Switching power supply

Abstract

Despite its relatively simple configuration, it prevents malfunction of the control unit due to discharge of the smoothing capacitor when the AC voltage is interrupted. [Solution] When an AC voltage is supplied, the switching power supply 1 charges the switching capacitor 23 to turn on the second switching element 22 and turns off the first switching element 21, thereby blocking the connection between the power terminal of the control unit 18 and ground using the first switching element 21. When the input of the AC voltage is stopped, the switching power supply 1 discharges the switching capacitor 23 to turn off the second switching element 22 and turns on the first switching element 21, connecting the power terminal of the control unit 18 to ground through the first switching element 21.

Description

Technical Field

[0001] The present invention relates to a switching power supply device that converts an AC voltage into a DC voltage and outputs the DC voltage.

Background Art

[0002] As a switching power supply device, there is one that includes a rectifier circuit for rectifying an AC voltage, a smoothing capacitor for smoothing the output of the rectifier circuit, a switching transformer having a primary coil and a secondary coil, and the smoothing capacitor is connected to the primary coil, a driving switching element for turning on and off the current flowing from the smoothing capacitor to the primary coil, and a control unit for controlling the driving switching element, etc., and smooths and outputs the secondary voltage of the secondary coil of the switching transformer. However, when the AC voltage is interrupted, malfunction of the control unit may be caused by the discharge of the smoothing capacitor.

[0003] Therefore, in the switching power supply device described in Patent Document 1, an AC input is rectified by a rectifier circuit, the output of the rectifier circuit is smoothed by a capacitor and supplied to a switching circuit, and the current flowing through the primary winding (primary coil) of a transformer is interrupted by a first switching element to generate an output in a secondary winding (secondary coil) and an auxiliary winding. The control IC operates by inputting the output of the auxiliary winding and performs on / off control of the first switching element. When the AC input is interrupted, the output voltage of the auxiliary winding of the transformer decreases and falls below a threshold value. In the discharge section, the fourth switching element turns off and the third and second switching elements turn on, and the charge of the capacitor is discharged through the second switching element. Thereby, the residual charge of the smoothing capacitor is surely discharged.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] However, Patent Document 1 assumes a configuration in which an auxiliary winding is provided on the transformer and the control IC is operated by the output of the auxiliary winding. Since a discharge section is added to this configuration, the discharge section is equipped with many elements such as a rectifier circuit, a Zener diode, and two switching elements, making the configuration of the discharge section complex.

[0006] Therefore, the present invention has been made in view of the above circumstances, and aims to prevent malfunction of the control unit due to discharge of the smoothing capacitor when the AC voltage is interrupted and the conversion to a DC voltage difference is stopped, while having a simple configuration. [Means for solving the problem]

[0007] A switching power supply device according to one aspect of the present invention includes: a rectifier circuit that converts an AC voltage to a DC voltage and outputs it; a smoothing capacitor that smooths the DC voltage; a switching transformer having a primary coil and a secondary coil, the smoothing capacitor connected to the primary coil; a driving switching element connected to the primary coil of the switching transformer that turns on and off the current flowing from the smoothing capacitor to the primary coil; a DC voltage output unit that smooths the secondary voltage of the secondary coil of the switching transformer, which is generated by the on / off switching of the current flowing to the primary coil by the driving switching element, and outputs a DC voltage; a control unit that controls the driving switching element to adjust the output of the secondary coil of the switching transformer; and a power terminal of the control unit to which the DC voltage from the DC voltage output unit is input. The power supply unit comprises: a first switching element inserted between the child and ground; a second switching element connected to the input side of the first switching element; a switching capacitor connected to the input side of the second switching element; and a power supply unit which, when an AC voltage is input, charges the switching capacitor, increases the voltage of the switching capacitor, turns on the second switching element, turns off the first switching element, and uses the first switching element to interrupt the connection between the power terminal of the control unit and ground; and when the input of the AC voltage is stopped, discharges the switching capacitor, decreases the voltage of the switching capacitor, turns off the second switching element, turns on the first switching element, and connects the power terminal of the control unit to ground through the first switching element. [Effects of the Invention]

