JP2010130881A - Switching power circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching power circuit for suppressing a decrease in efficiency and an increase in voltage in circuits at a light load. <P>SOLUTION: In the switching power circuit, a current intermittently flows to the primary winding of a transformer 38 from a capacitor 36 by switching a first switching element 40 connected to the capacitor 36 in parallel, thus rectifying a voltage generated at the secondary-winding side of the transformer 38 to obtain an output voltage. The switching power circuit includes: a choke coil 32 connected to one end of the capacitor 36 via a diode 34; a second switching element Q3 for switching the connection between the connecting point of the diode 34 and the choke coil 32 and the other end of the capacitor 36; and an oscillator X forcibly switching the second switching element Q3. When a drive signal VDD for performing switching control of the first switching element 40 is a preset threshold or below, switching the second switching element Q3 by the oscillator X is stopped. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a switching power supply circuit.

  In a switching power supply that supplies a DC voltage to an electric device such as a fluorescent lamp, power factor improvement is required in order to improve efficiency and prevent influence on other electric devices. In order to improve the power factor of the switching power supply, various power factor improvement circuits (PFC: Power Function Controller) have been considered.

  As shown in FIG. 8, the conventional boost chopper type power supply 100 multiplies the operational amplifier 10 that compares the output voltage Vout with the reference voltage Vref, the attenuator 14 that attenuates the output of the rectifier circuit 12, and the outputs of the operational amplifier 10 and the attenuator 14. A multiplier 16, a comparator 18 that changes a state according to a difference between a voltage value Vs obtained by converting a current flowing between the drain and source of the switching element Q into a voltage by a resistor Rs and an output value of the multiplier 16, and a choke coil L A comparator 20 that changes the state according to a voltage value Vc obtained by converting a change in the flowing current into a voltage, a flip-flop 22 that receives the output of the comparator 18 at a reset terminal, and receives the output of the comparator 20 at a set terminal; Controls the gate voltage of switching element Q in response to the output of flop 22 Configured to include a drive circuit 24 that.

  The output voltage Vout is a substantially constant DC voltage in a steady state, and the output from the attenuator 14 receives the output waveform of the rectifier circuit 12 and becomes a full-wave rectified waveform. Therefore, the output of the multiplier 16 also becomes a full-wave rectified waveform. The output of the multiplier 16 is used as a reference voltage for the comparator 18. On the other hand, the voltage Vs increases when the switching element Q is on, and the voltage Vs becomes 0 when the switching element Q is off. When this voltage Vs reaches the reference voltage from the multiplier 16, a reset signal is input to the flip-flop 22, and the switching element Q is turned off. When the switching element Q is turned off, the current flowing through the choke coil L changes, and the voltage Vc corresponding to the change is input to the comparator 20. When the non-inverting terminal (+) of the comparator 20 has a higher voltage than the inverting terminal (−), a set signal is input to the flip-flop 22 and the switching element Q is turned on again. At this time, the multiplier 16 sets the reference voltage of the comparator 18 to a full-wave rectified waveform corresponding to the output waveform of the rectifier circuit 12, thereby widening the conduction angle and improving the power factor of the boost chopper type power supply 100. A circuit configuration in which the input of the comparator 20 is replaced with an oscillation circuit (OSC) is also known.

  Patent Document 1 discloses a step-up type including an abnormal oscillation suppression circuit that reduces the pulse width for controlling the gate of the switching element and forcibly turns off the switching element when the output voltage of the rectifier circuit is extremely smaller than the maximum value. A switching power supply is disclosed.

Japanese Patent Laid-Open No. 10-164828

  By the way, the conventional power factor improvement circuit (PFC) is provided with an oscillator 26 for forcibly oscillating the switching element Q, and the drive circuit 24 is made to respond to a signal from the oscillator 26 when the switching power supply circuit is started. The function of forcibly driving and oscillating the power supply is provided.

