JP2599345Y2 - Soft start circuit - Google Patents

Description

[Detailed description of the invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a soft start circuit for a switching power supply.

[0002]

2. Description of the Related Art Conventionally, a method for controlling a dead time has been used as a soft start circuit of a switching power supply.

[0003]

The dead time control, which is a conventional method, is effective in a control circuit having an independent oscillator such as a pulse width control method, but cannot be used in a self-excited switching power supply.

[0004]

As shown in the circuit of FIG. 1, an embodiment of the present invention is a capacitor of a circuit for rectifying and smoothing an AC voltage generated in a winding 12 for supplying a bias current to a base of a switching transistor 11. The voltage charged to the capacitor 19 after being rectified and smoothed by appropriately selecting the values of the resistor 19 and the resistor 18 can be raised with a desired delay time.

A capacitor 14 connected between the base and the emitter of an NPN transistor 13 for controlling a forward bias current is charged and discharged by a current flowing through a series circuit including a resistor 15 and a PNP transistor 16. The earlier the charge, the shorter the time the transistor 13 is turned on, and the shorter the on period of the switching transistor 11.

[0006]

The capacitor 19 used in the rectifying and smoothing circuit
As the charging of PNP rises, the PNP transistor 16
The current flowing from the collector to the base increases, and the amount of charge flowing through the capacitor 14 during the reverse bias period increases. As a result, the time required to charge the capacitor 14 to the forward voltage between the base and the emitter of the NPN transistor 13 during the forward bias period is increased. become longer.

Since the PNP transistor 16 has characteristics as a reverse transistor, during the reverse bias period,
A current corresponding to the current amplification factor of the reverse transistor with respect to the current flowing from the collector to the base flows from the collector to the emitter. In other words, the PNP transistor 16 functions as a transistor for both currents for charging and discharging the capacitor 14.

The current for charging the capacitor 14 is:
Since it operates according to the intrinsic amplification factor of the transistor, it does not depend much on the voltage of the smoothing capacitor 19 and is constant at a value limited by the resistor 15, but the current when discharging increases as the voltage of the smoothing capacitor 19 rises. Therefore, the ON period of the switching transistor 11 is slowly increased by the delay time given by the time constant of the smoothing capacitor 19 and the resistor 18.

[0009]

FIG. 1 shows an embodiment of the present invention.

FIG. 2 shows another embodiment of the present invention, in which a MOSFET is applied to the switching transistor 11.

In FIG. 1, when the switching transistor 11 starts oscillating, the voltage of the capacitor 19 rises with a delay time corresponding to a time constant determined by its own capacitance and the value of the resistor 18.

In the embodiment shown in FIG. 1, the capacitor 19 is charged with a negative voltage.
The current flowing from the collector to the base of the transistor 16 increases, and therefore, the current flowing from the collector to the emitter also increases. The current flowing from the collector to the emitter of the PNP transistor 16 flows when a reverse bias voltage is generated in the winding 12, that is, when the switching transistor 11 is in the off period.

On the other hand, when a forward bias voltage is generated in the winding 12, a current flows from the emitter of the transistor 16 to the collector. The value of the current depends on the forward bias voltage of the winding 12 and the resistance 15. And is not greatly affected by the voltage of the capacitor 19.

Therefore, the current for charging the capacitor 14 is substantially constant, and the current for discharging increases as the voltage of the capacitor 19 rises. Therefore, the on-period of the switching transistor is a small value at the start of oscillation and gradually increases. Soft start characteristics can be obtained.

[0015]

According to the present invention, it is possible to easily adjust the delay time of the rise in the self-excited switching power supply, which has been difficult in the past.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram showing another embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Switching transistor 12 Winding 13 NPN transistor 14 Capacitor 15 Resistance 16 PNP transistor 17 Diode 18 Resistance 19 Capacitor 20 Resistance 30 Switching transformer 101 Capacitor 102 Resistance 103 DC power supply 104 Winding 105 Winding 106 Diode 107 Capacitor 108 Load

Claims (1)

(57) [Scope of request for utility model registration] 1. A switching transistor, a winding of a switching transformer for supplying a bias current to the switching transistor, a transistor for controlling a forward bias current supplied to the switching transistor, and controlling the forward bias current A capacitor connected between the base and the emitter of the transistor, a series circuit including a transistor and a resistor connected in series between the capacitor and the winding, and a diode for rectifying and smoothing a reverse bias voltage of the winding And a series circuit consisting of a resistor and a capacitor, the base of the transistor of the series circuit consisting of the transistor and the resistor, the diode for rectifying and smoothing the reverse bias voltage, and the reverse bias voltage of the capacitor of the series circuit consisting of the resistor and the capacitor Between the terminal to be charged A soft-start circuit comprising a connected resistor, whereby the on-duty of the switching transistor rises slowly at the time of startup.