JP2019022398A - Switching power supply device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform a foldback operation by using only one auxiliary winding required. SOLUTION: A primary winding L1 to which an input voltage Vin is applied by turning on a switching transistor MN1, a secondary winding L2 to which a load is connected, and a flyback voltage having the same polarity as a flyback voltage generated in the secondary winding. In a switching power supply device having a transformer 10 having an auxiliary winding L3 that generates a voltage, and a control circuit 20 that takes in a sense voltage Vs1 and a photocoupler current Ipc to control ON / OFF of the switching transistor MN1, the auxiliary winding L3 A decrease in the output voltage Vout is detected by the obtained voltage Vrise, and when the voltage Vrise obtained in the auxiliary winding L3 reaches a predetermined value or less, a feedback current Ifb that is inversely proportional to the voltage Vise obtained in the auxiliary winding L3 is generated. A current limiting feedback circuit 30 that is incorporated into the control circuit 20 instead of the photocoupler current Ipc is provided. [Selection diagram] Fig. 1

Description

  The present invention relates to a switching power supply device as a DC / DC converter using a transformer and a photocoupler.

  FIG. 7 shows a circuit of this type of conventional switching power supply device (Patent Document 1). A transformer 60 includes a primary winding L11, a first auxiliary winding L12, a secondary winding L13, and a second auxiliary winding L14. MN2 is an NMOS switching transistor, and 70 is a photocoupler including a photodiode PD2 and a phototransistor PT2. R11 to R19 are resistors, Rs2 is a sense resistor for detecting the drain current of the switching transistor MN2, and C11 to C15 are capacitors.

  In this switching power supply device, when the voltage obtained by dividing the output voltage Vout by the resistors R18 and R19 is higher than the reference voltage Vref11 of the voltage source VB11, the output voltage of the operational amplifier OP11 decreases according to the differential voltage. When the output voltage of the operational amplifier OP11 is equal to or lower than a predetermined value, a current flows through the photodiode PD2 of the photocoupler 70 according to the value of the output voltage, and the internal resistance of the phototransistor PT2 is determined according to the amount of light emitted.

  When the power supply voltage Vin is turned on, the capacitor C13 is charged so that the resistor R13 side becomes positive by the exciting current flowing through the first auxiliary winding L12 via the resistors R11 and R13. When the voltage on the resistor R13 side of the capacitor C13 reaches the threshold voltage of the switching transistor MN2, the switching transistor MN2 is turned on.

  As a result, when a current starts to flow in the primary winding L11 to which the switching transistor MN2 is connected due to the DC voltage Vin, an induced electromotive force is generated in each of the windings L12, L13, and L14 of the transformer 60, and energy is stored in the transformer 60. Is done. Since the induced voltage (the positive side is positive) generated in the first auxiliary winding L12 is superimposed on the voltage of the capacitor C13, the switching transistor MN2 maintains the gate voltage above its threshold voltage and continues to be in the ON state.

  At this time, the drain current of the switching transistor MN2 flows to the sense resistor Rs2, and the sense voltage generated there charges the capacitor C12 via the resistor R15. Since the exciting current flowing through the primary winding L11 increases almost linearly with time after the switching transistor MN2 is turned on, the voltage of the capacitor C12 also rises accordingly.

  Thereafter, when the voltage of the capacitor C12 reaches the threshold voltage of the transistor Q11, the transistor Q11 is turned on, and the switching transistor MN2 is turned off because its gate voltage falls below the threshold voltage.

  When the current flowing through the primary winding L11 is interrupted by turning off the switching transistor MN2, a flyback voltage is generated in each of the windings L11 to L14. At this time, the flyback voltage generated in the secondary winding L13 is rectified and smoothed by the diode D11 and the capacitor C14 and output as the output voltage Vout.

