Switching power supply

Abstract

The present invention provides a switching power supply device equipped with an auxiliary winding short circuit detection function and capable of switching to sleep mode. [Solution] A switching power supply device 100 comprising a transformer unit TR1, a primary side circuit 101, a control unit 102, a secondary side circuit 104, and an auxiliary winding side circuit 103, wherein the starting circuit of the control unit 200 has a function to stop the output of a drive signal when the number of times the power supply voltage rises from the stop voltage to the operating start voltage reaches a predetermined set number of times, thus fulfilling a stop condition, and the secondary side circuit 104 is characterized by comprising a detection circuit 105 that outputs a feedback signal according to the error between the output voltage and the target voltage, a voltage switching circuit 106 that lowers the target voltage and lowers the output voltage from the first voltage to the second voltage when a sleep signal is input, and a discharge circuit 107 that forms a discharge path for the smoothing capacitor C9 so that the stop condition is not met during the process of lowering to the second voltage.

Description

Technical Field

[0001] The present invention relates to a switching power supply device.

Background Art

[0002] As a control unit of a switching power supply device including a transformer unit, there is known one having a function (auxiliary winding short detection function) for detecting that an auxiliary winding of the transformer unit is short-circuited (for example, see Patent Document 1). When the auxiliary winding is short-circuited, the voltage of the power supply terminal of the control unit (hereinafter referred to as "power supply voltage Vdd") drops to the operation stop voltage, and a startup circuit (start circuit) between the high-voltage input startup terminal and the power supply terminal turns on to raise the power supply voltage Vdd to the operation start voltage. When the power supply voltage Vdd rises to the operation start voltage, the startup circuit turns off and the above operation is repeated. When the repetition exceeds a predetermined number of times, the control unit described in Patent Document 1 determines that the auxiliary winding is short-circuited, stops the switching operation of the primary side circuit, and latches and stops the power supply voltage Vdd.

[0003] As shown in FIG. 7, when the auxiliary winding is short-circuited at time t1, the power supply voltage Vdd varies within a voltage range of 6.5 [V] to 21 [V]. The control unit determines that the auxiliary winding is short-circuited at time t2 when the number of times of rising to 21 [V] after dropping to 6.5 [V] reaches 8 times, stops the switching operation, and latches and stops the power supply voltage Vdd. As a result, after time t2, the power supply voltage Vdd is limited to a relatively narrow voltage range.

[0004] Incidentally, some switching power supplies have both normal operation and sleep operation modes. For example, a switching power supply in normal operation outputs an output voltage of 24[V], while a switching power supply in sleep operation outputs an output voltage of 8[V]. If the above control unit is used with such a switching power supply, during the process of switching from normal operation to sleep operation, if the power supply voltage Vdd drops to 6.5[V] and then rises to 21[V] eight times, the control unit will mistakenly determine that the auxiliary winding has short-circuited. As a result, the above-mentioned latch stop occurs, and it becomes impossible to switch to sleep operation. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2016-158311 [Overview of the project] [Problems that the invention aims to solve]

[0006] The present invention has been made in view of the above circumstances, and its objective is to provide a switching power supply device that has an auxiliary winding short-circuit detection function and can be switched to sleep mode. [Means for solving the problem]

[0007] To solve the above problems, the switching power supply device according to the present invention is A transformer section comprising a primary winding, a secondary winding, and an auxiliary winding, A primary side circuit including a switching element connected to the primary side winding, A control unit for controlling the switching element, The secondary circuit connected to the aforementioned secondary winding, The auxiliary winding side circuit connected to the aforementioned auxiliary winding, A switching power supply device comprising, The control unit, The power supply terminal to which the power supply voltage is input from the auxiliary winding circuit, A constant voltage control circuit that performs constant voltage control to keep the output voltage of the secondary circuit constant, A drive circuit that generates and outputs a drive signal for driving the switching element based on a control signal output from the constant voltage control circuit, The system includes a starting circuit that, when the power supply voltage drops to a predetermined stop voltage, supplies voltage to the power supply terminals to raise the power supply voltage to a predetermined start voltage. The aforementioned startup circuit is The system includes a function to stop the output of the drive signal when the number of times the power supply voltage has risen from the stop voltage to the start voltage reaches a predetermined set number of times, thus fulfilling the stop condition. The aforementioned secondary circuit is A rectifier connected to the secondary winding, A smoothing section including a smoothing capacitor provided after the rectifier section, A detection circuit that outputs a feedback signal to the control unit corresponding to the error between the output voltage and a predetermined target voltage, A voltage switching circuit that, upon receiving a sleep signal, lowers the target voltage to reduce the output voltage from a first voltage to a second voltage lower than the first voltage, The present invention is characterized by comprising a discharge circuit that forms a discharge path for the smoothing capacitor such that the stop condition is not met during the process in which the output voltage decreases from the first voltage to the second voltage.

