JP2014064391A - Switching power supply device - Google Patents

Abstract

There is provided a switching power supply device capable of discharging a regenerated voltage of an electrolytic capacitor used for smoothing after rectification of an AC power supply without increasing power consumption during normal operation.
A rectifier circuit for rectifying an AC voltage, an electrolytic capacitor for smoothing the AC voltage rectified by the rectifier circuit, a transformer to which a voltage smoothed by the electrolytic capacitor is applied, and a switching element connected to the transformer T A control circuit Z1 for turning on and off the switching element. The control circuit Z1 supplies a voltage of the electrolytic capacitor to the Vcc terminal of the control circuit Z1 at the time of startup until the operation of the switching element is started. A discharge circuit Z14 is connected to the Vcc terminal and discharges the reactivation voltage of the electrolytic capacitor generated after the AC power supply is cut off via the starting circuit Z10.
[Selection] Figure 2

Description

  The present invention relates to a switching power supply apparatus that performs output voltage control by a switching operation.

  In a switching power supply device, an electrolytic capacitor used for rectifying and smoothing an AC power supply has a reoccurrence phenomenon in which the voltage between the terminals gradually returns when the charged voltage is discharged and the terminals are left with high impedance (Patent Literature). 1). For example, when the AC outlet is disconnected with an LCD-TV or the like, the voltage of the electrolytic capacitor is discharged and the operation is stopped. However, due to the reactivation phenomenon, the reactivation voltage is applied to the electrolytic capacitor even though the AC outlet is unplugged. Arise. Then, the re-start voltage may cause the power supply to operate for a moment and turn off immediately. As a result, the LED for turning on the power is turned on for a moment, or the transformer sounds. In addition, the secondary side microcomputer may malfunction, giving a misunderstanding to the user.

  Therefore, conventionally, as shown in FIG. 4, a high resistance R20 is connected between the terminals of the electrolytic capacitor C1 constituting the rectifying and smoothing circuit of the AC power supply AC with the rectifying circuit DB, and the regenerated voltage generated in the electrolytic capacitor C1 is discharged. I was letting. Also, a high resistance is connected between the terminals of the electrolytic capacitor C3 that supplies the power supply voltage to the control circuit Z20.

Japanese Patent Laid-Open No. 2004-111988

  However, in the prior art, since the high resistance R20 must be connected between the terminals of the electrolytic capacitor C1 used for smoothing after the rectification of the AC power supply, the number of parts increases and the discharge by the high resistance R20 is Since it is also performed during normal operation, there is a problem in that power consumption during normal operation increases.

  The object of the present invention is to solve the above-mentioned problems of the prior art in view of the above-mentioned problems, and to discharge the regenerated voltage of an electrolytic capacitor used for smoothing after rectification of an AC power supply without increasing the power consumption during normal operation. It is an object of the present invention to provide a switching power supply that can be made to operate.

The switching power supply of the present invention includes a rectifier circuit that rectifies an AC voltage input from an AC power supply, an electrolytic capacitor that smoothes the AC voltage rectified by the rectifier circuit, and a primary winding that is smoothed by the electrolytic capacitor. A transformer to which the applied voltage is applied, a switching element connected to the primary winding of the transformer, a control circuit for turning on / off the switching element, and a secondary winding of the transformer by the on / off operation of the switching element. A switching power supply device comprising a secondary side rectifying / smoothing circuit for rectifying and smoothing the induced pulse voltage, wherein the control circuit is configured to reduce the voltage of the electrolytic capacitor during startup until the operation of the switching element is started. A starting circuit for supplying power to the power supply terminal of the control circuit; and Characterized in that said recovery voltage of the electrolytic capacitor comprises a discharge circuit for discharging via the starting circuit occurring after.
Furthermore, in the switching power supply device of the present invention, the discharge circuit may include a switching circuit that reduces the impedance of the power supply terminal when the operation of the switching element is stopped.
Furthermore, in the switching power supply device of the present invention, the control circuit includes a comparison circuit that switches the switching circuit of the discharge circuit by comparing a voltage of the power supply terminal with a reference voltage, and the discharge circuit includes the switching circuit. The power supply terminal may be provided with a power supply maintenance circuit that maintains the voltage of the power supply terminal at or above the operating voltage of the comparison circuit in a state where the impedance of the power supply terminal is lowered.