  On the other hand, the flyback voltage generated in the first auxiliary winding L12 is proportional to the flyback voltage generated in the secondary winding L13, and the flyback voltage generated in the first auxiliary winding L12 (the negative side is the negative side). ), The capacitor C13 is charged via the resistors R12 and R13 so that the resistor R13 side becomes a positive electrode, and the running for turning on the switching transistor MN2 proceeds.

  Since the current of the primary winding L11 is cut off after the switching transistor MN2 is turned off, the voltage generated at the sense resistor Rs2 is zero, and the output voltage Vout is low and the phototransistor PT2 is operating. As a result, the voltage of the capacitor C12 continues to be discharged through the resistors R15 and Rs2. Thus, when the voltage of the capacitor C12 becomes equal to or lower than the threshold voltage of the transistor Q11, the transistor Q11 is turned off.

  By the way, since the base-collector of the transistor Q11 acts as an equivalent diode, the capacitor C13 is connected to the sense resistor Rs2, the resistor R15, the collector from the base of the transistor Q11, the resistor from the opposite side of the first auxiliary winding L12. The current flowing through R13 is also charged so that the resistor R13 side becomes a positive electrode.

  When the discharge of the electrical energy stored in the secondary winding L13 is finished by the flyback, the voltage of the primary winding L11 becomes the parasitic capacitance of the switching transistor MN2, the stray capacitance in the primary winding L11, and the primary Due to the inductance of the winding L11, free oscillation centered on the input voltage Vin is started, and its polarity is reversed with a voltage drop.

  The voltage on the capacitor C13 side of the first auxiliary winding L12 that oscillates in proportion to the free oscillation of the voltage of the primary winding L11 changes in the same manner. When the polarity returns after the flyback voltage disappears, the voltage is switched. It acts as a forward voltage on the gate of the transistor MN2. Further, since the voltage of the capacitor C13 charged so far is added to this voltage, when the total voltage exceeds the threshold voltage of the switching transistor MN2, the switching transistor MN2 is turned ON again. In this way, a series of self-oscillation operations are repeated.

  Until now, since the output voltage Vout is low and the photocoupler 70 is not operating, the phototransistor PT2 does not affect the base voltage of the switching transistor MN2, and the switching transistor MN2 has a maximum ON period determined by the resistance value of the sense resistor Rs2. Operate. Thereafter, the output voltage Vout increases every time oscillation is repeated. When the output voltage Vout exceeds the set voltage corresponding to the reference voltage Vref11, the comparison operation by the operational amplifier OP11 is started, and the operation shifts to the normal operation in which the photocoupler 70 operates.

  In this normal operation, when the output voltage Vout is higher than the set voltage, the voltage of the capacitor C12 is charged by the current flowing through the phototransistor PT2 of the photocoupler 70 in addition to charging by the voltage generated in the sense resistor Rs2. For this reason, the higher the output voltage Vout, the earlier the ON timing of the transistor Q11, and the earlier the OFF timing of the switching transistor MN2. That is, the ON period of the switching transistor MN2 is shortened.

  When the switching transistor MN2 is turned off, the switching transistor MN2 continues to be turned off until the voltage on the resistor R13 side of the capacitor C13 charged by the flyback voltage of the first auxiliary winding L12 reaches the threshold voltage of the switching transistor MN2.

  In this switching power supply device, when the voltage obtained by dividing the input voltage Vin by the resistors R11 and R12 is less than a predetermined value, the bias voltage of the switching transistor MN2 becomes low, and the switching transistor MN2 does not perform ON / OFF operation.

JP 2005-027412 A

  However, the switching power supply device of FIG. 7 requires the first auxiliary winding L12 specially in order to generate the ON timing of the switching transistor MN2. Further, since the voltage on the resistor R13 side of the capacitor C13 controls the gate of the switching transistor MN2, there is a problem that the ON timing of the switching transistor MN2 is affected by variations in the threshold value of the switching transistor MN2. Further, since the switching transistor MN2 is turned off when the charging voltage of the capacitor C12 reaches the threshold value of the transistor Q11, there is a problem that the OFF timing of the switching transistor MN2 is affected by the variation of the threshold value of the transistor Q11. Further, the second auxiliary winding L14 is specially required to obtain a photocoupler current. Furthermore, the current limiting function when the load current increases is insufficient.