[0008] In this configuration, when a sleep signal is input, the discharge circuit of the secondary circuit forms a discharge path for the smoothing capacitor. This shortens the time it takes to switch to sleep mode (the time it takes for the output voltage to drop from the first voltage to the second voltage), allowing the system to switch to sleep mode before the conditions for stopping the auxiliary winding short circuit detection function are met.

[0009] In the above switching power supply device, The aforementioned discharge circuit is A first series circuit of a Zener diode and a first resistor, configured such that the discharge current of the smoothing capacitor flows through the first resistor via the Zener diode.

[0010] In the above switching power supply device, The detection circuit includes a photocoupler and a shunt regulator, The secondary side circuit, Can be configured to include a protection circuit that raises the reference voltage of the shunt regulator when the discharge circuit is short-circuited.

[0011] In the above switching power supply device, The protection circuit, A second series circuit of a second resistor and a diode, configured such that one end is connected to the reference terminal of the shunt regulator and the other end is connected to the connection point of the Zener diode and the first resistor.

[0012] In the above switching power supply device, The Zener diode, Preferably, the Zener voltage is smaller than the first voltage and larger than the second voltage.

Effect of the Invention

[0013] According to the present invention, it is possible to provide a switching power supply device having an auxiliary winding short-circuit detection function and capable of switching to a sleep operation.

Brief Description of the Drawings

[0014] [Figure 1] A circuit diagram of a switching power supply device according to a first embodiment of the present invention. [Figure 2] A block diagram of the control unit of the first embodiment. [Figure 3] A diagram showing the output voltage waveform when switching to the sleep operation, where (A) is a diagram of a comparative example and (B) is a diagram of the first embodiment. [Figure 4]This is a circuit diagram of a switching power supply device according to the second embodiment of the present invention. [Figure 5] This is a circuit diagram of a switching power supply device according to the third embodiment of the present invention. [Figure 6] This is a circuit diagram of a switching power supply device according to the fourth embodiment of the present invention. [Figure 7] This is a diagram for explaining the auxiliary winding short-circuit detection function of the control unit.

Embodiments for Carrying Out the Invention

[0015] Hereinafter, embodiments of a switching power supply device according to the present invention will be described with reference to the accompanying drawings.

[0016] [First Embodiment] FIG. 1 shows a switching power supply device 100 according to the first embodiment of the present invention. The switching power supply device 100 is a flyback converter and includes a transformer unit TR1, a primary-side circuit 101, a control unit 102, an auxiliary winding-side circuit 103, and a secondary-side circuit 104.

[0017] The transformer unit TR1 includes a primary-side winding N1, a secondary-side winding N2, and an auxiliary winding N3. The primary-side winding N1 is connected to the primary-side circuit 101, the secondary-side winding N2 is connected to the secondary-side circuit 104, and the auxiliary winding N3 is connected to the auxiliary winding-side circuit 103.

[0018] The primary-side circuit 101 includes terminals T1, T2, diodes D1 to D6, resistors R1 to R9, capacitors C1, C2, and switching elements Q1, Q2.

[0019] The terminals T1, T2 are AC input terminals to which an AC voltage is input. Although not shown in the figure, an X capacitor for reducing normal-mode noise may be provided between the terminals T1, T2. Also, a filter circuit composed of a common-mode choke coil may be provided between the terminals T1, T2 and the diode D1.