  According to the present invention, since the discharge circuit is configured to discharge the regenerated voltage of the electrolytic capacitor via the starting circuit, the voltage of the electrolytic capacitor is not discharged by the discharge circuit during normal operation. Therefore, it is possible to discharge the regenerated voltage of the electrolytic capacitor used for smoothing after rectification of the AC power supply without increasing the power consumption during normal operation.

It is a circuit block diagram which shows the circuit structure of embodiment of the switching power supply device which concerns on this invention. FIG. 2 is a circuit configuration diagram illustrating a circuit configuration of a control circuit illustrated in FIG. 1. It is a wave form diagram which shows the signal waveform and operation | movement waveform of each part of FIG. It is a circuit block diagram which shows the circuit structure of the conventional switching power supply apparatus.

  Referring to FIG. 1, the switching power supply according to the present embodiment includes a rectifier circuit DB, electrolytic capacitors C1, C2, and C3, a switching element Q1, a transformer T, rectifier diodes D1 and D2, and resistors R1 and R2. And capacitors C4 and C5, a control circuit Z1, a snubber circuit Z2, and a feedback circuit Z3.

  The AC power supply AC is connected to the AC input terminals ACin1 and ACin2 of the rectifier circuit DB in which the diodes are bridged, and the AC voltage input from the AC power supply AC is full-wave rectified and output from the rectifier circuit DB. An electrolytic capacitor C1 is connected between the rectified output positive terminal and the rectified output negative terminal of the rectifier circuit DB. As a result, a DC power source obtained by rectifying and smoothing the AC power source AC with the rectifier circuit DB and the electrolytic capacitor C1 is obtained.

  Between the positive electrode terminal and the negative electrode terminal of the electrolytic capacitor C1, the primary winding P1, the switching element Q1, and the resistor R1 of the transformer T are connected in series. The switching element Q1 is composed of an N-type power MOSFET (Metal Oxide Semiconductor Field Effect Transistor). The drain terminal of the switching element Q1 is connected to the primary winding P1 of the transformer T, and the resistance is connected to the source terminal of the switching element Q1. R1 is connected, and the gate terminal of the switching element Q1 is connected to the drive output (DRIVE) terminal of the control circuit Z1. The positive terminal of the electrolytic capacitor C1 is connected to the starting voltage input (START) terminal of the control circuit Z1, and the negative terminal of the electrolytic capacitor C1 is connected to the ground (GND) terminal of the control circuit Z1.

  The control circuit Z1 is a circuit for performing switching control that causes the switching element Q1 to oscillate (ON / OFF). By the on / off operation of the switching element Q1 connected via the line P1, it is output to the secondary winding S1 of the transformer T during the off period. An electrolytic capacitor C2 is connected between both terminals of the secondary winding S1 of the transformer T via a rectifier diode D1, and a pulse voltage induced in the secondary winding S1 of the transformer T is connected to the rectifier diode D1. It is rectified and smoothed by a secondary side rectifying and smoothing circuit comprising an electrolytic capacitor C2, and supplied as a DC output voltage to a load (not shown) connected between the positive electrode output terminal OUT + and the ground output terminal OUT−. Note that a line connected to the positive output terminal OUT + is a power supply line, and a line connected to the ground output terminal OUT− is a GND line. A capacitor C4 for reducing common mode noise is connected between the negative electrode terminal of the primary electrolytic capacitor C1 and the secondary GND line.

  An electrolytic capacitor C3 is connected between both terminals of the auxiliary winding P2 of the transformer T via a rectifier diode D2 and a resistor R2, and a connection point between the rectifier diode D2 and the electrolytic capacitor C3 is a control circuit power supply for the control circuit Z1. It is connected to a voltage input (Vcc) terminal. As a result, the voltage generated in the auxiliary winding P2 is rectified and smoothed by the rectifier diode D2 and the electrolytic capacitor C3, then supplied to the Vcc terminal of the control circuit Z1, and used as a control circuit power supply for the control circuit Z1.