  The object of the present invention is that the second auxiliary winding for obtaining the photocoupler current is not essential, the switching transistor is not affected by the variation of the threshold value of the switching transistor, and the load current is further increased. A switching power supply device having a current limiting function is provided.

In order to achieve the above object, a switching power supply according to a first aspect of the present invention includes a switching transistor, a sense resistor that generates a sense voltage when the switching transistor is turned on, and an input by turning on the switching transistor. A transformer having a primary winding to which a voltage is applied, a secondary winding to which a load is connected, and an auxiliary winding, and a photocoupler that generates a photocoupler current according to an output voltage supplied from the secondary winding to the load And a switching power supply device having a control circuit that takes in the photocoupler current and controls ON / OFF of the switching transistor to detect a decrease in the output voltage by a voltage obtained by the auxiliary winding, When the voltage obtained by the auxiliary winding reaches a predetermined value or less, the voltage obtained by the auxiliary winding is obtained. And a current limit feedback circuit that generates a feedback current inversely proportional to the generated voltage and causes the control circuit to capture the feedback current instead of the photocoupler current.

  According to a second aspect of the present invention, in the switching power supply device according to the first aspect, when the voltage generated in the auxiliary winding during the OFF period of the switching transistor is equal to or less than a predetermined value, the current limiting feedback circuit is the auxiliary winding. The feedback current inversely proportional to the voltage generated on the line is generated over an ON period and an OFF period of the switching transistor.

  The invention according to claim 3 is the switching power supply device according to claim 1 or 2, wherein the control circuit takes in the sense voltage and outputs a sense current corresponding to a load current during an OFF period of the switching transistor. An ON period in which an OFF timing signal for turning off the switching transistor after the switching transistor is turned on is generated earlier as the photocoupler current is larger, the feedback current is smaller, and the sense voltage is larger. A control circuit and an ON timing signal for turning on the switching transistor after the switching transistor is turned off, the smaller the photocoupler current and the larger the feedback current, and the smaller the sense current. The OFF period control circuit generated at an earlier timing, and the switching transistor is turned off by the ON timing signal output from the OFF period control circuit, and the switching transistor is turned OFF by the OFF timing signal output from the ON period control circuit. And an SRFF circuit to be turned on.

  According to a fourth aspect of the present invention, in the switching power supply device according to the first, second, or third aspect, the ON period control circuit is a path through which the photocoupler current or the feedback current flows when the switching transistor is ON. And a first comparator that generates the OFF timing signal when the second voltage generated on the photocoupler current or feedback current introduction side of the second resistor becomes the same voltage as the sense voltage. It is characterized by providing.

  According to a fifth aspect of the present invention, in the switching power supply device according to the first, second, third, or fourth aspect, the OFF period control circuit is charged with the sense current when the switching transistor is OFF. A fourth capacitor that is discharged when ON, a third resistor inserted to cause a voltage drop from the voltage of the fourth capacitor by the photocoupler current or the feedback current, and the fourth capacitor of the third resistor And a second comparator that generates the ON timing signal when the voltage at the terminal opposite to the terminal reaches a predetermined value.

  The invention according to claim 6 is the switching power supply device according to claim 5, further comprising an inversion detection circuit that shapes a voltage generated in the auxiliary winding provided in the transformer to generate a pulse signal, The ON timing signal of the OFF period control circuit is retimed by the pulse signal.

  According to the present invention, the photocoupler current, the sense current according to the ON period of the switching transistor, the sense voltage, and the feedback current indicating the output voltage drop obtained by the auxiliary winding are respectively captured and processed, and the ON timing signal of the switching transistor, Since an OFF timing signal can be generated, only one auxiliary winding is required. Also, since the SRFF circuit is driven by the ON timing signal and the OFF timing signal to control the ON / OFF of the switching transistor, the ON / OFF timing of the switching transistor is not affected by the variation of the threshold value. Furthermore, since the sense current is a current corresponding to the output current, and the feedback current is a current that increases in inverse proportion to the decrease in the output voltage, the foldback operation is performed by taking these, and overcurrent protection is realized. can do.