[0008] According to the present invention, despite its simple configuration, it is possible to prevent malfunction of the control unit due to discharge of the smoothing capacitor when the AC voltage is interrupted. [Brief explanation of the drawing]

[0009] [Figure 1] This is a circuit diagram showing a switching power supply according to one embodiment of the present invention. [Figure 2] (A) and (B) are graphs showing the voltage Vcc and sleep signal SP, which are the output of the switching power supply in this embodiment. [Figure 3] This is a circuit diagram showing a switching power supply as an example. [Figure 4] This graph shows the voltage Vcc and sleep signal SP when the AC plug is disconnected from the commercial AC power supply after the power switch is turned off without being held down for a long time in the comparative example switching device. [Figure 5] This graph shows the voltage Vcc and sleep signal SP when the AC plug is disconnected from the commercial AC power supply after the power switch is turned off by pressing and holding the switch in the comparative example switching device. [Figure 6] This graph shows the voltage Vcc and sleep signal SP when the AC plug is disconnected from the commercial AC power supply without turning off the power switch in the comparative example switching device. [Figure 7] (A) is a graph showing the voltage Vhv input to the HV terminal of the control unit, the voltage Vcc input to the VCC terminal (power terminal) of the control unit, and the secondary voltage Vout of the secondary coil of the switching transformer in the comparative example switching power supply device, and (B) is a table showing the operating conditions of the control unit. [Modes for carrying out the invention]

[0010] Hereinafter, a switching power supply according to one embodiment of the present invention will be described with reference to the drawings. Figure 1 is a circuit diagram showing a switching power supply according to one embodiment of the present invention. The switching power supply of this embodiment is applied to an image forming apparatus such as an MFP, and is used as a power supply for the backlight of the display unit of the image forming apparatus.

[0011] As shown in Figure 1, the switching power supply unit 1 includes an EMI filter 11, a rectifier circuit 12, two smoothing capacitors 13 and 14, a switching transformer 15, a driving switching element 16, a DC voltage output unit 17, a control unit 18, a power supply unit 19, and a discharge unit 20. In Figure 1, the five straight arrows indicate that the ends of the arrows are electrically short-circuited.

[0012] The EMI filter 11 takes the AC voltage input from the commercial AC power supply, removes noise from the AC voltage, converts it back into AC voltage, and outputs it to the rectifier circuit 12.

[0013] The rectifier circuit 12 is a diode bridge that rectifies an AC voltage and outputs a DC voltage. The smoothing capacitors 13 and 14 are large-capacity electrolytic capacitors that smooth the DC voltage output from the rectifier circuit 12 to generate a nearly constant DC voltage. The rectifier circuit 12 and the smoothing capacitors 13 and 14 constitute a so-called voltage doubler rectifier circuit.

[0014] The switching transformer 15 has a primary coil 15A and a secondary coil 15B, and outputs the current from each smoothing capacitor 13 and 14 to the primary coil 15A through an inductor element FBL or the like.

[0015] The driving switching element 16 is connected to the primary coil 15A of the switching transformer 15, and is also connected to ground through the resistor R0 and capacitor C0. The driving switching element 16 is switched on and off by the control unit 18, intermittently supplying current from the smoothing capacitors 13 and 14 to ground through the primary coil 15A, the resistor R0, and the capacitor C0.

[0016] When a secondary voltage is generated in the secondary coil 15B of the switching transformer 15 as the driving switching element 16 turns on and off, the DC voltage output unit 17 smoothes the secondary voltage with the diode D3 and the capacitor C1, inputs the smoothed voltage to a constant voltage circuit composed of the transistor T2 and a Zener diode, generates a constant voltage Vcc, and outputs this voltage Vcc.