  The snubber circuit Z2 is a protection circuit that is connected between the primary windings P1 of the transformer T and absorbs a transient high voltage generated when the switching element Q1 is cut off. The snubber circuit Z2 includes a diode D3, a capacitor C6, and a resistor R3. The anode of the diode D3 is connected to the connection point between the drain terminal of the switching element Q1 and the primary winding P1 of the transformer T. The capacitor C6 and the resistor R3 are connected in parallel between the cathode of the diode D3 and the connection point between the positive terminal of the electrolytic capacitor C1 and the primary winding P1 of the transformer T.

  The feedback circuit Z3 includes a photocoupler PC, an error amplifier EA, resistors R4, R5, R6, and R7, and a capacitor C7. Between the power supply line and the GND line, the resistor R4, the light emitting side element (light emitting diode) of the photocoupler PC, and the error amplifier EA are connected in series, and the resistor R4 and the photo resistor R5 are connected in series. The light emitting side element (light emitting diode) of the coupler PC is connected in parallel. Further, a voltage dividing resistor R6 and a resistor R7 are connected in series between the power supply line and the GND line, and a connection point between the resistor R6 and the resistor R7 is connected to the control terminal a of the error amplifier EA. Yes. Further, a capacitor C7 is connected between a connection point between the resistor R6 and the resistor R7 and a connection point between the light emitting side element (light emitting diode) of the photocoupler PC and the error amplifier EA. As a result, the output voltage output between the positive output terminal OUT + and the ground output terminal OUT− is divided by the resistors R6 and R7, and the divided output voltage is input to the control terminal a of the error amplifier EA. The The divided output voltage is compared with a reference voltage (not shown) incorporated in the error amplifier EA, and the difference is used as a feedback signal from the light emitting side element (light emitting diode) of the secondary photocoupler PC to the primary side. Is fed back to the light receiving side element (light receiving transistor) of the photocoupler PC.

  The light receiving side element (light receiving transistor) of the photocoupler PC is connected in parallel with the capacitor C5 between the feedback signal input (FB) terminal of the control circuit Z1 and the negative electrode terminal of the electrolytic capacitor C1, and the feedback signal is Input to the FB terminal of the control circuit Z1. The connection point between the source terminal of the switching element Q1 and the resistor R1 is connected to the overcurrent detection (OCP) terminal of the control circuit Z1, and the drain current flowing through the switching element Q1 is detected as a voltage signal by the resistor R1. The detected voltage signal is input to the OCP terminal of the control circuit Z1.

  Referring to FIG. 2, the control circuit Z1 includes a start-up circuit Z10, a low voltage malfunction prevention (UVLO) circuit COMP1, a constant voltage circuit Z11, a switch element Q2 made of an N-type MOSFET, a Zener diode array ZD1, Constant voltage control circuit COMP2, overcurrent limiting circuit COMP3, OR circuit OR1, oscillation circuit Z12, flip-flop FF1, AND circuit AND1, drive circuit Z13, variable voltage VR, reference voltage Vref, resistance R8 and a discharge circuit Z14 are provided.

  The starting circuit Z10 is connected between the START terminal connected to the positive terminal of the electrolytic capacitor C1 and the Vcc terminal connected to the positive terminal of the electrolytic capacitor C3, and the electrolytic circuit connected to the Vcc terminal at the time of starting. This is a constant current circuit for supplying a constant current to the capacitor C3. The start-up circuit Z10 includes a switch element Q3 made of an N-type MOSFET, an NPN transistor TR1, resistors R9 and R10, and a diode D4. A switch element Q3, a resistor R9, and a diode D4 are connected in series between the START terminal and the Vcc terminal, and a resistor R10 is connected between the START terminal and the gate terminal of the switch element Q3. The resistor R10 is a high-resistance bias resistor. The transistor TR1 has a collector terminal connected to the gate terminal of the switch element Q3, a base terminal connected to the source terminal of the switch element Q3, and an emitter terminal connected to a connection point between the resistor R9 and the diode D4. As a result, when the switch element Q3 is turned on, a base current flows through the transistor TR1, and a constant current is supplied to the electrolytic capacitor C3 connected to the Vcc terminal.