1 is a configuration block diagram of a switching power supply device according to an embodiment of the present invention. FIG. 2 is a circuit diagram of an ON period control circuit of the switching power supply device of FIG. 1. It is a circuit diagram of the OFF period control circuit of the switching power supply device of FIG. FIG. 2 is a circuit diagram of an inversion detection circuit and a current limit feedback circuit of the switching power supply device of FIG. 1. It is a U-shaped characteristic diagram showing the relationship between the output current and the output voltage. It is an operation | movement waveform diagram at the time of foldback control of the switching power supply device of FIG. It is a circuit diagram of the conventional switching power supply device.

<First embodiment>
FIG. 1 shows the configuration of a switching power supply device according to a first embodiment of the present invention. Reference numeral 10 denotes a transformer having a primary winding L1, a secondary winding L2, and an auxiliary winding L3. The input DC voltage Vin is input to the primary winding L1 after being stabilized by the capacitor C1, and electromagnetic energy generated by the ON / OFF operation of the NMOS switching transistor MN1 is supplied to the secondary winding L2 and the auxiliary winding L3. Tell. In the secondary winding L2, a diode D1 and a capacitor C2 form a rectifying / smoothing circuit, and an output DC voltage Vout is extracted from the rectifying / smoothing circuit. In the auxiliary winding L3, a diode D2 and a capacitor C3 form a rectifying / smoothing circuit, and a power supply voltage VDD is generated from the rectifying / smoothing circuit.

  A control circuit 20 controls ON / OFF of the switching transistor MN1. In the control circuit 20, 21 is an ON period control circuit that outputs the OFF timing voltage Voff by controlling the time during which the switching transistor MN <b> 1 is kept ON, and 22 is the time during which the switching transistor MN <b> 1 is kept OFF. And an OFF period control circuit for outputting the ON timing voltage Von. The OFF period control circuit 22 includes an external capacitor C4.

  Reference numeral 23 denotes an SRFF circuit, which is reset when the OFF timing voltage Voff output from the ON period control circuit 21 becomes “H”, and sets the drive voltage Vdrv output from the Q terminal to “L”. Further, the ON timing voltage Von output from the OFF period control circuit 22 is set to “H”, and the drive voltage Vdrv output from the Q terminal is set to “H”.

  Reference numeral 24 denotes a drive circuit that inputs a drive voltage Vdrv output from the Q terminal of the SRFF circuit 23 and generates a gate voltage Vg for turning on / off the switching transistor MN1. When the drive voltage Vdrv is “H”, the gate voltage Vg is set. “H” is set to turn on the switching transistor MN1, and when the drive voltage Vdrv is “L”, the gate voltage Vg is set to “L” and the switching transistor MN1 is turned off.

  Reference numeral 25 denotes a current sense circuit which inputs a sense voltage Vs1 generated in a sense resistor Rs1 for detecting a drain current flowing in the switching transistor MN1, and immediately before the drive voltage Vdrv becomes “L”, that is, the switching transistor MN1 is turned off. A sense current Ioff proportional to the sense voltage Vs1 generated in the sense resistor Rs1 is generated and held immediately before the output, and output to the OFF period control circuit 22

  Reference numeral 26 denotes an inversion detection circuit which takes in the pulsating voltage Vrise generated in the auxiliary winding L3 through the resistor R1, generates a pulse voltage Vp having a waveform, and outputs it to the OFF period control circuit 22 for retiming. .

  Reference numeral 30 denotes a current limit feedback circuit for detecting a decrease in the output voltage Vout, and sucks a feedback current Ifb that is inversely proportional to the negative voltage component of the pulsating voltage Vrise generated in the auxiliary winding L3. The feedback current Ifb is a current that shows a larger value as the output voltage Vout is lower.