  Such a forced oscillation circuit is designed to function when the switching element Q is lightly loaded. Therefore, the forced oscillation circuit is forcibly even when the switching element Q becomes lightly loaded and the synchronization signal from the switching element Q is lost. Oscillation is performed. Such oscillation has caused a decrease in circuit efficiency and a voltage increase.

  One aspect of the present invention includes a capacitor, and a series circuit of a transformer and a first switching element connected in parallel to the capacitor, and switching the first switching element, thereby switching the transformer from the capacitor. A switching power supply circuit that obtains an output voltage by rectifying a voltage generated on the secondary winding side of the transformer by intermittently passing a current through the primary winding, and is connected to one end of the capacitor via a diode. A choke coil, a second switching element for switching a connection between a connection point of the diode and the choke coil and the other end of the capacitor, and an oscillator for forcibly switching the second switching element; And a drive signal for controlling the switching of the first switching element. Switching that changes according to an output voltage, controls switching of the second switching element by the drive signal, and stops switching of the second switching element by the oscillator when the drive signal is equal to or lower than a predetermined threshold value It is a power supply circuit.

  Here, it is preferable to forcibly switch the second switching element using an oscillation signal from the oscillator when starting the circuit.

  Here, it is preferable to control so that the increase / decrease in the on-time of the first switching element and the on-time of the second switching element coincide with each other.

  In addition, it is preferable to have a circuit that controls switching of the second switching element in accordance with a current flowing through the second switching element. For example, a resistor that converts a current flowing through the second switching element into an overcurrent detection voltage, and a circuit that blocks input of the drive signal to the second switching element according to the overcurrent detection voltage. Therefore, it is preferable to stop the switching when an overcurrent flows through the second switching element.

  It is preferable to have a circuit that controls switching of the second switching element in accordance with a voltage input to the choke coil. For example, a resistor that divides a voltage input to the choke coil and converts the divided voltage into an input detection voltage, and a circuit that blocks input of the drive signal to the second switching element according to the input detection voltage. By providing, it is preferable to stop the switching of the second switching element when the voltage input to the choke coil becomes high.

  In addition, the switching power supply circuit in the present invention may be an RCC switching power supply, a quasi-resonant switching method, or a PWM control switching power supply.

  Furthermore, an overcurrent protection circuit that controls the current flowing through the first switching element so as not to exceed a predetermined value may be provided.

  ADVANTAGE OF THE INVENTION According to this invention, the switching power supply which suppressed the efficiency fall and voltage rise of the circuit at the time of a light load can be provided.

  As shown in the block diagram of FIG. 1, the switching power supply circuit 200 according to the embodiment of the present invention includes a rectifier circuit 30, a choke coil 32, a diode 34, a capacitor 36, a transformer 38, a switching element 40, a drive circuit 42, and an adjustment circuit. 44, a secondary side diode 46, a secondary side capacitor 48, an output voltage detection circuit 50, a power factor improvement circuit (PFC circuit) 52, and a forced oscillation stop circuit 56.

  The switching power supply circuit 200 in the present embodiment is configured by a combination of a PFC circuit 52 and a general ringing choke converter (RCC) circuit.

  The rectifier circuit 30 is configured by combining four diodes, and full-wave rectifies and outputs the AC power input to the switching power supply circuit 200. The rectifier circuit 30 may include an inrush current prevention resistor connected in series between the output terminals T1 and T2. The rectifier circuit 30 may include a smoothing capacitor connected in parallel between the output terminals T1 and T2.

  The output terminal T1 of the rectifier circuit 30 is connected to one end of the choke coil 32. The other end of the choke coil 32 is connected to the anode of the diode 34. The cathode of the diode 34 is connected to one end of the primary winding L1 of the transformer 38. The other end of the primary winding L1 of the transformer 38 is connected to the output terminal T2 via the switching element 40 and a resistor. In this way, a series circuit is formed between the output terminals T1 and T2 of the rectifier circuit 30 via the choke coil 32, the diode 34, the primary winding L1 of the transformer 38, the switching element 40, and the resistor. A capacitor 36 is connected between the cathode of the diode 34 and the output terminal T2 of the rectifier circuit 30.

