JPH0541391U - Switching power supply soft-start circuit - Google Patents

Abstract

(57) [Summary] (Correction) [Purpose] It is an object of the present invention to provide circuit means for removing the overshoot voltage generated on the secondary DC output side at the time of starting the switching power supply. [Structure] A soft start circuit is configured by a diode, a capacitor, and a resistor added to the secondary side.

Description

[Detailed description of the device]

[0001]

[Explanation of the field to which the device belongs]

 The present invention relates to circuit means for preventing an overshoot voltage generated at an output end of a switching power supply when it is started.

[0002]

[Prior art and its problems]

 If the overshoot voltage is generated at the rise of the DC output voltage, the load may malfunction or the IC, which is the main load recently, may break over and be destroyed. Conventionally, it was necessary to use a switching power supply IC having a soft start circuit function in order to remove the overshoot voltage.

[0003]

[Purpose of the present invention]

 A switch that damages the current main load of switching power supplies such as ICs and LSIs. (2) Overshoot voltage at startup of the switching power supply is removed by several additional parts such as resistors, diodes and capacitors. Is its purpose.

[0004]

[Means of the Invention for Solving Problems]

 By adding a soft start circuit consisting of resistors, diodes, capacitors, etc. to the secondary side of the switching power supply.

[0005]

【Example】

 FIG. 1 shows an embodiment of the present invention. In FIG. 1, 1 is a transformer, 2 is a switching element, 3 is a primary side control circuit, 4 is a secondary side rectifying diode, 5 is a smoothing capacitor, 6 is a reference voltage element, and 7 and 8 are output voltages. Is a resistor for setting, 9 is a photocoupler, 9-1 is a photo diode which is an input part thereof, and 9-2 is a photo transistor which is an output part thereof. The constant voltage control of the conventional normal switching power supply is performed by each element from 1 to 9.

In the present invention, a soft start circuit is constructed by adding 10 diodes, 11 capacitors and 12 resistors to them.

In the operation, when the DC output voltage starts to rise on the secondary side when the switching power supply is activated, a charging current flows through the photo diode 9-1 and the diode 10 to the capacitor 11 to charge the capacitor. At this time, the light excited in the diode by the current flowing in the photodiode 9-1 is transmitted as a signal to the primary side phototransistor as if the output voltage exceeds a preset value (reference voltage). Since the start-up is started while limiting the switching frequency or conduction width of the primary side switching element 2, it is possible to prevent the generation of a sudden overshoot voltage. (3)

When the DC output voltage rises, the electronic comparison element 6 operates, a current is passed through the photodiode 9-1 so as to keep the output voltage value set in advance, and the operation shifts to a steady operation. Will not flow and a reverse voltage will be applied to the diode 10, and the capacitor 11 will be in an electrically disconnected state. The resistor 12 is necessary to discharge the charge stored in the capacitor 11 when the switching operation is stopped. This is because the effect of the soft start circuit of the present invention cannot be expected unless the capacitor 11 is in the discharged state when the switching power supply is turned on again.

[0009]

[Effect of the present invention]

 By adding the circuit of the present invention, the secondary side output voltage has a rising waveform with an overshoot as shown in FIG. As shown, it is improved to a smooth rising waveform.

[Brief description of drawings]

[Figure 1] Main circuit diagram of switching power supply

[Figure 2] DC output voltage rising waveform of switching power supply (conventional example)

[Figure 3] DC output voltage rising waveform of switching power supply (Example of the present invention)

[Explanation of symbols]

 VI input VO output 1 transformer 2 switch element 3 primary side control circuit 4 secondary side rectifying diode (4) 5 smoothing capacitor 6 reference voltage element 7 resistor 8 resistor 9 photo coupler 9-1 photo diode 9-2 Phototransistor 10 Diode 11 Capacitor 12 Resistance

Claims (1)

[Scope of utility model registration request] 1. In a switching power supply, when a preset value set by a voltage comparison element such as a shunt regulator is exceeded in order to maintain the secondary side DC output voltage at a constant voltage, a signal transmission such as a photo coupler is transmitted. In a switching power supply configured to control the amount of switching on the primary side via an element, the positive electrode of the input side terminal of the signal transmission element such as the photocoupler is connected to the positive electrode of the output end of the secondary side. A negative electrode of the signal transmission element is connected to an output terminal of the voltage comparison element and is also connected to one end of a capacitor through a diode, and the other end of the capacitor is a negative electrode of the secondary side DC output terminal. And a resistor is inserted between the connection point of the diode and the capacitor and the positive electrode of the secondary side output terminal. Soft start circuit.