[0020] Diode D1 is a bridge diode formed by bridging four diodes. Diode D1 has a first input terminal, a second input terminal, a first output terminal, and a second output terminal. The first input terminal is connected to terminal T1, and the second input terminal is connected to terminal T2. The first output terminal is connected to one end of the primary winding N1, and the second output terminal is connected to the other end of the primary winding N1 via the current paths of resistor R9 and switching elements Q1 and Q2. A smoothing capacitor C1 is also connected between the first and second output terminals.

[0021] Diodes D2 and D3 and resistor R1 constitute an input circuit that supplies voltage to the control unit 102. Diode D2 has its anode connected to the first input terminal of diode D1, and its cathode connected to the eighth terminal (HV) of the control unit 102 via resistor R1. Diode D3 has its anode connected to the second input terminal of diode D1, and its cathode connected to the eighth terminal (HV) of the control unit 102 via resistor R1.

[0022] The switching element Q1, resistors R2 to R4, and diode D4 constitute the first switching circuit. In this embodiment, the switching element Q1 is an N-channel MOSFET. The gate of the switching element Q1 is connected to the fifth terminal (GATE) of the control unit 102 via a series circuit of resistors R2 and R3. Diode D4 is connected in parallel to resistor R2, and resistor R4 is connected between the gate and source of the switching element Q1.

[0023] The switching element Q2, resistors R5 to R7, and diode D5 constitute a second switching circuit. In this embodiment, the switching element Q2 is an N-channel MOSFET. The gate of the switching element Q2 is connected to the fifth terminal (GATE) of the control unit 102 via a series circuit of resistors R5 and R6. Diode D5 is connected in parallel to resistor R5, and resistor R7 is connected between the gate and source of the switching element Q2.

[0024] The first switching circuit and the second switching circuit have the same circuit configuration as described above. The switching element Q1 of the first switching circuit and the switching element Q2 of the second switching circuit are driven in parallel under the control of the control unit 102, and if the characteristics of the elements are the same, they are switched on and off at the same timing. Note that the primary side circuit 101 may consist of only one of the first switching circuit or the second switching circuit.

[0025] The resistor R8, capacitor C2, and diode D6 constitute a snubber circuit. The snubber circuit is located between one end and the other end of the primary winding N1.

[0026] The control unit 102 includes a first terminal (ADJ), a second terminal (FB), a third terminal (CS), a fourth terminal (GND), a fifth terminal (GATE), a sixth terminal (VDD), a seventh terminal (NC), and an eighth terminal (HV). In Figure 1, reference numerals 1 to 8 within the control unit 102 indicate the first terminal (ADJ) to the eighth terminal (HV).

[0027] For the control unit 102, for example, a flyback PWM control IC (model: MM3663) manufactured by MinebeaMitsumi Inc. can be used. The first terminal (ADJ) is a setting terminal, the second terminal (FB) is a feedback terminal, the third terminal (CS) is a current detection terminal, the fourth terminal (GND) is a ground terminal, the fifth terminal (GATE) is a gate output terminal, the sixth terminal (VDD) is a power supply terminal, the seventh terminal (NC) is an unconnected terminal, and the eighth terminal (HV) is a high-voltage start terminal.

[0028] The auxiliary winding circuit 103 includes diodes D7 and D8, capacitors C3 to C8, switching element Q3, resistors R10 to R13, and a photo-receiving element (phototransistor in this embodiment) of the photocoupler PC1.

[0029] Diode D7 and capacitor C3 constitute a rectifier-smoothing circuit and are connected to auxiliary winding N3. Switching element Q3, diode D8, resistor R10, and capacitor C4 constitute a series regulator. Switching element Q3 is an NPN transistor, and diode D8 is a Zener diode. The voltage induced in auxiliary winding N3 is stepped down through the above-mentioned rectifier-smoothing circuit and series regulator and input as the power supply voltage Vdd to terminal 6 (VDD) of the control unit 102.