  Reference numeral 40 denotes an output voltage feedback circuit for detecting the output voltage Vout, which is similar to the circuit including the operational amplifier OP11, the voltage source VB11, the capacitor C15, and the resistors R16, R17, R18, and R19 described with reference to FIG. The output voltage feedback circuit 40 increases the current flowing through the photodiode PD1 of the photocoupler 50 as the output voltage Vout increases. This photodiode PD1 constitutes a photocoupler 50 with the phototransistor PT1. The phototransistor PT1 generates a photocoupler current Ipc proportional to the light emission amount of the photodiode PD1, that is, the output voltage Vout, and outputs the photocoupler current Ipc to the ON period control circuit 21 and the OFF period control circuit 22 of the control circuit 20 as a sink current.

  FIG. 2 shows a detailed diagram of the ON period control circuit 21. The ON period control circuit 21 is turned on when the voltage source VB1 of the reference voltage Vref1, the buffer BF1 for impedance conversion, the resistor R2, and the drive voltage Vdrv is “H”, and the photocoupler current Ipc or feedback current Ifb is supplied to the resistor R2. A switch SW1 for passing the current, a buffer BF2 for impedance-converting the sense voltage Vs1, and a comparator CP1 for comparing the sense voltage Vs1 with the voltage Vr2 dropped by the resistor R2 when the photocoupler current Ipc or the feedback current Ifb flows.

  While the switching transistor MN1 is OFF, since the switching SW1 is OFF, the voltage Vr2 at the inverting input terminal of the comparator CP1 is VB1. However, when the switch SW1 is turned on, the photocoupler current Ipc or the feedback current Ifb flows through the resistor R2, so that the voltage Vr2 at the inverting input terminal of the comparator CP1 decreases to “Vref1−R2 × (Ipc or Ifb)”. When the voltage Vr2 drops below the sense voltage Vs1, the OFF timing voltage Voff, which is the output of the comparator CP1, changes from “L” to “H”.

  In this way, the ON period control circuit 21 switches the OFF timing voltage Voff1 to “H” earlier by switching the OFF timing voltage Voff1 to “H” as the photocoupler current Ipc is larger, the feedback current Ifb is larger, the sense voltage Vs1 is higher. The transistor MN1 is controlled so as to shorten the ON time.

  FIG. 3 shows a detailed view of the OFF period control circuit 22. The OFF period control circuit 22 includes a switch SW4 that is turned on when the drive voltage Vdrv is “H”, the external capacitor C4 that is charged by the sense current Ioff of the current sense feedback circuit 25, and an impedance conversion buffer BF3. A resistor R3, a comparator CP2 in which the reference voltage Vref2 of the voltage source VB2 is set, and a DFF circuit 221 that generates an ON timing voltage Von by retiming the output voltage of the comparator CP2 with the output pulse Vp of the inversion detection circuit 26. With. Note that the DFF circuit 221 can be omitted.

  In the OFF period control circuit 22, when the drive voltage Vdrv becomes “L” and the switching transistor MN 1 is turned OFF, the switch SW 2 is turned OFF, and the capacitor C 4 is charged by the sense current Ioff of the current sense circuit 25. Become. Since the sense current Ioff increases as the output current Iout increases, the voltage Vc4 also increases. A photocoupler current Ipc or a feedback current Ifb flows through the resistor R3. Therefore, the voltage Vr3 at the common connection point between the resistor R3 and the inverting input terminal of the comparator CP2 increases as the sense current Ioff of the current sense circuit 25 increases, and decreases as the photocoupler current Ipc or the feedback current Ifb increases. When the voltage Vr3 exceeds the reference voltage Vref2, the output voltage of the comparator CP2 changes from “L” to “H”. The “H” voltage of the comparator CP2 is retimed in the DFF circuit 221 at the rising edge of the pulse Vp output from the inversion detection circuit 26, and becomes the ON timing voltage Von. The generation timing of the ON timing voltage Von at this time becomes slower as the photocoupler current Ipc is larger, the feedback current Ifb is larger, and the sense current Ioff of the current sense feedback circuit 25 is smaller, and the OFF period becomes longer.