  A junction point between the choke coil 32 and the diode 34 is switching-controlled by a PFC circuit 52 described later, and a voltage applied to the capacitor 36 via the choke coil 32 and the diode 34 can be controlled.

  The transformer 38 forms an electromagnetic coupling between the primary winding L1 and the secondary winding L2, and outputs a change in voltage applied between the terminals of the primary winding L1 from between the terminals of the secondary winding L2. Is converted to a voltage to be output. The transformer 38 forms an electromagnetic coupling between the primary winding L1 and the feedback winding L3, and outputs a change in voltage applied between the terminals of the primary winding L1 from between the terminals of the feedback winding L3. Is converted to a voltage to be output. The output voltage of the feedback winding L3 is converted by the drive circuit 42 into the drive signal VDD (gate voltage) of the switching element 40.

  The anode of the secondary diode 46 is connected to one end of the secondary winding L2 of the transformer 38. The output terminal T3 of the switching power supply circuit 200 is connected to the cathode of the secondary diode 46. The output terminal T4 of the switching power supply circuit 200 is connected to the other end of the secondary winding L2 of the transformer 38. The secondary side capacitor 48 is connected in parallel between the output terminals T3 and T4 of the switching power supply circuit 200.

  In the RCC circuit, the current flowing from the capacitor 36 to the primary winding L1 of the transformer 38 is intermittently switched by the switching element 40, and the voltage generated in the secondary winding L2 of the transformer 38 due to the switching operation is switched to the secondary side. The current is rectified by the diode 46 and the secondary side capacitor 48 and output between the output terminals T3 and T4 of the switching power supply circuit 200.

  The output voltage detection circuit 50 outputs the control voltage FB to the adjustment circuit 44 according to the output voltage Vout between the output terminals T3 and T4. The control voltage FB is used to control the drive signal VDD applied to the gate of the switching element 40 when the input voltage of the switching power supply circuit 200 increases or the output voltage decreases. The output voltage detection circuit 50 can be configured by a circuit in which a Zener diode and a resistor are connected in series, and the output voltage Vout is divided and output.

  The switching element 40 includes a transistor. The forward voltage of the transformer 38 is fed back to the drive circuit 42 by the feedback winding L3 of the transformer 38, and the drive signal VDD applied to the gate of the switching element 40 is controlled by the drive circuit 42.

  As shown in FIG. 1, the drive circuit 42 can be composed of a parallel circuit of a capacitor CT and a diode DT and a resistor RT. The voltage generated in the feedback winding L3 of the transformer 38 is output via the capacitor CT and the resistor RT. When the voltage across the capacitor CT reaches the forward voltage of the diode DT, the voltage generated in the feedback winding L3 of the transformer 38 is also output through the diode DT. The output of the drive circuit 42 is applied to the gate of the switching element 40 as the drive signal VDD.

  As a result, the switching element 40 is controlled to be in the on state only for the time until the drain current Id of the switching element 40 reaches its peak value, and the primary current L1 and the secondary winding L2 of the transformer 38 are used. The output voltage Vout of the switching power supply circuit 200 is held almost constant.

  The adjustment circuit 44 controls the drive signal VDD applied to the gate of the switching element 40 in accordance with the control voltage FB output from the output voltage detection circuit 50. As shown in FIG. 2, the adjustment circuit 44 includes transistors Q1 and Q2. The drain of the transistor Q1 is connected to the drain of the switching element 40, and the source is connected to the base of the transistor Q2 via a resistor. The drive signal VDD is applied to the gate of the transistor Q1. The collector of the transistor Q2 is connected to the gate of the switching element 40 and the gate of the transistor Q1, and the drive signal VDD is applied. The emitter of the transistor Q2 is connected to the source of the switching element 40. A control voltage FB is applied to the base of the transistor Q2.