[0030] Resistor R11 has one end connected to the source of switching elements Q1 and Q2, and the other end connected to one end of resistor R12 and the third terminal (CS) of the control unit 102. The other end of resistor R12 is connected to the fourth terminal (GND) of the control unit 102. Capacitor C5 is connected in parallel with resistor R12 between the third terminal (CS) and the fourth terminal (GND). Capacitor C6 is connected between the fourth terminal (GND) and the sixth terminal (VDD).

[0031] The phototransistor of photocoupler PC1 receives an optical signal from the light-emitting element (light-emitting diode in this embodiment) of photocoupler PC1, which is provided in the secondary circuit 104, and causes the current path to conduct. One end of the phototransistor's current path is connected to the second terminal (FB) of the control unit 102, and the other end is connected to the fourth terminal (GND). A capacitor C7 is connected in parallel to the phototransistor's current path.

[0032] Resistor R13 has one end connected to the first terminal (ADJ) of the control unit 102 and the other end connected to the fourth terminal (GND). Capacitor C8 is connected in parallel to resistor R13. By changing the resistance value of resistor R13, the voltage value input to the first terminal (ADJ) can be changed.

[0033] The secondary circuit 104 comprises diodes D9 to D11, capacitor C9, resistors R14 to R21, light-emitting diodes of photocoupler PC1, shunt regulator IC1, switching elements Q4 and Q5, and terminals T3 to T5.

[0034] Diodes D9 and D10 constitute the "rectifier section" of the present invention, and capacitor C9 constitutes the "smoothing section" of the present invention. Diodes D9 and D10 are connected in parallel, with their anodes connected to one end of the secondary winding N2. Capacitor C9 is connected between the cathodes of diodes D9 and D10 and the other end of the secondary winding N2.

[0035] The photocoupler PC1, shunt regulator IC1, and resistors R14 to R17 constitute the detection circuit 105. In the detection circuit 105, one end of resistor R14 is connected to the high-potential terminal and terminal T3 of capacitor C9, and the other end of resistor R14 is connected to the cathode terminal of shunt regulator IC1 via the light-emitting diode of photocoupler PC1. Resistor R15 is connected in parallel to the light-emitting diode of photocoupler PC1. The anode terminal of shunt regulator IC1 is connected to the low-potential terminal and terminal T4 of capacitor C9, and the reference terminal of shunt regulator IC1 is connected to the voltage divider point of the voltage divider circuit consisting of resistors R16 and R17. In the voltage divider circuit, one end on the side of resistor R16 is connected to terminal T3, and the other end on the side of resistor R17 is connected to terminal T4. The detection circuit 105 outputs a feedback signal to the second terminal (FB) of the control unit 102 according to the error between the DC output voltage output from terminals T3 and T4 and a predetermined target voltage.

[0036] Switching elements Q4, Q5 and resistors R18~R20 constitute the voltage switching circuit 106. Switching element Q4 is a PNP transistor, with one end of its current path (emitter) connected to one end of a voltage divider circuit consisting of resistors R16 and R17 (the side of resistor R16), and the other end of its current path (collector) connected to the voltage divider point of the voltage divider circuit via resistor R18. Resistor R19 is connected between the base and emitter of switching element Q4. Switching element Q5 is an NPN transistor, with one end of its current path (collector) connected to the base of switching element Q4 via resistor R20, the other end of its current path (emitter) connected to terminal T4, and its control terminal (base) connected to terminal T5. Terminal T4 is connected to ground (GND), and an external sleep signal is input to terminal T5.

[0037] The sleep signal is a signal used to switch the operation of the switching power supply unit 100 from normal operation to sleep operation (power saving operation). The sleep signal has an on (high level) state and an off (low level) state. When the sleep signal is on, the switching power supply unit 100 is in sleep operation, and when the sleep signal is off, the switching power supply unit 100 is in normal operation. During normal operation, the switching power supply unit 100 maintains the output voltage at a first voltage (24[V] in this embodiment), and during sleep operation, the switching power supply unit 100 maintains the output voltage at a second voltage lower than the first voltage (8[V] in this embodiment).