  FIG. 4 is a detailed diagram of the inversion detection circuit 26 and the current limit feedback circuit 30. The inversion detection circuit 26 includes a comparator CP3 in which the voltage Vref3 is set to the inverting input terminal by the voltage source VB3, a diode-connected NPN transistor Q1 having a collector connected to the auxiliary winding L3 via the resistor R1, and an auxiliary winding. A transistor Q2 whose emitter is connected to the line L3 via a resistor R1 and a voltage source VB4 that applies a reference voltage Vref4 to the base of the transistor Q2 are provided.

  The transistor Q1 constitutes a maximum voltage regulating circuit, and when the voltage Vrise having a positive polarity on the auxiliary winding L3 is generated, the maximum voltage at the non-inverting input terminal of the comparator CP3 is limited to Vbe (Q1). Alternatively, a resistor may be connected instead of the transistor Q1, and a divided voltage of the voltage Vrise with the resistor R1 may be input as a maximum voltage to the non-inverting input terminal of the comparator CP3. The transistor Q2 constitutes a minimum voltage regulation circuit, and when the voltage Vrise having the negative side on the side of the auxiliary winding L3 is generated, the minimum voltage of the non-inverting input terminal of the comparator CP3 is limited to “Vref4-Vbe (Q2)”. To do. Vbe (Q1) is the base-emitter voltage of the transistor Q1, and Vbe (Q2) is the base-emitter voltage of the transistor Q2. The comparator CP3 outputs an “H” signal as the voltage Vp when the voltage at the non-inverting input terminal exceeds the voltage Vref3, and an “L” signal when the voltage decreases.

  By this inversion detection circuit 26, the ON timing signal can be retimed with a pulse signal obtained by shaping the pulsation voltage Vrise of the auxiliary winding L3, and the switching transistor MN1 is turned on at the valley of the free oscillation of the drain voltage to perform a pseudo resonance operation. Therefore, it is possible to eliminate the influence of the variation in the threshold value of the switching transistor during the quasi-resonant operation.

  The current limiting feedback circuit 30 includes a first current mirror circuit 31 that converts and outputs a current Ia that flows when the side of the auxiliary winding L3 becomes a negative polarity to a current Ib that is m times larger, a current source 33 of a current Ic, A current Id (= Ic−Ib) that is input and converted into a current Ie multiplied by n and output; and a current Ie that is input and sucks a feedback current Ifb that is the same value A hold circuit 34 and a timing generation circuit 35 for applying a hold timing voltage Vsp to the current hold circuit 34 are provided. Since the timing generation circuit 35 sets the hold timing voltage Vsp to “H” every predetermined time after the ● side of the auxiliary winding L3 becomes negative, the current hold circuit 34 changes the voltage Vsp from “H” to “L”. The feedback current Ifb at the timing of changing to is held.

  The current Ic of the current source 33 is set so that Ic = Ib when the output voltage Vout becomes a voltage Vout2 described later. That is, when the current Ib exceeds the current Ic, Id does not flow and the feedback current Ifb does not flow.

<Normal operation>
In the normal operation, the switching power supply device is controlled at a constant voltage so that the output voltage Vout becomes the target voltage Vout1 as shown in FIG. During this constant voltage control, the voltage Vrise generated in the auxiliary winding L3 is high, and when the ● side of the auxiliary winding L3 is negative, the current Ia input to the current feedback circuit 30 becomes large, so that the current Ib becomes the current The current Ic of the source 33 becomes larger, the currents Id and Ie become zero, and the feedback current Ifb also becomes “0”. Therefore, in this normal operation, the ON period obtained by the ON period control circuit 21 and the OFF period obtained by the OFF period control circuit 22 are exclusively controlled by the photocoupler current Ipc.