  With this configuration, when the control voltage FB applied to the base of the transistor Q2 increases, the transistor Q2 draws current from the switching element 40 and the gate of the transistor Q1, and the drive signal VDD applied to the gate of the switching element 40 is Reduce. Since the control voltage FB changes with changes in the input voltage and output voltage of the switching power supply circuit 200, the switching operation of the switching element 40 can be controlled according to the input voltage and output voltage of the switching power supply circuit 200. As a result, the output voltage Vout of the switching power supply circuit 200 is controlled to be constant.

  The PFC circuit 52 includes a switching element Q3 that opens and closes a connection point between the choke coil 32 and the diode 34 and the output terminal T2 of the rectifier circuit 30 by switching. During normal operation, the control circuit 54 controls the switching operation of the switching element Q3 in accordance with the output signal from the flip-flop circuit FF.

  That is, the drive signal VDD is input to the set terminal of the flip-flop circuit FF, and the flip-flop circuit FF is set when the drive signal VDD becomes higher than a predetermined threshold value. The control circuit 54 receives the output signal from the output terminal Q of the flip-flop circuit FF, applies a high level voltage to the gate of the switching element Q3, and turns on the switching element Q3. Thereby, the connection point between the choke coil 32 and the diode 34 and the output terminal T2 of the rectifier circuit 30 are connected via the resistor R02.

  On the other hand, the reset terminal of the flip-flop circuit FF has a drive signal VDD, an input detection voltage V1 obtained by resistance-dividing the output voltage between the output terminals T1 and T2 of the rectifier circuit 30 with resistors R1L and R2L, and An AND element A is provided that receives the logical product of the overcurrent detection voltage V2 obtained by converting the current flowing through the switching element Q3 into a voltage by the resistor R02. That is, when the drive signal VDD is higher than a predetermined value, the input detection voltage V1 is higher than a predetermined value, and the overcurrent detection voltage V2 is lower than a predetermined value, the flip-flop circuit FF is reset. The control circuit 54 receives the output signal from the output terminal Q of the flip-flop circuit FF, applies a low level voltage to the gate of the switching element Q3, and turns off the switching element Q3. Thereby, the connection point between the choke coil 32 and the diode 34 and the output terminal T2 of the rectifier circuit 30 are disconnected.

  FIG. 3 shows changes in the gate signals of the switching element 40 and the switching element Q3 when the input detection voltage V1 is higher than a predetermined value and the overcurrent detection voltage V2 is lower than the predetermined value, that is, when the reset signal is low (L). FIG. Both the switching element 40 and the switching element Q3 are subjected to switching control in accordance with the change of the drive signal VDD.

  FIG. 4 shows the switching element 40 and the switching element when the input detection voltage V1 becomes lower than a predetermined value or the overcurrent detection voltage V2 becomes higher than a predetermined value, that is, when the reset signal becomes high (H). It is the figure which showed the change of the gate signal of Q3. The switching element 40 is switching-controlled in accordance with the change of the drive signal VDD, but is abnormal when the input detection voltage V1 becomes lower than a predetermined value or when the overcurrent detection voltage V2 becomes higher than a predetermined value. It is turned off regardless of the drive signal VDD as an operating state.

  In this way, the charging voltage applied to the capacitor 36 via the diode 34 is adjusted by switching the current flowing through the choke coil 32 with the switching element Q3.

  That is, when the AC power supply voltage Vin input from the input terminal Tin is low or the output load power is large, the drive signal VDD is set so that the ON time of the switching element 40 is increased by the output voltage detection circuit 50 and the adjustment circuit 44. Be controlled. When the on-time of the switching element 40 increases, the on-time (Ton) of the switching element Q3 also increases, and the off-time (Toff) decreases.