[0038] In the voltage switching circuit 106, when a sleep signal is input to terminal T5 (the sleep signal turns on), switching element Q5 turns on and the current path conducts, and switching element Q4 turns on and the current path conducts. As a result, resistor R18 is connected in parallel with resistor R16, and the voltage V1 at the reference terminal of shunt regulator IC1 (hereinafter, reference voltage V1) changes. Specifically, the reference voltage V1 changes so that the target voltage of the detection circuit 105 changes from the first voltage (24[V]) to the second voltage (8[V]).

[0039] Diode D11 and resistor R21 constitute the discharge circuit 107. Diode D11 is a Zener diode, with its cathode connected to the collector of switching element Q4 and its cathode connected to the collector of switching element Q5 via resistor R21. When a sleep signal is input to terminal T5, the discharge circuit 107 forms a discharge path for capacitor C9.

[0040] Diode D11 is set to a Zener voltage that is lower than the first voltage and slightly higher than the second voltage (8.2[V] in this embodiment). Therefore, after the output voltage drops to the second voltage (8[V]), no current flows through diode D11, and the discharge circuit 107 does not operate. This suppresses power consumption in the discharge circuit 107 during sleep operation.

[0041] As shown in Figure 2, the control unit 102 includes a setting circuit 108, a constant voltage control circuit 109, a drive circuit 110, a switching element 111, a regulator 112, and a start circuit 113.

[0042] The setting circuit 108 includes a comparator 108a. The comparator 108a compares the voltage of the first terminal (ADJ) with a predetermined threshold voltage and outputs a comparison result signal to the constant voltage control circuit 109.

[0043] The constant voltage control circuit 109 includes an oscillator circuit 109a, a waveform generation circuit 109b, an amplifier 109c, comparators 109d and 109e, an OR gate 109f, and an RS flip-flop 109g.

[0044] The oscillator circuit 109a generates an oscillation signal that oscillates at a frequency determined based on the comparison result signal of the comparator 108a and the feedback signal (voltage at the second terminal (FB), feedback voltage), and generates a clock signal based on this oscillation signal. The oscillator circuit 109a stores, for example, two data points related to the feedback voltage-frequency characteristics, and selects one of the data points depending on whether the comparison result signal is high level or low level, and determines the frequency of the oscillation signal based on the selected data point. The clock signal is a signal for determining the turn-on timing of the switching elements Q1 and Q2.

[0045] The waveform generation circuit 109b generates a voltage signal by superimposing a predetermined ramp voltage on the feedback voltage. The ramp voltage is superimposed to prevent subharmonic oscillation, i.e., for slope compensation. The amplifier 109c amplifies the voltage at the third terminal (CS) and outputs it. The voltage signal output from amplifier 109c is, for example, a right-sloping triangular waveform or a trapezoidal waveform. The comparator 109d compares the voltage signal from the waveform generation circuit 109b with the voltage signal from amplifier 109c, and the comparator 109e compares the voltage signal from amplifier 109c with a predetermined threshold voltage. The OR gate 109f outputs a reset signal which is the logical OR of the outputs of comparators 109d and 109e. The reset signal is used to determine the turn-off timing of switching elements Q1 and Q2.

[0046] The RS flip-flop 109g generates a PWM control signal (corresponding to the "control signal" of this invention) to turn switching elements Q1 and Q2 on and off based on the clock signal and reset signal, and outputs it to the drive circuit 110.

[0047] The drive circuit 110 generates drive signals to turn switching elements Q1 and Q2 on and off in response to the PWM control signal and outputs them to the fifth terminal (GATE).

[0048] The switching element 111 is an N-channel MOSFET, with one end of the current path (drain) connected to terminal 8 (HV) and the other end of the current path (source) connected to terminal 6 (VDD). The switching element 111 is switched on and off under the control of the startup circuit 113.

[0049] The regulator 112 is a circuit for generating the internal power supply voltage of the control unit 102. When it starts operating under the control of the startup circuit 113, it supplies the internal power supply voltage to each part (each circuit) of the control unit 102.