  The UVLO circuit COMP1 is a comparison circuit that compares the voltage Vcc of the electrolytic capacitor C3 (Vcc terminal) with the variable voltage VR. The UVLO circuit COMP1 has a non-inverting input terminal connected to the Vcc terminal and an inverting input terminal connected to the variable voltage VR. The output signal output from the output terminal of the UVLO circuit COMP1 is Hi when the voltage Vcc exceeds the variable voltage VR. When the voltage Vcc becomes lower than the variable voltage VR, the level becomes Low level. The output signal from the UVLO circuit COMP1 is input to the variable voltage VR, and the variable voltage VR is set to the first reference voltage Va (for example, 15V) when the output signal from the UVLO circuit COMP1 is at a low level. When the output signal from COMP1 is at the Hi level, the second reference voltage Vb (for example, 10V) lower than the first reference voltage Va is set. As a result, the output signal of the UVLO circuit COMP1 has a hysteresis characteristic. When the voltage Vcc exceeds the first reference voltage Va, the output signal becomes Hi level, and when the voltage Vcc becomes lower than the second reference voltage Vb, the output signal becomes Low level. Become.

  The output terminal of the UVLO circuit COMP1 is connected to the constant voltage circuit Z11. The constant voltage circuit Z11 operates when the output signal of the UVLO circuit COMP1 is at the Hi level, and supplies a power supply voltage for operating each part of the control circuit Z1. That is, the output signal of the UVLO circuit COMP1 is a signal for controlling the on / off of the control circuit Z1, and during the steady operation of the control circuit Z1 (when the switching operation is on), the output signal of the UVLO circuit COMP1 is high level. Become. Therefore, the first reference voltage Va of the variable voltage VR is an operation start voltage of the control circuit Z1, and the second reference voltage Vb of the variable voltage VR is an operation stop voltage of the control circuit Z1.

  The output terminal of the UVLO circuit COMP1 is connected to the gate terminal of the switch element Q2. The source terminal of the switch element Q2 is connected to the GND line connected to the GND terminal, the drain terminal of the switch element Q2 is connected to the gate terminal of the switch element Q3 in the activation circuit Z10, and the source terminal and drain terminal of the switch element Q2 A Zener diode array ZD1 is connected between the two. The switch element Q2 is turned on when the output signal of the UVLO circuit COMP1 is at a high level, and is turned off when the output signal of the UVLO circuit COMP1 is at a low level. When the switch element Q2 is turned on, the gate terminal of the switch element Q3 is grounded to the GND line, the switch element Q3 is turned off, and the starting circuit Z10 is stopped, that is, the electrolytic capacitor C3 connected to the Vcc terminal is connected. The constant current supply is stopped. On the other hand, when the switch element Q2 is turned off, a voltage is applied to the gate terminal of the switch element Q3 via the resistor R10, the switch element Q3 is turned on, and the current flowing through the resistor R9 is detected between the base and emitter of the transistor TR1. Thus, the activation circuit Z10 starts to operate so that a constant current flows through the resistor R9 as the gate voltage of the switch element Q3. That is, supply of a constant current to the electrolytic capacitor C3 connected to the Vcc terminal is started.

  The constant voltage control circuit COMP2 is a comparison circuit that compares the voltage VFB at the FB terminal with the voltage VOCP at the OCP terminal. The constant voltage control circuit COMP2 has a non-inverting input terminal connected to the OCP terminal and an inverting input terminal connected to the FB terminal, and the output signal output from the output terminal of the constant voltage control circuit COMP2 is such that the voltage VOCP exceeds the voltage VFB. When the voltage VOCP becomes equal to or lower than the voltage VFB, the low level is obtained.

  The overcurrent limiting circuit COMP3 is a comparison circuit that compares the reference voltage Vref with the voltage VOCP at the OCP terminal. In the overcurrent limiting circuit COMP3, the non-inverting input terminal is connected to the OCP terminal, the inverting input terminal is connected to the reference voltage Vref, and the output signal output from the output terminal of the overcurrent limiting circuit COMP3 is that the voltage VOCP is the reference voltage Vref. Exceeds HICP, and when the voltage VOCP falls below the reference voltage Vref, it goes low.