  Since the voltage Vc applied from the choke coil 32 to the capacitor 36 via the diode 34 is expressed as Vc = (Ton + Toff) / Toff × (voltage of the output terminal T1 of the rectifier circuit 30), the OFF time of the switching element Q3 ( When Toff) decreases, the voltage Vc increases.

  When the voltage Vc rises, the output voltage detection circuit 50 and the adjustment circuit 44 decrease the on-time of the switching element 40 to keep the output voltage constant. When the ON time of the switching element 40 decreases, the ON time (Ton) of the switching element Q3 also decreases and the OFF time (Toff) increases. As a result, the voltage Vc decreases.

  By repeating such switching control, the voltage Vc can be stabilized in synchronization with the switching control of the switching element 40 during normal operation.

  FIG. 5 is a diagram showing the improvement of the power factor in the present embodiment. The solid line in FIG. 5 is an input current waveform in a circuit in which the PFC circuit 52 is provided, and the broken line is an input current waveform in a circuit in which the PFC circuit 52 is not provided. As is apparent from the figure, the current conduction angle is increased in the circuit provided with the PFC circuit 52, and the power factor is improved.

  Moreover, FIG. 6 is a figure which shows the time change of the output voltage in this Embodiment. The solid line in FIG. 6 represents the time change of the output voltage in the circuit provided with the PFC circuit 52, and the broken line represents the time change of the output voltage in the circuit not provided with the PFC circuit 52. As is clear from the figure, the circuit provided with the PFC circuit 52 can obtain a stable output voltage as in the case where the PFC circuit 52 is not provided.

  Moreover, FIG. 7 is a figure which shows the time change of the terminal voltage of the capacitor | condenser 36 in this Embodiment. The solid line in FIG. 7 represents the time change of the terminal voltage Vc in the circuit provided with the PFC circuit 52, and the broken line represents the time change of the terminal voltage Vc in the circuit not provided with the PFC circuit 52. In the circuit provided with the PFC circuit 52, the terminal voltage Vc increased by about 1.4 times compared to the circuit without the PFC circuit 52.

  Next, the startup time of the switching power supply circuit 200 will be described. When the switching power supply circuit 200 is activated, the control circuit 54 receives an activation control signal from the outside, and controls the switching operation of the switching element Q3 according to a predetermined oscillation signal output from the oscillator X.

  That is, when receiving the activation control signal for activating the switching power supply circuit 200, the control circuit 54 switches from the control based on the output signal from the flip-flop circuit FF during the normal operation to the control based on the output signal from the oscillator X. In this state, the control circuit 54 receives the self-oscillation signal of the oscillator X, applies a high level voltage to the gate of the switching element Q3 when the oscillation signal is equal to or higher than a predetermined threshold, and the oscillation signal is predetermined. When it is less than the threshold value, a low level voltage is applied to the gate of the switching element Q3. Thus, the output can be forcibly increased when the switching power supply circuit 200 is activated.

  In the conventional switching power supply circuit, such switching control of the switching element Q3 by the oscillator X is performed in a state where the drive signal VDD is not stable. That is, when the load of the switching power supply circuit is lightened, the output voltage Vout becomes smaller than a predetermined value, and the drive signal VDD that becomes the synchronization signal of the switching element 40 cannot be obtained, the output signal from the oscillator X is forcibly It was switched to control by.

  In contrast, the switching power supply circuit 200 according to the present embodiment stops forced oscillation when the drive signal VDD decreases when the switching power supply circuit 200 is not activated.

  Specifically, a forced oscillation stop circuit 56 that smoothes the drive signal VDD and inputs it to the control circuit 54 as the control signal ST may be provided. As shown in FIG. 1, the forced oscillation stop circuit 56 can be configured to include a diode 56a, a low-pass filter circuit 56b, and a switching element 56c.

  In the forced oscillation stop circuit 56, the drive signal VDD is smoothed by the diode 56a and the low-pass filter circuit 56b, and the switching element 56c is turned on / off. When the signal obtained by smoothing the drive signal VDD is equal to or higher than the on-voltage of the switching element 56c, the switching element 56c is turned on, and otherwise, the switching element 56c is turned off.