[0050] The startup circuit 113 turns on the switching element 111 when an AC voltage is input to terminals T1 and T2, supplying voltage from terminal 8 (HV) to terminal 6 (VDD) and charging capacitors C3 and C4 connected to terminal 6 (VDD). The startup circuit 113 monitors the voltage at terminal 6 (VDD) (power supply voltage Vdd), and when the power supply voltage Vdd reaches the operating start voltage (21[V] in this embodiment), it turns off the switching element 111 and starts the operation of the regulator 112. After the regulator 112 starts operating, a voltage is induced in the auxiliary winding N3, and this voltage is input to terminal 6 (VDD) via the rectifier and smoothing circuit and series regulator of the auxiliary winding side circuit 103.

[0051] If the auxiliary winding N3 is short-circuited, no voltage is input from the auxiliary winding N3 to the sixth terminal (VDD), so the power supply voltage Vdd at the sixth terminal (VDD) decreases when the switching element 111 is off. When the power supply voltage Vdd drops to the stop voltage (6.5[V] in this embodiment), the startup circuit 113 turns on the switching element 111 and stops the operation of the regulator 112. When the switching element 111 is turned on, voltage is supplied from the eighth terminal (HV) to the sixth terminal (VDD), and the power supply voltage Vdd rises again. When the power supply voltage Vdd reaches the operating start voltage (21[V]), the startup circuit 113 turns off the switching element 111 as described above and restarts the operation of the regulator 112. At this time, if the short-circuit condition of the auxiliary winding N3 has not been resolved, the power supply voltage Vdd will drop again.

[0052] The startup circuit 113, like the control unit described in Patent Document 1 in the background art, is equipped with an auxiliary winding short circuit detection function. Specifically, the startup circuit 113 determines that the auxiliary winding N3 is short-circuited when the number of times the voltage rises from the stop voltage (6.5[V]) to the operating start voltage (21[V]) reaches a predetermined set number of times (8 times in this embodiment), which is a stop condition. Upon determining that the auxiliary winding N3 is short-circuited, the startup circuit 113 stops the output of the drive signal from the drive circuit 110 and latches the power supply voltage Vdd.

[0053] Latching stops refer to limiting the power supply voltage Vdd to a voltage range smaller than the stop voltage - start voltage (in this embodiment, 10.5[V] to 15[V]) (see Figure 7 from time t2 onwards). The startup circuit 113 latches the power supply voltage Vdd by turning on the switching element 111 when the power supply voltage Vdd drops to 10.5[V], and turning off the switching element 111 when the power supply voltage Vdd rises to 15[V].

[0054] Here, the transient period during which the switching power supply 100 switches from normal operation to sleep operation is the period during which it switches from constant voltage control of 24[V] to constant voltage control of 8[V]. During the transient period, the oscillation of the PWM control stops and the switching elements Q1 and Q2 are turned off. Since the power supply voltage Vdd depends on the voltage (charge amount) of capacitors C3 and C4, the power supply voltage Vdd decreases over time.

[0055] The startup circuit 113 raises the power supply voltage Vdd to the start voltage (21[V]) when it drops to the stop voltage (6.5[V]). When the number of times the voltage has been raised reaches 8 (the stop condition is met), the startup circuit 113 mistakenly determines that the auxiliary winding N3 is short-circuited, stops the output of the drive signal from the drive circuit 110, and latches the power supply voltage Vdd. In this case, constant voltage control of 8[V] (sleep operation) becomes impossible.

[0056] In this regard, in the switching power supply 100, the discharge circuit 107 forms a discharge path for the capacitor C9 during the transient period when switching from normal operation to sleep operation. This shortens the transient period, that is, the period during which the output voltage drops from 24[V] to 8[V].

[0057] Figure 3 shows the output voltage waveform during the transient period when switching from normal operation to sleep operation. Figure 3(A) shows the output voltage waveform of a comparative example switching power supply without the discharge circuit 107, and Figure 3(B) shows the output voltage waveform of the switching power supply 100 of this embodiment. The comparative example switching power supply has the same configuration as the switching power supply 100 of this embodiment, except that it does not have the discharge circuit 107.