  The output terminal of the constant voltage control circuit COMP2 and the output terminal of the overcurrent limiting circuit COMP3 are respectively connected to the input terminal of the OR circuit OR1, and the output terminal of the OR circuit OR1 is connected to the reset (R) terminal of the flip-flop FF1. It is connected. The set (S) terminal of the flip-flop FF1 is connected to the oscillation circuit Z12, and the output (Q) terminal of the flip-flop FF1 is connected to the input terminal of the AND circuit AND1. Further, the other input terminal of the AND circuit AND1 is connected to the output terminal of the UVLO circuit COMP1. As a result, when the output of the UVLO circuit COMP1 is Hi level and the flip-flop FF1 is set and the output signal of the Q terminal is Hi level, the output signal of the AND circuit AND1 becomes Hi level and passes through the drive circuit Z13. Thus, the switching element Q1 is turned on.

  The discharge circuit Z14 is connected between the Vcc terminal connected to the positive terminal of the electrolytic capacitor C3 and the GND terminal, and discharges the regenerated voltage of the electrolytic capacitor C1 via the starting circuit Z10. The discharge circuit Z14 includes a switching element Q4 made of an N-type MOSFET, an NPN transistor TR2, resistors R11 and R12, and a Zener diode ZD2. A resistor R11 and a switching element Q4 are connected in series between the Vcc terminal and the GND terminal, and a resistor R12, a Zener diode ZD2, and an NPN transistor TR2 are connected in series. That is, the drain terminal of the switch element Q4 is connected to the Vcc terminal via the resistor R11, and the source terminal of the switch element Q4 is connected to the GND terminal. The collector terminal of the transistor TR2 is connected to the anode of the Zener diode ZD2, the cathode of the Zener diode ZD2 is connected to the Vcc terminal via the resistor R12, and the emitter terminal of the transistor TR2 is connected to the GND terminal. The gate terminal of the switch element Q4 is connected to the output terminal of the UVLO circuit COMP1, and the base terminal of the transistor TR2 is connected to the drain terminal of the switch element Q4.

  During the steady operation of the control circuit Z1, the output signal of the UVLO circuit COMP1 is at the Hi level, so that the switch element Q4 is in an on state. When the switch element Q4 is on, the base terminal of the transistor TR2 is grounded to the GND terminal, and the transistor TR2 is turned off. When the transistor TR2 is off, the Vcc terminal is connected to the GND terminal via the resistor R11. The resistor R11 is set to a high impedance such as 5 MΩ. On the other hand, when the output signal of the UVLO circuit COMP1 becomes low level, the switch element Q4 is turned off. Since the base terminal of the transistor TR2 flows when the switch element Q4 is turned off, the transistor TR2 is turned on. When the transistor TR2 is on, the Vcc terminal is connected to the GND terminal via the resistor R12 and the Zener diode ZD2. The resistor R12 is set to a low impedance such as 68 kΩ compared to the resistor R11. As described above, the discharge circuit Z14 is a circuit that lowers the impedance of the Vcc terminal when the switching operation is turned off rather than when the switching operation is turned on according to the output signal of the UVLO circuit COMP1, that is, when the switching operation is turned on and off. The element Q4 functions as a switching circuit that lowers the impedance of the Vcc terminal when the switching operation is off.

  3 is a timing chart showing signals at various parts of the control circuit Z1 shown in FIG. 2, where (a) shows the voltage of the electrolytic capacitor C1, (b) shows the voltage Vcc of the electrolytic capacitor C3, and (c) shows the UVLO circuit COMP1. (D) is an output signal from the DRIVE terminal of the control circuit Z1, (e) is an on / off state of the switch element Q2, (f) is an on / off state of the switch element Q3, and (g) is an on / off state of the switch element Q4. The state (g) shows the on / off state of the transistor TR2.