  The control circuit 54 controls the switching element Q3 according to the output of the flip-flop circuit FF when the switching element 56c is in the ON state. The control circuit 54 controls the switching element Q3 according to the oscillation signal of the oscillator X when the activation control signal of the switching power supply circuit 200 is changed from off to on when the switching element 56c is in the off state. Unless the activation control signal of the switching power supply circuit 200 is changed from OFF to ON, the switching element Q3 is not controlled according to the oscillation signal of the oscillator X.

  In this way, when the output voltage Vout is small, the control by the oscillator X is stopped except when the switching power supply circuit 200 is started, so that it is possible to suppress a decrease in circuit efficiency and a voltage increase at light loads.

  The PFC circuit 52 in the present embodiment can be applied not only to an RCC switching power supply circuit but also to other switching power supply circuits such as a pseudo-resonance method and a PWM control method.

It is a figure which shows the structure of the switching power supply circuit in embodiment of this invention. It is a figure which shows the circuit example of the switching power supply circuit in embodiment of this invention. It is a figure explaining control of a switching element in an embodiment of the invention. It is a figure explaining control of a switching element in an embodiment of the invention. It is a figure which shows the time change of the input current of the switching power supply circuit in embodiment of this invention. It is a figure which shows the time change of the output voltage of the switching power supply circuit in embodiment of this invention. It is a figure which shows the time change of the terminal voltage of the capacitor | condenser of the switching power supply circuit in the modification of implementation of this invention. It is a figure which shows the structure of the conventional switching power supply circuit.

Explanation of symbols

  10 operational amplifiers, 12 rectifier circuits, 14 attenuators, 16 multipliers, 18 comparators, 20 comparators, 22 flip-flops, 24 drive circuits, 30 rectifier circuits, 32 choke coils, 34 diodes, 36 capacitors, 38 transformers, 40 switching elements, 42 drive circuit, 44 overcurrent protection circuit, 46 secondary side diode, 48 secondary side capacitor, 50 output voltage detection circuit, 52 power factor improvement circuit, 54 control circuit, 56 forced oscillation stop circuit, 100, 200 switching power supply circuit .

Claims (6)

A capacitor, and a series circuit of a transformer and a first switching element connected in parallel with the capacitor,
Switching that obtains an output voltage by switching the first switching element to intermittently flow current from the capacitor to the primary winding of the transformer and rectify the voltage generated on the secondary winding side of the transformer A power circuit,
A choke coil connected to one end of the capacitor via a diode;
A second switching element for switching a connection between a connection point between the diode and the choke coil and the other end of the capacitor;
An oscillator for forcibly switching the second switching element,
The drive signal for controlling the switching of the first switching element is changed according to the output voltage, and the switching of the second switching element is controlled by the drive signal,
A switching power supply circuit that stops switching of the second switching element by the oscillator when the drive signal is equal to or lower than a predetermined threshold value. The switching power supply circuit according to claim 1,
A switching power supply circuit for forcibly switching the second switching element by using an oscillation signal from the oscillator when starting the circuit. The switching power supply circuit according to claim 1 or 2,
A switching power supply circuit, wherein the increase / decrease in the on-time of the first switching element and the on-time of the second switching element are matched. A switching power supply circuit according to any one of claims 1 to 3,
A switching power supply circuit comprising a circuit for controlling switching of the second switching element in accordance with a current flowing through the second switching element. A switching power supply circuit according to any one of claims 1 to 4,
A switching power supply circuit comprising a circuit for controlling switching of the second switching element in accordance with a voltage input to the choke coil. A switching power supply circuit according to any one of claims 1 to 5,
A switching power supply circuit comprising: an overcurrent protection circuit that controls the current flowing through the first switching element so as not to exceed a predetermined value.

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