[0017] The control unit 18 is, for example, an IC (integrated circuit), and includes an HV terminal, a VCC terminal, an OUT terminal, an FB terminal, a COMP terminal, a CS terminal, and a GND terminal connected to the ground.

[0018] The HV terminal is a terminal to which a startup current is input from the output side of the EMI filter 11 through the diodes D1 and D2. When a startup current is input to the HV terminal, the control unit 18 starts operating.

[0019] The VCC terminal is a power supply terminal of the control unit 18 to which the voltage Vcc is input. The control unit 18 operates with the voltage Vcc input to the VCC terminal.

[0020] The FB terminal is a terminal to which the output from the transistor T4 on the light-receiving side of the photocoupler FC2 is input. The diode D1 on the light-emitting side of the photocoupler FC2 changes the light emission amount according to the current value of the current flowing through the diode D1. The output of the transistor T1 on the light-receiving side of the photocoupler FC corresponding to the current value of the current flowing through the diode D1 is input to the control unit 18 from the FB terminal. The control unit 18 executes a protection operation of the switching power supply device 1 according to the current and the current value.

[0021] The CS terminal is a terminal to which the current flowing through the driving switching element 16 is input. The control unit 18 limits the current flowing through the driving switching element 16 according to the current input to the CS terminal.

[0022] The COMP terminal is the terminal to which the output of the light-receiving transistor T1 of the photocoupler FC1 is input. The amount of light emitted by the light-emitting diode D5 of the photocoupler FC1 changes according to the value of the current flowing through the load L1 of the secondary coil 15B. The output of the light-receiving transistor T1 indicates the value of the current flowing through the load L1 of the secondary coil 15B. The control unit 18 receives the output of the light-receiving transistor T1 of the photocoupler FC1, which indicates the value of the current flowing through the load L1, from the COMP terminal. The control unit 18 generates an on / off signal with a pulse width corresponding to the value of the current of the load L1, and outputs this on / off signal to the driving switching element 16 from the OUT terminal. The driving switching element 16 is PWM controlled by its on / off signal, the secondary voltage of the secondary coil 15B is adjusted, and the voltage Vcc of the DC voltage output section 17 is maintained constant.

[0023] Therefore, when a starting current is input to the HV terminal, the control unit 18 starts operating and generates an on / off signal with a pulse width corresponding to the current value of the secondary coil 15B based on the output of the light-receiving transistor T1 of the photocoupler FC1 input to the COMP terminal, and outputs it to the drive switching element 16. The control unit 18 then uses PWM control to output a constant voltage Vcc from the DC voltage output unit 17. In addition, the control unit 18 operates stably when a constant voltage Vcc is input to the VCC terminal (power supply terminal).

[0024] On the other hand, the discharge unit 20 includes a first switching element 21 inserted between the VCC terminal (power terminal) of the control unit 18 and ground, a second switching element 22 connected to the gate of the first switching element 21 to switch the first switching element 21 on or off, a switching capacitor 23 to switch the second switching element 22 on or off, and a discharge resistor 24 inserted between the smoothing capacitor 14 and ground. The discharge resistor 24 is connected in parallel to the output side of the second switching element, and the voltage across this discharge resistor 24 is input to the gate of the first switching element 21.

[0025] The second switching element 22 turns on or off depending on the terminal voltage of the switching capacitor 23, turning the first switching element 22 on or off. When the first switching element 21 is turned off, it disconnects the VCC terminal (power supply terminal) of the control unit 18 from ground. When the first switching element 21 is turned on, the VCC terminal (power supply terminal) of the control unit 18 is connected to ground through the first switching element 21.

[0026] The power supply unit 19 is a separate switching power supply unit provided in the image forming apparatus, which generates a low voltage and supplies it to the image forming apparatus. This power supply unit 19 is equipped with a separate rectifier circuit 31 that converts the AC voltage supplied from the commercial AC power supply into a DC voltage and outputs it. The DC voltage output from the separate rectifier circuit 31 is supplied to the switching capacitor 23 of the discharge unit 20, which charges the switching capacitor 23.