  At time t1, when the AC power supply AC is connected to the AC input terminals ACin1 and ACin2 of the rectifier circuit DB, the voltage of the electrolytic capacitor C1 rises as shown in FIG. When the voltage of the electrolytic capacitor C1 rises, the switch element Q2 is turned off, so the voltage of the gate terminal of the switch element Q3 in the startup circuit Z10 also rises. When the voltage at the gate terminal of the switch element Q3 rises and exceeds the threshold voltage, the switch element Q3 is turned on as shown in FIG. As a result, the starting circuit Z10 starts to operate as a constant current circuit, and a constant current is supplied from the starting circuit Z10 to the electrolytic capacitor C3. As shown in FIG. 3B, the voltage Vcc of the electrolytic capacitor C3 increases. I will do it.

  Note that when the voltage Vcc of the electrolytic capacitor C3 starts to rise at time t1, the base current of the transistor TR2 in the discharge circuit Z14 also starts to flow, and the transistor TR2 is turned on. As a result, the discharge circuit Z14 is switched to a low impedance, but since a sufficient start-up current (constant current) is supplied from the start-up circuit Z10, even if the transistor TR2 is turned on, FIG. ), The voltage Vcc of the electrolytic capacitor C3 increases.

  When the voltage Vcc of the electrolytic capacitor C3 exceeds the reference voltage Va, which is the operation start voltage of the control circuit Z1, at time t2, the output signal of the UVLO circuit COMP1 becomes Hi level as shown in FIG. 3 (c). As shown in FIG. 3D, the switching operation is turned on.

  When the output signal of the UVLO circuit COMP1 becomes Hi level, as shown in FIG. 3E, the switch element Q2 is turned on and the gate terminal of the switch element Q3 is grounded. Thus, the switch element Q3 is turned off. As a result, the operation of the starting circuit Z10 as a constant current circuit is stopped, and the voltage generated in the auxiliary winding P2 is supplied to the electrolytic capacitor C3 during the steady operation thereafter.

  Further, when the output signal of the UVLO circuit COMP1 becomes Hi level, as shown in FIG. 3G, the switch element Q4 in the discharge circuit Z14 is turned on, and the base current of the transistor TR2 does not flow. As shown, the transistor TR2 is turned off. Thus, the discharge circuit Z14 is switched to a high impedance that does not affect the steady operation.

  Next, when the AC power supply AC is cut off at time t3, as shown in FIG. 3A, the voltage of the electrolytic capacitor C1 decreases and the voltage is supplied from the auxiliary winding P2 to the electrolytic capacitor C3. Therefore, as shown in FIG. 3B, the voltage Vcc of the electrolytic capacitor C3 decreases.

  When the voltage Vcc of the electrolytic capacitor C3 becomes the reference voltage Vb that is the operation stop voltage of the control circuit Z1 at time t4, as shown in FIG. 3C, the output signal of the UVLO circuit COMP1 becomes a low level, As shown in FIG. 3D, the switching operation is turned off.

  Further, when the output signal of the UVLO circuit COMP1 becomes a low level, the switch element Q2 is turned off as shown in FIG. When the regenerative voltage is generated in the electrolytic capacitor C1 due to the reactivation phenomenon as shown in FIG. 3A with the switch element Q2 turned off, the reactivation voltage of the electrolytic capacitor C1 causes the gate terminal of the switch element Q3 in the starting circuit Z10. The voltage also rises. When the voltage at the gate terminal of the switch element Q3 rises and exceeds the threshold voltage, the switch element Q3 is turned on as shown in FIG. As a result, the reactivation voltage generated by the reactivation phenomenon in the electrolytic capacitor C1 is supplied to the Vcc terminal by the starting circuit Z10. If the voltage of the electrolytic capacitor C1 during normal operation is 320 V, the constant current supplied from the starting circuit Z10 to the Vcc terminal during normal operation is about 2.5 mA. On the other hand, since the reactivation voltage of the electrolytic capacitor C1 is lower than the voltage during normal operation, the current supplied from the starting circuit Z10 to the Vcc terminal by the reactivation voltage of the electrolytic capacitor C1 is about several μA to several hundred μA. Get smaller.

