative example will be described in detail with reference to FIG. The switching power supply device of the comparative example is different from the switching power supply device according to the present invention in that the determination unit 33 shown in FIGS. 1 to 3 is not provided.
[0083]
As shown in FIG. 6A, in a light load state, the switching power supply of the comparative example is in a choke discontinuous state. As shown in FIG. 6C, the period t3 during which the voltage supplied from the third coil N3 to the gate of the three-terminal commutation element SW2 is positive, and the three-terminal rectification element SW1 is in the ON state. The period t1 overlaps.
[0084]
Since the switching power supply device of the comparative example does not include the determination unit 33, the switching power supply device controls the three-terminal commutation element SW2 based on the voltage supplied from the third coil N3. For this reason, as shown in FIG. 6D, in a period in which the period t3 and the period t1 overlap, the three-terminal rectifying element SW1 and the three-terminal commutating element SW2 are simultaneously turned on. .
[0085]
In this case, as shown in FIG. 6E, a through current flows through the three-terminal commutation element SW2, the three-terminal rectification element SW1, and the second coil N2 during a period in which the period t3 and the period t1 overlap. .
[0086]
Since the through current is a large current, a large surge voltage is generated in the second coil N2 by the through current, and the surge voltage is generated between the drain and the source of the three-terminal commutation element SW2 as shown in FIG. Appears at voltage Vds. Therefore, when the surge voltage exceeds the drain-source breakdown voltage of the three-terminal commutation element SW2, the three-terminal commutation element SW2 may be damaged.
[0087]
On the other hand, in the switching power supply according to the present embodiment, as described above, there is no possibility that the three-terminal commutation element SW2 is damaged.
[0088]
FIG. 7 is an electric circuit diagram showing another embodiment of the switching power supply device according to the present invention. The switching power supply shown in the drawing differs from the switching power supply shown in FIGS. Hereinafter, description will be made mainly on this point.
[0089]
7, the determining means 33 includes a voltage detection coil L3 in addition to the configuration shown in FIG. The voltage detection coil L3 is a coil for detecting the voltage VL1 generated in the choke coil L1 and supplying the detected voltage signal to the diode D41.
[0090]
As described above, the waveform of the voltage VL1 of the choke coil L1 is shown in FIGS. 4F and 5F, and is similar to the voltage waveform of the second coil N2. For this reason, the determination unit 33 can determine whether the load is light or not by using the voltage VL1 of the choke coil L1 as a voltage signal having a time width corresponding to the intermittent switching output. .
[0091]
Further, the switching power supply device shown in FIG. 7 includes the same components as those of the switching power supply device described with reference to FIGS.
[0092]
As described above, the content of the present embodiment has been specifically described with reference to the preferred embodiment. However, based on the basic technical idea and teaching of the present embodiment, those skilled in the art can adopt various modifications. Is self-evident.
【The invention's effect】
As described above, according to the present invention, the following effects can be obtained.
(A) A high-efficiency switching power supply device using a synchronous rectifier circuit can be provided.
(B) It is possible to provide a switching power supply device capable of avoiding damage to the three-terminal commutation element in a light load state.
(C) It is possible to provide a switching power supply device capable of reliably detecting a light load state without causing a large loss.
[Brief description of the drawings]
FIG. 1 is an electric circuit diagram showing one embodiment of a switching power supply device according to the present invention.
FIG. 2 is an electric circuit diagram showing the switching power supply device shown in FIG. 1 in further detail.
FIG. 3 is another electric circuit diagram showing the switching power supply device shown in FIG. 1 in further detail.
FIG. 4 is an operation waveform diagram of the switching power supply device shown in FIGS. 2 and 3 under a normal load.
FIG. 5 is an operation waveform diagram of the switching power supply device shown in FIGS. 2 and 3 under a light load.
FIG. 6 is an operation waveform diagram of the switching power supply device of the comparative example when the load is light.
FIG. 7 is an electric circuit diagram showing another embodiment of the switching power supply device according to the present invention.
[Explanation of symbols]
7 Switch circuit
T transformer
N1 first coil
N2 Second coil
3 Output circuit
33 Judgment means
SW1 3-terminal rectifying element
SW2 3-terminal commutation element
L1 choke coil

Claims (3)

A switching power supply including a switch circuit, a transformer, and an output circuit,
The switch circuit switches a DC voltage to generate a periodically intermittent switching output,
The transformer includes a first coil and a second coil, and outputs a switching output supplied from the switch circuit to the first coil to an electromagnetic wave between the first coil and the second coil. By coupling, the signal is transmitted to the second coil,
The output circuit includes a synchronous rectifier circuit and a determination unit,
The synchronous rectifier circuit includes a three-terminal rectifying element and a three-terminal commutating element, and the three-terminal rectifying element and the three-terminal commutating element are provided with a synchronous control signal supplied from the transformer to a control electrode. With this, the voltage induced in the second coil is synchronously rectified,
The determination means monitors a voltage signal having a time width corresponding to the intermittent switching output on the output circuit side, and a time width corresponding to an ON period of the three-terminal rectifying element becomes equal to or less than a certain value. A switching power supply that sometimes determines a light load and turns off the three-terminal commutation element. The switching power supply device according to claim 1,
The switching power supply device, wherein the determination unit determines whether or not the load is light by integrating a voltage generated in the second coil. The switching power supply device according to claim 1,
In addition, including a choke coil,
The main terminals of the three-terminal commutation element are connected in parallel to a series circuit of the main terminals of the three-terminal rectifying element and a second coil, and one of the main electrodes is connected to the output terminal via the choke coil. Connected to
The switching power supply device, wherein the determination unit determines whether the load is a light load or not by integrating a voltage generated in the choke coil.

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