[0027] Here, when a commercial AC power supply is connected to the image forming apparatus and the power switch (not shown) of the image forming apparatus is turned on, the AC voltage is supplied to the rectifier circuit 12 through the EMI filter 11, the AC voltage is rectified by the rectifier circuit 12 to obtain a DC voltage, and this DC voltage is smoothed by the smoothing capacitors 13 and 14. At the same time, a starting current is input from the output side of the EMI filter 11 to the HV terminal of the control unit 18 through the diodes D1 and D2, and the control unit 18 starts operating and controls the drive switching element 16 by turning it on and off. As a result, current flows intermittently from the smoothing capacitors 13 and 14 to the primary coil 15A, and a secondary voltage is generated in the secondary coil 15B of the switching transformer 15. In the DC voltage output section 17, the secondary voltage is smoothed by the diode D3 and capacitor C1, and a constant voltage Vcc is generated by a constant voltage circuit consisting of a transistor T2 and a Zener diode. This voltage Vcc is input to the VCC terminal of the control unit 18, and the control unit 18 continues to operate.

[0028] Furthermore, when a commercial AC power supply is connected to the image forming apparatus and the power switch (not shown) of the image forming apparatus is turned on, the AC voltage is supplied to another rectifier circuit 31 of the power supply unit 19, where the AC voltage is rectified by the other rectifier circuit 31 to obtain a DC voltage, and this DC voltage is applied from the other rectifier circuit 31 to the switching capacitor 23 of the discharge unit 20, thereby charging the switching capacitor 23. When the switching capacitor 23 is charged, the terminal voltage of the switching capacitor 23 rises, the second switching element 22 turns on, the voltage of the discharge resistor 24 on the gate side of the first switching element 21 (gate voltage of the first switching element 21) drops, the first switching element 21 turns off, and the first switching element 21 disconnects the connection between the VCC terminal (power supply terminal) of the control unit 18 and ground. In this case, a constant voltage Vcc is maintained at the VCC terminal of the control unit 18, and the control unit 18 operates stably.

[0029] When the commercial AC power supply is disconnected from the image forming apparatus and the supply of AC voltage is stopped, the DC voltage output from the rectifier circuit 12 is also stopped. Immediately before this DC voltage output is stopped, the smoothing capacitors 13 and 14 are charged, so from the moment the AC voltage supply is stopped, the current released by the discharge of the smoothing capacitors 13 and 14 flows into the primary coil 15A of the switching transformer 15.

[0030] At this time, the AC voltage supply to the power supply unit 19 is stopped, and the DC voltage output from another rectifier circuit 31 is also stopped. As a result, the DC voltage is no longer applied to the switching capacitor 23 of the discharge unit 20, the switching capacitor 23 discharges through the resistor R1, the terminal voltage of the switching capacitor 23 drops, and the second switching element 22 turns off. Then, current flows from the smoothing capacitor 14 through the resistor R2 to the discharge resistor 24 on the gate side of the first switching element 21, the gate voltage of the first switching element 21 rises, the first switching element 21 turns on, and the VCC terminal (power supply terminal) of the control unit 18 is connected to ground through the resistor R3 and the first switching element 21. As a result, the voltage at the VCC terminal of the control unit 18 drops to 0V, and the control unit 18 stops. Therefore, the driving switching element 16 is not turned on or off and controlled by the control unit 18, no secondary voltage is generated in the secondary coil 15B of the switching transformer 15, and the DC voltage output unit 17 does not output a voltage Vcc.

[0031] Furthermore, current flows from the smoothing capacitor 14 to ground through resistor R2 and the discharge resistor 24 on the gate side of the first switching element 21, causing the smoothing capacitor 14 to discharge. As a result, the amount of current input from the smoothing capacitor 14 to the HV terminal of the control unit 18 through diode D2 is reduced. Therefore, this current does not become the starting current that restarts the operation of the control unit 18.