[0058] As shown in Figure 3(A), in the comparative example switching power supply, when a sleep signal is input to terminal T5 at time t10 (the sleep signal turns on), switching elements Q5 and Q4 conduct and the output voltage begins to decrease. However, in the comparative example switching power supply, the stop condition is met at time t30, before the output voltage drops to 8[V], resulting in a latch stop. As a result, the comparative example switching power supply is unable to perform constant voltage control at 8[V].

[0059] On the other hand, in the switching power supply device 100 of this embodiment, as shown in Figure 3(B), when a sleep signal is input to terminal T5 at time t10 (the sleep signal turns ON), the switching elements Q5 and Q4 conduct and the output voltage begins to decrease. At the same time, the discharge circuit 107 forms a discharge path for capacitor C9. As a result, the degree of output voltage decrease is greater than in the comparative example, and the output voltage drops to 8[V] at time t20, before time t30, without satisfying the stop condition. After time t20, a constant voltage control of 8[V] is performed.

[0060] As described above, the switching power supply 100 of this embodiment avoids latch stoppage during the transition to sleep mode (transient period). In this embodiment, the stop condition is that the number of times the power supply voltage Vdd rises from the stop voltage (6.5[V]) to the start voltage (21[V]) reaches 8. Therefore, the discharge circuit 107 should be designed so that the number of such occurrences during the transient period is 7 or less.

[0061] Alternatively, the capacitances of capacitors C3 and C4 may be increased to prevent the power supply voltage Vdd from dropping to the stop voltage (6.5[V]) during the transient period. In the switching power supply device 100 of this embodiment, the transient period is shortened by the discharge circuit 107, so by slightly increasing the capacitances of capacitors C3 and C4, it is possible to avoid the power supply voltage Vdd dropping to the stop voltage (6.5[V]) during the transient period.

[0062] [Second Embodiment] Figure 4 shows a switching power supply 100A according to a second embodiment of the present invention. The switching power supply 100A has the same configuration as the first embodiment, except that it includes a secondary circuit 104A instead of the secondary circuit 104. The secondary circuit 104A has the same configuration as the secondary circuit 104 of the first embodiment, except that it further includes a protection circuit 114.

[0063] The protection circuit 114 consists of a resistor R22 and a diode D12, and increases the reference voltage V1 of the shunt regulator IC1 when the diode D11 (Zener diode) of the discharge circuit 107 is short-circuited. One end of the resistor R22 is connected to the reference terminal of the shunt regulator IC1, and the other end is connected to the cathode of diode D12. The anode of diode D12 is connected to the anode of diode D11 (Zener diode).

[0064] If diode D11 (Zener diode) in discharge circuit 107 is short-circuited, protection circuit 114 (resistor R22 and diode D12) conducts, increasing the reference voltage V1 of shunt regulator IC1. As a result, control unit 102 operates to reduce the output voltage.

[0065] According to the switching power supply device 100A of the second embodiment, in addition to the effects of the first embodiment, abnormal heat generation of resistor R21 during a short circuit of the discharge circuit 107 can be suppressed, and damage to resistor R21 can be avoided. As a result, a relatively inexpensive resistor with a low rating can be used as resistor R21. Similarly, a relatively inexpensive resistor with a low rating can also be used as resistor R22.

[0066] [Third Embodiment] Figure 5 shows a switching power supply device 100B according to a third embodiment of the present invention. The switching power supply device 100B has the same configuration as the second embodiment, except that it includes a secondary circuit 104B instead of a secondary circuit 104A. The secondary circuit 104B has the same configuration as the second embodiment, except that it includes a discharge circuit 107B instead of a discharge circuit 107.

[0067] The discharge circuit 107B has the same configuration as the second embodiment, except that the resistor R21 is connected to terminal T4 without passing through the current path of the switching element Q5. Therefore, the switching power supply device 100B according to the third embodiment has the same effects as the second embodiment.

