JPS6229991B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a switching regulator type power supply device, and even if the power supply voltage repeatedly turns on and off, a large current will not flow through the switching transistor and the transistor will not be destroyed. This is what we are trying to do.

An embodiment of the present invention will be described below in relation to a conventional device. As shown in Figure 1, AC power supply 1
A rectifying and smoothing circuit is connected to the power switch 2 through the power switch 2. 3 is a full-wave rectifier circuit, and 4 is a smoothing capacitor. The output obtained from this smoothing circuit is applied to a series circuit of the primary winding of the transformer 5 and the switching transistor 6. A rectifying and smoothing circuit is connected to the secondary winding of the transformer 5. 7 is a rectifying diode, and 8 is a smoothing capacitor. The voltage obtained by dividing the DC voltage obtained across this smoothing capacitor is applied to terminal 9 and a reference voltage is applied to terminal 10, and the error detection circuit 11 compares the divided voltage, and generates an output obtained by subtracting the voltage at terminal 9 from the voltage at terminal 10, for example. . This output is pulse width modulation circuit 1
2 to control the width of the resulting pulse. It is set so that the larger the output voltage applied to the pulse width modulation circuit 12, the larger the pulse width of the output pulse. A triangular wave is applied to terminal 13 to create a pulse. The obtained pulse is applied to the base of the transistor 6 via the drive circuit 14. When the output voltage, which is the voltage of capacitor 8, increases, the voltage at terminal 9 increases. Then, the output of the error detection circuit 11 becomes smaller and the pulse width becomes smaller. As a result, the conduction time of the transistor 6 becomes shorter, and the output voltage becomes smaller and stabilized. When the voltage at terminal 9 becomes small, the opposite operation occurs and stabilization is achieved in the same way. However, when the power switch 2 is turned on, the voltage at the terminal 9 is 0, so the output voltage of the error detection circuit 11 is large, and the pulse width modulation circuit 12
The pulse width of the output pulse becomes extremely large.
As a result, a large current flows through the transistor 6, which may destroy the transistor 6. Therefore, a soft start circuit 16 is provided so that the voltage slowly charged to the capacitor 15 when the power is turned on is applied to the pulse width modulation circuit 12 instead of the output of the error detection circuit 11. This soft start circuit 16 can basically be constructed from a switching circuit. When power switch 2 is turned on,
The capacitor 15 is slowly charged from the DC power source via the resistor. Since this voltage is initially smaller than the output voltage of the error detection circuit 11, this small voltage is applied to the pulse width modulation circuit 12. As time passes, the output voltage of the error detection circuit 11 becomes smaller, so eventually the error detection circuit 11
The output voltage of is now applied to the pulse width modulation circuit 12. However, when the power switch 2 is turned on and then turned off, the voltage across the capacitors 4 and 8 increases relatively quickly, but the voltage across the capacitor 15 does not disappear easily. Unless the time constant provided by the capacitor 15 is made quite large, it will no longer function as a soft start operation. If the power switch 2 is turned on again before the voltage of the capacitor 15 has completely decreased, a large voltage will be applied to the pulse width modulation circuit 12, leading to destruction of the transistor 6.

The present invention attempts to prevent the transistor 6 from being destroyed even in this case. For this purpose, a switch 17 is connected in parallel to the capacitor 15, and the switch 17 is controlled on/off by the output of the detection circuit 18. The input of the detection circuit 18 is the voltage across a resistor 19 connected in series to the smoothing capacitor 4.

From power switch 2 on state, power switch 2
Suppose you cut it off and then put it back in immediately. At this time, since the charge in the capacitor 4 has been discharged quickly, a transient current flows through the capacitor 4 and the resistor 19, and the voltage across the resistor 19 during this transient period is large.
This voltage is detected by the detection circuit 18, and the detection output closes the switch 17 for a short time to instantly discharge the charge in the capacitor 15. As a result, a large current will not flow through the transistor 6.

Figures 2 and 3 specifically show the detection circuit, and Figure 2 shows a transistor 20 and a resistor 2.
1 and 22 constitute a detection circuit 18 and a switch 17, and in FIG. 3, a Zener diode 23 and a resistor 24 constitute a detection circuit 18 and a switch 17.

As described above, according to the present invention, it is possible to prevent a large current from flowing through the switching transistor even when the power switch is turned off and then immediately turned on again when the power switch is on, and the switching transistor can be protected. can.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a power supply device according to an embodiment of the present invention, and FIGS. 2 and 3 are circuit diagrams each showing a part of the same device as a specific circuit. DESCRIPTION OF SYMBOLS 1... AC power supply, 2... Power switch, 3... Rectifier circuit, 4... Smoothing circuit, 5... Transformer, 6... Switching transistor, 7... Rectifier diode, 8...
Smoothing capacitor, 11...Error detector, 12...Pulse width modulation circuit, 16...Soft start circuit, 18
...Detection circuit, 15...Capacitor, 17...Switch, 19...Resistor.

Claims (1)

[Claims] 1 Connect the AC power source to a rectifying and smoothing circuit via a power switch, adding the output of this rectifying and smoothing circuit to the series circuit of the primary winding of the transformer and the switching transistor, and applying the rectifying and smoothing circuit to the secondary winding of the transformer. An error detection circuit is provided to detect the difference between the output of this rectifying and smoothing circuit and a reference voltage, and the pulse width modulation circuit is controlled by the output voltage of this error detection circuit to adjust the width according to this output voltage. The device is configured to generate a pulse voltage of , and control the on/off of the switching transistor by this pulse voltage, and is provided with a capacitor that starts charging when the power supply is switched on, and the error is maintained until the voltage of this capacitor reaches a certain value. a soft start circuit that supplies the voltage of the capacitor to the pulse width modulation circuit instead of the output voltage of the detection circuit, and a resistor connected in series to the smoothing capacitor of the rectification and smoothing circuit that rectifies and smoothes the output of the AC power supply; A power supply device characterized in that a voltage across this resistor is detected, and a switch connected in parallel to the capacitor is turned on when the voltage across this resistor exceeds a predetermined value.