[0032] Therefore, in the switching power supply device 1 according to this embodiment, when the AC voltage is no longer supplied, the control unit 18 does not perform operation control based on the current input by the discharge of the smoothing capacitor 14 (i.e., the control unit 18 does not malfunction due to the current input by the discharge of the smoothing capacitor 14). Furthermore, the discharge unit 20 is composed of a small number of elements such as the first switching element 21, the second switching element 22, the switching capacitor 23, and the discharge resistor 24, so its configuration is relatively simple.

[0033] Figures 2(A) and (B) are graphs showing the voltage Vcc and sleep signal SP, which are the output of the switching power supply 1 according to this embodiment. The sleep signal SP is generated by the control unit 18 or the engine board to which the voltage Vcc of the switching power supply 1 is supplied.

[0034] The graph in Figure 2(A) shows the voltage Vcc and sleep signal SP when the power switch of the image forming apparatus is turned off and the AC plug of the image forming apparatus is disconnected from the commercial AC power supply. The graph in Figure 2(B) shows the voltage Vcc and sleep signal SP when the AC plug of the image forming apparatus is disconnected from the commercial AC power supply without turning off the power switch of the image forming apparatus. As is clear from the graphs in Figures 2(A) and (B), when the supply of AC voltage is stopped, the voltage Vcc decreases and the sleep signal SP also decreases rapidly. Therefore, no malfunction of the control unit 18 occurs when the AC voltage is interrupted. <Comparative Example>

[0035] Figure 3 is a circuit diagram showing a comparative example switching power supply 1A. As shown in Figure 3, the comparative example switching power supply 1A has a configuration in which the power supply unit 19 and the discharge unit 20 of the switching power supply 1 of the embodiment shown in Figure 1 are omitted.

[0036] In this comparative example switching power supply 1A, when a commercial AC power supply is connected to the image forming apparatus and the power switch (not shown) of the image forming apparatus is turned on, the AC voltage is applied to the rectifier circuit 12 through the EMI filter 11, similar to the switching power supply 1 of the embodiment, and the DC voltage output from the rectifier circuit 12 is smoothed by the smoothing capacitors 13 and 14. At the same time, a starting current is input from the output side of the EMI filter 11 to the HV terminal of the control unit 18 through the diodes D1 and D2, causing the control unit 18 to start operating and turn the driving switching element 16 on and off. As a result, current flows intermittently from the smoothing capacitors 13 and 14 to the primary coil 15A, generating a secondary voltage in the secondary coil 15B of the switching transformer 15, and a constant voltage Vcc is output from the DC voltage output unit 17. This voltage Vcc is applied to the VCC terminal of the control unit 18, and the control unit 18 continues to operate.

[0037] When the commercial AC power supply is disconnected from the image forming apparatus and the supply of AC voltage is stopped, the DC voltage output from the rectifier circuit 12 is stopped, but the current generated by the discharge of each smoothing capacitor 13 and 14 is supplied to the primary coil 15A of the switching transformer 15.

[0038] Furthermore, current is input from the smoothing capacitor 14 through the diode D2 to the HV terminal of the control unit 18. This current acts as a starting current, causing the control unit 18 to start operating and switch the drive switching element 16 on and off. As a result, current flows intermittently from each smoothing capacitor 13 and 14 to the primary coil 15A, generating a secondary voltage in the secondary coil 15B of the switching transformer 15, and a voltage is output from the DC voltage output unit 17.

[0039] At this time, the voltage Vcc output from the DC voltage output section 17 is generated solely by the discharge from each smoothing capacitor 13 and 14 while the output from the rectifier circuit 12 is stopped, so its voltage value is unstable and gradually decreases. For this reason, even if the control unit 18 operates temporarily due to the input of a starting current, the control unit 18 does not operate continuously and stops. However, the current input from the smoothing capacitor 14 through the diode D2 to the HV terminal of the control unit 18 becomes the starting current again, causing the control unit 18 to operate temporarily again, and then stop again without operating continuously. This temporary operation of the control unit 18 is repeated until the smoothing capacitor 14 is sufficiently discharged, and each time the temporary operation is repeated, a voltage is output from the DC voltage output section 17.