  When the output voltage Vout is higher than the target value, the photocoupler current Ipc is increased, whereby the voltage Vr2 of the ON period control circuit 21 is controlled to be low, and the OFF timing voltage Voff output from the comparator CP1 is “ H ", and the ON period is shortened. Also, since the ON period is short, the sense current Ioff is small and the photocoupler current Ipc is large. Therefore, the voltage Vr3 of the OFF period control circuit 22 is controlled low, and the ON timing voltage Von output from the comparator CP2 is “H” at a late timing. Therefore, the OFF period becomes longer. For this reason, the output voltage Vout is controlled to be low.

  When the output voltage Vout is lower than the target value, the reverse operation is performed. That is, since the OFF timing voltage Voff is output from the ON period control circuit 21 at a late timing Mig and the ON timing voltage Von is output from the OFF control circuit 22 at an early timing, the output voltage Vout is controlled to be high.

<Current limiting operation>
When the output current Iout increases and reaches the maximum value Ioutmax in FIG. 5, the output voltage Vout begins to decrease, the photodiode PD1 of the photocoupler 40 does not emit light, and the photocoupler current Ipc does not flow. Further, since the output voltage Vout is larger than Vout2, the currents Ib and Ic in the current limiting feedback circuit 30 satisfy Ib> Ic, the current Id does not flow, and the feedback current Ifb does not flow.

  At this time, since the photocoupler current Ipc and the feedback current Ifb do not flow in the ON period control circuit 21, the voltage drop at the resistor R2 does not occur, the voltage Vr2 = Vref1, and the time until the voltage Vr2 becomes equal to the sense voltage Vs1 is It becomes the maximum, that is, the ON period becomes the maximum. In the OFF period control circuit 22, since the ON period is maximized, the sense voltage Vs1 and the sense current Ioff are increased, and the photocoupler current Ipc and the feedback current Ifb are zero. Therefore, the voltage Vr3 is equal to the charging voltage Vc4 of the capacitor C4. Equally, Vr3 = Vc4, and the comparator CP2 compares it with the voltage Vref2.

  Therefore, the OFF period is determined by the charging time of the capacitor C4 and the retiming in the DFF circuit 221. At this time, since the ON period is the maximum, the OFF period is substantially constant regardless of the output state. Since the ON period and the OFF period are constant, the output power is constant, the further increase in the output current Iout is suppressed and the overcurrent is limited, and at the same time, the output voltage Vout is lower than the voltage Vout1.

<Foldback operation>
When the output voltage Vout decreases to the voltage Vout2 as shown in FIG. 5, the current Ia flowing therethrough becomes small when the ● side of the auxiliary winding L3 becomes negative, and the current Ib of the current limiting feedback circuit 30 becomes the current source 33. Therefore, the feedback current Ifb flowing into the current hold circuit 34 increases in inverse proportion to the current Ia. As a result, the feedback current Ifb flows in the ON period control circuit 21 and the OFF period control circuit 22 instead of the photocoupler current Ipc.

  At this time, in the ON period control circuit 21, the voltage Vr2 is generated by the feedback current Ifb flowing through the resistor R2. However, when the output voltage Vout decreases, the voltage Vr2 increases and the feedback current Ifb increases. The timing at which the OFF timing voltage Voff output from the comparator OP1 becomes “H” is advanced, and the ON period is shortened. In the OFF period control circuit 22, since the ON period of the switching transistor MN1 is shortened, the sense current Ioff of the current sense circuit 25 is decreased and the feedback current Ifb is increased, so that the voltage Vr3 of the resistor R3 reaches the voltage Vref2. The rising time becomes longer, the timing when the ON timing voltage Von becomes “H” is delayed, and the OFF period becomes longer.

