JPH0785647B2 - Inrush current prevention circuit for switching power supply - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a closing current prevention circuit for a switching power supply, and more particularly to a control method (circuit method) for controlling an inrush current.

2. Description of the Related Art Conventionally, a rush current prevention circuit for a switching power supply has a circuit configuration as shown in FIG. That is, in FIG. 4, an AC full-wave rectified waveform is input to the input power source in, and first,
Electric charges are accumulated in the capacitor C1 through the resistor R1 and the transistor Q starts to turn “ON / OFF” when the voltage of the capacitor C1 becomes a certain voltage. "Turn on this transistor,
When "OFF", a voltage is generated in the temporary winding L of the timer output circuit, and the timer circuit operates. Since the gate signal of the thyristor S'is generated by this timer circuit output, the thyristor S'is turned "on" and the capacitor is turned on. Charging starts.

Problems to be Solved by the Invention The conventional inrush current prevention circuit for the switching power supply described above controls the "on" of the thyristor S'by the output of the timer circuit based on the charging voltage of the capacitor C1. Thyristor S'begins "on", regardless of.

Therefore, when the thyristor S'is turned on when the phase angle is relatively small, an excessive current flows, the thyristor S'is destroyed, noise is generated due to a rapid rise of the current, and the life of the capacitor C is shortened. There is a drawback that

The present invention has been made in view of the above-mentioned conventional circumstances,
Therefore, it is an object of the present invention to provide a novel inrush current prevention circuit for a switching power supply, which can eliminate the above-mentioned drawbacks inherent in the prior art.

Differences from the Prior Art of the Invention In contrast to the inrush current prevention circuit of the conventional switching power supply described above, the present invention charges the primary side smoothing capacitor through the current limiting resistor and the second transistor drive resistor until the timer circuit output is output. After the timer circuit output is performed, the primary side smoothing capacitor is charged by the above-mentioned two resistors and the first transistor. At this time, the first transistor is turned “on” by [0 <( Input voltage)-
The difference is that the relationship of (charging voltage) <(specified voltage)] is satisfied only for a limited time.

Means for Solving the Problems In order to achieve the above object, the inrush current prevention circuit for a switching power supply according to the present invention turns on only under a certain condition of a current limiting resistor and a second transistor drive resistor. Timer output consisting of 1 transistor, 2nd transistor turned on by 2nd transistor drive resistor, primary side smoothing capacitor, primary side winding, primary side winding control transistor, diode, capacitor and resistor And a circuit.

EXAMPLES Next, the present invention will be specifically described with reference to the drawings for each of its preferable examples.

FIG. 1 is a circuit configuration diagram showing an embodiment of the present invention, and FIG. 2 is an operation timing chart of the circuit configuration shown in FIG.

Referring to FIG. 1, R1 to R3 are resistors, C1 and C2 are capacitors, Q1 to Q3 are transistors, and D is a diode.

First, when the input voltage is applied to point B, current limiting resistor R2,
Charging of the capacitor C1 starts through the transistor driving resistor R3, and the voltage at the point A rises. This charging current generates a voltage in the transistor driving resistor R3,
Q2 does "on, off". When the capacitor C1 exceeds a certain voltage, the transistor Q1 starts “on / off”, so that a voltage is generated in the capacitor C2 and the timer output circuit operates. This timer output circuit allows
Q3 also tries to "turn on", but since the base current is bypassed by the transistor Q2, the transistor Q2 and the transistor Q2 are inverted. Since the transistor Q2 is "off" when the charging current is small (the charging voltage is small), when the point B voltage-the point A voltage (point B voltage> point A voltage) <specified voltage (determined by the resistor R3). Only the transistor Q3 turns on. At this time, the charging current is limited by the h _FE of the transistor Q3.

FIG. 3 is a circuit configuration diagram showing a second embodiment according to the present invention.

Referring to FIG. 3, the basic part of this second embodiment is:
Although the same as the first embodiment, a voltage detecting section including a resistor 4, a Zener diode D2, and a photocoupler P is further provided in parallel with the capacitor C1, and a resistor 5, a thyristor S, and a photocoupler P are provided according to the detection signal. The switch circuit of 2 is closed.

In the second embodiment, when the voltage of the capacitor C1 becomes a certain voltage or more, the photocoupler P of the voltage detection section operates, the gate signal of the thyristor S is supplied, and the second switch circuit is turned on. The advantage is that the time to reach the charging voltage can be shortened.

As described above, according to the present invention, the timer output circuit charges the primary side smoothing capacitor through the current limiting resistor and the second transistor driving resistor until the timer output circuit operates. After the operation, the primary side smoothing capacitor is charged by the two resistors and the first transistor (transistor Q3). At this time, the first transistor turns on only when the relation of [0 <input voltage-charging voltage <specified voltage] is satisfied, and the current can be limited, so that the inrush current of the primary side smoothing capacitor can be prevented. In addition, the phase angle of the input current can be limited.

Further, according to the present invention, compared with the conventional circuit, the rapid rise of the current is suppressed, so that it is possible to obtain the effects of reducing the noise and extending the life of the capacitor.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram showing a first embodiment of a rush current prevention circuit for a switching power supply according to the present invention, FIG. 2 is an operation timing chart of the first embodiment, and FIG. 3 is a switching timing according to the present invention. FIG. 4 is a circuit configuration diagram showing a second embodiment of an inrush current prevention circuit for a power supply, and FIG. 4 is a conventional inrush current prevention circuit for a switching power supply. C1 …… Smoothing capacitor on the primary side of the timer output circuit, C2 ……
Output smoothing capacitor of timer output circuit, Q1 …… Primary winding drive transistor of timer output circuit, Q2 …… First transistor of switch circuit Inversion transistor, Q3 …… First transistor of switch circuit, R1 …… Timer Output limiting resistance of output circuit, R2 ... Current limiting resistance of primary smoothing capacitor of switch circuit, R3 ... Second transistor driving resistance of switch circuit, L ... Timer primary winding of output circuit, D ... Timer Rectifier diode of output circuit, R4 ... current limiting resistor of voltage detecting unit, R5 ... thyristor gate current limiting resistor of second switch circuit, D2 ... constant voltage diode of voltage detecting unit, P ... voltage detecting unit Photo coupler for optically coupling the second switch circuit, S ... Second switch circuit thyristor, S '... Thyristor

Claims (1)

[Claims] 1. In a switching power supply, a collector terminal of a first transistor is connected to the diode bridge + output between a primary side diode bridge + output and a primary side smoothing capacitor + terminal, and an emitter terminal of the first transistor. Is connected to the primary side smoothing capacitor + terminal, the base terminal of the first transistor is connected to a timer output circuit, and a current limiting resistor and a second terminal are provided in parallel with the collector terminal and the emitter terminal of the first transistor.
Connected in series with the transistor drive resistor of, the current limiting resistor is connected as the primary side diode bridge side,
The collector terminal of the second transistor is connected to the base terminal of the first transistor, the emitter terminal of the second transistor is connected to the primary side smoothing capacitor + terminal, and the base terminal of the second transistor is connected to the base terminal of the second transistor. A rush current prevention circuit for a switching power supply, comprising a switch circuit connected to a midpoint between a current limiting resistance and the second transistor driving resistance.