[0040] For example, if the switching power supply 1A of the comparative example is used as the power supply for the backlight of the display unit of the image forming apparatus, when the supply of AC voltage to the image forming apparatus is stopped, a voltage is temporarily and repeatedly output from the DC voltage output unit 17 to the backlight, causing the backlight to repeatedly light up, which is a malfunction.

[0041] Figures 4, 5, and 6 are graphs showing the voltage Vcc and sleep signal SP, which are the output values ​​of the comparative example switching power supply 1. The graph in Figure 4 shows the voltage Vcc and sleep signal SP when the power switch of the image forming apparatus is turned off without pressing and holding it, and then the AC plug of the image forming apparatus is disconnected from the commercial AC power supply. The graph in Figure 5 shows the voltage Vcc and sleep signal SP when the power switch of the image forming apparatus is turned off by pressing and holding it, and then the AC plug of the image forming apparatus is disconnected from the commercial AC power supply. The graph in Figure 6 shows the voltage Vcc and sleep signal SP when the AC plug of the image forming apparatus is disconnected from the commercial AC power supply without turning off the power switch of the image forming apparatus.

[0042] As is clear from the graphs in Figures 4, 5, and 6, when the AC voltage supply is stopped, the voltage Vcc decreases, but an abnormal waveform is generated in the voltage Vcc due to the temporary repetitive output from the DC voltage output unit 17, and at the same time, an abnormal waveform is also generated in the sleep signal SP.

[0043] Figure 7(A) is a graph showing the voltage Vhv input to the HV terminal of the control unit 18, the voltage Vcc input to the VCC terminal (power terminal) of the control unit 18, and the secondary voltage Vout of the secondary coil 15B of the switching transformer 15 in the comparative example switching power supply device 1A.

[0044] As is clear from the graph in Figure 7(A), when current is input from the smoothing capacitor 14 through the diode D2 to the HV terminal of the control unit 18, the voltage Hvh at the HV terminal temporarily rises and exceeds Hvhmin in the table in Figure 7(B) (start current flows), the control unit 18 starts operating, the driving switching element 16 is turned on and off, current flows intermittently through the primary coil 15A of the switching transformer 15, the secondary voltage Vout of the secondary coil 15B rises, and the voltage Vcc input to the VCC terminal (power supply terminal) rises.

[0045] When the voltage Vcc at the VCC terminal exceeds the UVLO(OFF) voltage value (15V) of the UVLO (Undervoltage Lockout) function in the table in Figure 7(B), the control unit 18 continues to operate. When the voltage Vcc at the VCC terminal falls below the UVLO(OFF) voltage value (5.5V), the control unit 18 stops.

[0046] As described above, the voltage Vcc output from the DC voltage output unit 17 is generated solely by the discharge from each smoothing capacitor 13 and 14. Therefore, its voltage value is unstable and gradually decreases, and when the voltage Vcc at the VCC terminal falls below the UVLO (OFF) voltage value (5.5V), the control unit 18 stops.

[0047] The temporary operation of the control unit 18 is repeated until the smoothing capacitor 14 is fully discharged, and each time the temporary operation is repeated, a voltage Vcc is output from the DC voltage output unit 17.

[0048] As is clear from the graphs in Figures 4 to 7, in the comparative example switching power supply 1A, when the commercial AC power supply is disconnected from the image forming apparatus and the supply of AC voltage is stopped, a starting current is repeatedly input from the smoothing capacitor 14 through the diode D2 to the HV terminal of the control unit 18 until the smoothing capacitor 14 is sufficiently discharged, and a voltage Vcc is repeatedly output from the DC voltage output unit 17.