[0068] [Fourth Embodiment] Figure 6 shows a switching power supply 100C according to a fourth embodiment of the present invention. The switching power supply 100C has the same configuration as the second embodiment, except that it includes a secondary circuit 104C instead of a secondary circuit 104A. The secondary circuit 104C has the same configuration as the second embodiment, except that it includes a discharge circuit 107C instead of a discharge circuit 107.

[0069] The discharge circuit 107C has the same configuration as the second embodiment, except that the cathode of diode D11 (Zener diode) is connected to terminal T3 without passing through the current path of switching element Q4. Therefore, the switching power supply device 100C according to the fourth embodiment has the same effects as the second embodiment.

[0070] Although embodiments of the switching power supply device according to the present invention have been described above, the present invention is not limited to the above embodiments.

[0071] The discharge circuit according to the present invention can be modified as appropriate, as long as the discharge path of the smoothing capacitor (capacitor C9 in the above embodiment) is formed so that the stop condition of the auxiliary winding short detection function is not met during the process in which the output voltage drops from the first voltage during normal operation to the second voltage during sleep operation. For example, a switching element may be used instead of the diode D11. [Explanation of Symbols]

[0072] 100V, 100A~100C Switching Power Supply 101 Primary side circuit 102 Control Unit 103 Auxiliary winding side circuit 104, 104A~104C Secondary circuit 105 Detection circuit 106 Voltage switching circuit 107, 107B, 107C discharge circuit 108 Setting Circuit 109 Constant Voltage Control Circuit 110 Drive Circuit 111 Switching elements 112 Regulator 113 Startup Circuit 114 Protection circuit

Claims

1. A transformer section comprising a primary winding, a secondary winding, and an auxiliary winding, A primary side circuit including a switching element connected to the primary side winding, A control unit for controlling the switching element, The secondary circuit connected to the secondary winding, The auxiliary winding side circuit connected to the aforementioned auxiliary winding, A switching power supply device comprising, The control unit, The power supply terminal to which the power supply voltage is input from the auxiliary winding circuit, A constant voltage control circuit that performs constant voltage control to keep the output voltage of the secondary circuit constant, A drive circuit that generates and outputs a drive signal for driving the switching element based on a control signal output from the constant voltage control circuit, The system includes a starting circuit that, when the power supply voltage drops to a predetermined stop voltage, supplies voltage to the power supply terminals to raise the power supply voltage to a predetermined start voltage. The aforementioned startup circuit is The system includes a function to stop the output of the drive signal when the number of times the power supply voltage has risen from the stop voltage to the start voltage reaches a predetermined set number of times, thus fulfilling the stop condition. The aforementioned secondary circuit is A rectifier connected to the secondary winding, A smoothing section including a smoothing capacitor provided after the rectifier section, A detection circuit that outputs a feedback signal to the control unit corresponding to the error between the output voltage and a predetermined target voltage, A voltage switching circuit that, upon receiving a sleep signal, lowers the target voltage to reduce the output voltage from a first voltage to a second voltage lower than the first voltage, The discharge circuit includes a discharge circuit that forms a discharge path for the smoothing capacitor so that the stop condition is not met during the process in which the output voltage decreases from the first voltage to the second voltage. A switching power supply device characterized by the following features.

2. The aforementioned discharge circuit is This is a first series circuit consisting of a Zener diode and a first resistor, where the discharge current of the smoothing capacitor flows through the Zener diode to the first resistor. The switching power supply device according to feature 1.

3. The detection circuit includes a photocoupler and a shunt regulator. The aforementioned secondary circuit is The system includes a protection circuit that increases the reference voltage of the shunt regulator when the discharge circuit is short-circuited. The switching power supply device according to feature 2.

4. The aforementioned protection circuit is This is a second series circuit consisting of a second resistor and a diode, with one end connected to the reference terminal of the shunt regulator and the other end connected to the connection point between the Zener diode and the first resistor. The switching power supply device according to feature 3.

5. The Zener diode is The Zener voltage is less than the first voltage and greater than the second voltage. A switching power supply device according to any one of claims 2 to 4.

Images