  By such an operation, after the output voltage Vout is reduced to the voltage Vout2, the output power is reduced. Therefore, the output current Iout is sequentially reduced, and at the same time, the output voltage Vout is also sequentially reduced. The characteristics of the current voltage are shown in FIG. It becomes the characteristic to show. As described above, in this embodiment, the U-shaped characteristic shown in FIG. 5 can be realized in the power supply limitation. An operation waveform diagram in the case of the above foldback operation is shown in FIG. Vrise_L indicates the magnitude of the negative voltage generated in the auxiliary winding L3.

10: Transformer, L1: Primary winding, L2: Secondary winding, L3: Auxiliary winding 20: Control circuit, 21: ON period control circuit, 22: OFF period control circuit, 221: DFF circuit, 23: SRFF Circuit 24: drive circuit 25: current sense circuit 26: inversion detection circuit 30: current limit feedback circuit 31: first current mirror circuit 32: second current mirror circuit 33: current source 34: current hold Circuit 35: timing generation circuit 40: output voltage feedback circuit 50: photocoupler

Claims (6)

A switching transistor, a sense resistor that generates a sense voltage when the switching transistor is turned on, a primary winding to which an input voltage is applied by turning on the switching transistor, a secondary winding to which a load is connected, and an auxiliary A transformer having a winding; a photocoupler that generates a photocoupler current corresponding to an output voltage supplied from the secondary winding to the load; and taking on the sense voltage and the photocoupler current to turn on / off the switching transistor In a switching power supply device having a control circuit to control,
A drop in the output voltage is detected by the voltage obtained from the auxiliary winding, and when the voltage obtained from the auxiliary winding reaches a predetermined value or less, a feedback current inversely proportional to the voltage obtained from the auxiliary winding is generated. A switching power supply apparatus comprising a current limit feedback circuit that causes the control circuit to capture in place of the photocoupler current. The switching power supply device according to claim 1,
The current limit feedback circuit is configured to turn on the switching transistor when the voltage generated in the auxiliary winding during the OFF period of the switching transistor is equal to or lower than a predetermined voltage. A switching power supply device characterized by generating over a period and an OFF period. The switching power supply device according to claim 1 or 2, wherein the control circuit includes:
A current sense circuit that takes in the sense voltage and outputs a sense current corresponding to a load current during an OFF period of the switching transistor;
An ON period control circuit for generating an OFF timing signal for turning off the switching transistor after the switching transistor is turned on, at an earlier timing as the photocoupler current is larger, the feedback current is smaller, and the sense voltage is larger;
An OFF period control circuit that generates an ON timing signal for turning on the switching transistor after the switching transistor is turned off at an earlier timing as the photocoupler current is smaller, the feedback current is larger, and the sense current is smaller;
An SRFF circuit for turning off the switching transistor by the OFF timing signal output from the ON period control circuit and turning on the switching transistor by the ON timing signal output from the OFF period control circuit;
A switching power supply device comprising: In the switching power supply device according to claim 1, 2, or 3,
The ON period control circuit includes a second resistor inserted in a path through which the photocoupler current or the feedback current flows when the switching transistor is ON, and introduction of the photocoupler current or the feedback current of the second resistor. And a first comparator that generates the OFF timing signal when the second voltage generated on the side becomes the same voltage as the sense voltage. In the switching power supply device according to claim 1, 2, 3, or 4,
The OFF period control circuit includes a fourth capacitor that is charged with the sense current when the switching transistor is OFF and discharged when the switching transistor is ON, and the photocoupler current or the feedback current from the voltage of the fourth capacitor. A third resistor inserted so as to cause a voltage drop, and a second comparator that generates the ON timing signal when a voltage at a terminal opposite to the fourth capacitor side of the third resistor reaches a predetermined value; A switching power supply device comprising: In the switching power supply device according to claim 5,
An inversion detection circuit for generating a pulse signal by shaping a voltage generated in an auxiliary winding provided in the transformer, and the ON timing signal of the OFF period control circuit is retimed by the pulse signal A switching power supply device.

Images