[0049] In contrast to this comparative example, in the switching power supply device 1 according to the embodiment of the present invention, when the supply of AC voltage from the image forming apparatus is stopped, the commercial AC power supply turns on as described above, the first switching element 21 turns on, the VCC terminal (power terminal) of the control unit 18 is connected to ground through the first switching element 21, no voltage Vcc is applied to the VCC terminal of the control unit 18, and the control unit 18 is shut down.

[0050] Furthermore, since the smoothing capacitor 14 is discharged through the resistor R2 and the discharge resistor 24 on the gate side of the first switching element 21, the amount of current flowing from the smoothing capacitor 14 through the diode D2 to the HV terminal of the control unit 18 is suppressed, and the control unit 18 does not operate temporarily.

[0051] Therefore, the DC voltage output unit 17 will not repeatedly output voltage temporarily, and the problem of the backlight repeatedly turning on will not occur.

[0052] Furthermore, the configuration and processing of the above embodiments described with reference to Figures 1 and 2 are merely examples of the present invention, and the present invention is not intended to be limited to such configurations and processing. [Explanation of Symbols]

[0053] 1. Switching power supply 11 EMI filter 12 Rectifier circuit 13, 14 Smoothing Capacitors 15 Switching Transformer 16. Driving switching element 17 DC voltage output section 18 Control Unit 19 Power supply section 20 Discharge section 21 First switching element 22 Second switching is possible. 23 Switching capacitor 24 Discharge Resistance

Claims

1. A rectifier circuit that converts AC voltage to DC voltage and outputs it, A smoothing capacitor for smoothing the DC voltage, A switching transformer having a primary coil and a secondary coil, wherein the smoothing capacitor is connected to the primary coil, A driving switching element connected to the primary coil of the switching transformer, which switches the current flowing from the smoothing capacitor to the primary coil on and off, A DC voltage output unit that smooths the secondary voltage of the secondary coil of the switching transformer, which is generated by the on / off switching of the current flowing through the primary coil by the driving switching element, and outputs a DC voltage, A control unit that controls the drive switching element to adjust the output of the secondary coil of the switching transformer, A first switching element is inserted between the power terminal of the control unit, to which the DC voltage from the DC voltage output unit is input, and ground. A second switching element connected to the input side of the first switching element, A switching capacitor connected to the input side of the second switching element, When the AC voltage is input, the switching capacitor is charged, the voltage of the switching capacitor is increased, the second switching element is turned on, the first switching element is turned off, and the first switching element isolates the connection between the power terminal of the control unit and the ground. A switching power supply device comprising: a power supply unit that, when the input of the AC voltage is stopped, discharges the switching capacitor, lowers the voltage of the switching capacitor, turns off the second switching element, turns on the first switching element, and connects the power terminal of the control unit to the ground through the first switching element.

2. A discharge resistor is inserted between the smoothing capacitor and the ground, and the discharge resistor is connected in parallel to the output side of the second switching element, and the voltage of the discharge resistor is applied to the input side of the first switching element. When the AC voltage is input, the switching capacitor is charged, and as the voltage of the switching capacitor rises, the second switching element turns on, causing the voltage of the discharge resistor to drop, and the first switching element turns off. The switching power supply device according to claim 1, wherein when the AC voltage is stopped being input, the switching capacitor is discharged, the voltage of the switching capacitor decreases and the second switching element turns off, a current flows from the smoothing capacitor through the discharge resistor to the ground, the voltage of the discharge resistor increases and the first switching element turns on.

3. The power supply unit is further equipped with another rectifier circuit that converts AC voltage to DC voltage and outputs it. The aforementioned switching capacitor is When the AC voltage is input to the other rectifier circuit, it is charged by the DC voltage output from the other rectifier circuit. The switching power supply device according to claim 1 or claim 2, wherein when the AC voltage is not input to the other rectifier circuit, the other rectifier circuit does not output a DC voltage and is charged.

Images