  Further, when the output signal of the UVLO circuit COMP1 becomes low level, as shown in FIG. 3G, the switch element Q4 in the discharge circuit Z14 is turned off, and the base current of the transistor TR2 flows. Then, as shown in FIG. 3 (h), the transistor TR2 is turned on, and the discharge circuit Z14 is switched to a low impedance. Thereby, the current supplied from the starting circuit Z10 to the Vcc terminal by the reactivation voltage of the electrolytic capacitor C1 is discharged by the discharge circuit Z14 (low impedance resistor R12), and the reference voltage Vcc is the operation start voltage of the control circuit Z1. The rise to the voltage Va is prevented.

  In the discharge circuit Z14, the Zener diode ZD2 functions as a voltage maintaining (clamping) circuit that maintains the voltage Vcc at the Vcc terminal when the discharge circuit Z14 operates. The Zener voltage of the Zener diode ZD2 is set higher than the operating voltage of the UVLO circuit COMP1. Thus, even if the voltage Vcc is discharged by the discharge circuit Z14 (resistor R12), the voltage Vcc is maintained higher than the operating voltage of the UVLO circuit COMP1 while the reactivation phenomenon of the electrolytic capacitor C1 continues (several seconds to several thousand seconds). The Further, the voltage Vcc is directly supplied as an operating voltage to the UVLO circuit COMP1. As a result, while the electrolytic capacitor C1 is restarting (several seconds to thousands of seconds), the output level of the UVLO circuit COMP1 is not indefinite and the low level is maintained. The Zener voltage of the Zener diode ZD2 is set to be lower than at least the first reference voltage Va (operation start voltage of the control circuit Z1) of the UVLO circuit COMP1. In order to obtain a sufficient discharge effect, the Zener voltage of the Zener diode ZD2 is preferably set lower than the second reference voltage Vb (operation stop voltage of the control circuit Z1) of the UVLO circuit COMP1.

  As described above, according to the present embodiment, the rectifier circuit DB that rectifies the AC voltage input from the AC power supply AC, the electrolytic capacitor C1 that smoothes the AC voltage rectified by the rectifier circuit DB, and the primary Transformer T in which voltage smoothed by electrolytic capacitor C1 is applied to winding P1, switching element Q1 connected to primary winding P1 of transformer T, control circuit Z1 for switching on and off switching element Q1, and switching A secondary side rectifying / smoothing circuit (rectifying diode D1, electrolytic capacitor C2) for rectifying and smoothing a pulse voltage induced in the secondary winding S1 of the transformer T by the on / off operation of the element Q1 is provided. When starting up the operation of Q1, the voltage of the electrolytic capacitor C1 is supplied to the Vcc terminal of the control circuit Z1. A circuit Z10, is connected to the Vcc terminal, the AC power source AC is provided with a discharge circuit Z14 for discharging the recovery voltage of the electrolytic capacitor C1 caused after being cut through the starting circuit Z10. With this configuration, when the switching element Q1 is operating (at the time of steady operation), the operation of the starting circuit Z10 is stopped, so that the voltage of the electrolytic capacitor C1 is not discharged by the discharge circuit Z14. Therefore, it is possible to discharge the regenerated voltage of the electrolytic capacitor used for smoothing after rectification of the AC power supply without increasing the power consumption during normal operation. Note that the voltage of the electrolytic capacitor C1 does not necessarily drop to 0 V when the AC power supply AC is cut off, and the voltage of the electrolytic capacitor C1 remaining halfway can be quickly discharged by the discharge circuit Z14. In addition, a discharge circuit Z14 (resistance) for discharging the regenerative voltage of the electrolytic capacitor C1 can be taken into the control circuit Z1, and the number of parts can be reduced. Furthermore, the discharge circuit Z14 taken into the control circuit Z1 is basically composed of a resistor, a transistor, and a MOSFET, and is not an element that is not particularly difficult to integrate. Therefore, an increase in the chip cost of the control circuit Z1 can be suppressed.

  Further, according to the present embodiment, the discharge circuit Z14 includes the switching circuit (switch element Q4) that reduces the impedance of the Vcc terminal when the operation of the switching element Q1 is stopped. With this configuration, it is possible to quickly and surely discharge the regenerative voltage of the electrolytic capacitor C1, and it is possible to suppress power consumption during steady operation.

  Further, according to the present embodiment, the control circuit Z1 switches the discharge circuit Z14 switching circuit (switch element Q4) by comparing the voltage Vcc and the reference voltage (first reference voltage Va, second reference voltage Vb). The discharge circuit Z14 includes a power supply maintenance circuit (maintaining a voltage Vcc higher than the operating voltage of the UVLO circuit COMP1 in a state where the impedance of the Vcc terminal is lowered by the switching circuit (switch element Q4). A zener diode ZD2). With this configuration, while the electrolytic capacitor C1 is restarting (several seconds to thousands of seconds), the output level of the UVLO circuit COMP1 that controls the switching circuit (switch element Q4) does not become indefinite, and the low level is maintained. Maintained.

  As mentioned above, although this invention was demonstrated by specific embodiment, the said embodiment is an example and it cannot be overemphasized that it can change and implement in the range which does not deviate from the meaning of this invention. For example, a power factor correction (PFC) circuit may be provided in addition to the rectifying / smoothing circuit including the rectifying circuit DB and the electrolytic capacitor C1 of the present embodiment. Further, a half-wave rectifier circuit, a double-wave rectifier circuit, a double-wave voltage doubler rectifier circuit, or the like may be used instead of the rectifier circuit DB having a diode-bridged configuration. In the present embodiment, an example using a flyback transformer as shown in FIG. 1 has been described, but a forward transformer or a half-bridge transformer may be used. Moreover, although the circuit example which controls an output voltage with a feedback signal from the secondary side is shown, you may use for the system controlled indirectly with the auxiliary winding voltage of a primary side.

AC AC power supply AND1 AND circuit C1, C2, C3 Electrolytic capacitor C4, C5, C6, C7 Capacitor COMP1 Low voltage malfunction prevention (UVLO) circuit COMP2 Constant voltage control circuit COMP3 Overcurrent limiting circuit D1, D2 Rectifier diode
D3, D4 Diode DB Rectifier circuit EA Error amplifier FF1 Flip-flop OR1 OR circuit PC Photocoupler Q1 Switching element Q2, Q3, Q4 Switch element R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11 , R12 resistance T transformer TR1, TR2 transistor VR variable voltage Vref reference voltage Z1 control circuit Z2 snubber circuit Z3 feedback circuit Z10 start-up circuit Z11 constant voltage circuit Z12 oscillation circuit Z13 drive circuit Z14 discharge circuit ZD1 Zener diode array ZD2 Zener diode

Claims (3)

A rectifier circuit that rectifies an AC voltage input from an AC power supply; an electrolytic capacitor that smoothes the AC voltage rectified by the rectifier circuit; and a transformer in which a voltage smoothed by the electrolytic capacitor is applied to a primary winding. A switching element connected to the primary winding of the transformer, a control circuit for turning on / off the switching element, and rectifying and smoothing a pulse voltage induced in the secondary winding of the transformer by the on / off operation of the switching element A switching power supply device comprising a secondary side rectifying and smoothing circuit,
The control circuit is a startup circuit that supplies the voltage of the electrolytic capacitor to the power supply terminal of the control circuit at the time of startup until the operation of the switching element is started,
A switching power supply device comprising: a discharge circuit connected to the power supply terminal and for discharging a regenerated voltage of the electrolytic capacitor generated after the AC power supply is cut off via the starting circuit.   The switching power supply device according to claim 1, wherein the discharge circuit includes a switching circuit that reduces the impedance of the power supply terminal when the operation of the switching element is stopped. The control circuit comprises a comparison circuit that switches the switching circuit of the discharge circuit by comparing the voltage of the power supply terminal and a reference voltage;
2. The discharge circuit according to claim 1, further comprising a power supply maintaining circuit that maintains a voltage of the power supply terminal equal to or higher than an operating voltage of the comparison circuit in a state where the impedance of the power supply terminal is lowered by the switching circuit. 3. The switching power supply device according to 1 or 2.

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