JP2515394Y2 - Overcurrent protection circuit for switching power supply - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a protection circuit for detecting an overcurrent on the primary side of a switching power supply used for communication equipment or OA equipment.

The overcurrent detection circuit of the switching power supply with small output capacity is
The method of directly detecting the voltage of the current detection resistor inserted in the primary side with a transistor is common and inexpensive. This example is shown in FIGS. 1 and 2. FIG. 1 is a conventional circuit diagram, and FIG. 2 is its output characteristic diagram. In the figure, V ₁ is the input power supply, T
Is a voltage conversion transformer, D is a rectifying diode, R ₁ is a load, Q ₁ is a main switch, 1 is a current detection resistor, 2 is a transistor, COP is a comparator, OSC is a reference oscillator, R ₁ and R ₂ are output voltages. A detection resistor, Dp is a light emitting diode, and Tp is a phototransistor. In this circuit operation, the output voltage detection signal is input to the comparator COP via the transistor Tp to control the ON / OFF of the main switch Q ₁ to make the output voltage V ₀ constant via the transformer T and detect the current. The current flowing through the main switch Q ₁ is detected by the resistor 1, the collector side of the transistor 2 is used to change the detection side (+) potential of the comparator COP to control the main switch Q _1, and in FIG. The overcurrent drooping operation indicated by the point is reached. However, in this example, if an overcurrent is detected and the protection function operates, the output current I _o decreases as the output voltage decreases.
Shows an increasing (I _o ′) characteristic. Therefore, the current capacity of the rectifying diode D, etc. on the secondary side must be able to withstand the above-mentioned excessive current (in some cases, 3 to 5 times the rated output current) for a long time, and when considering the switching power supply as a whole, It cannot be cheap.

The present invention, when an overcurrent is detected and the protection function is activated,
The circuit is configured so that the detection current level on the primary side is automatically lowered so that the output current does not increase even if the output voltage decreases.
It is intended to provide a circuit in which the cost is reduced by downsizing the secondary side rectifying diode and the like, and the reliability of a load device is improved by preventing an excessive current from flowing. 3 and 4
The figure is a circuit diagram of one embodiment of the present invention and its output characteristic diagram.
The same reference numerals as those in the conventional example indicate the same parts.

In the present invention, in addition to the resistor 1 and the transistor 2 used in the conventional example, a second resistor 3 is provided between the base of the transistor 2 and the resistor 1 to provide a delay time when the transistor 1 detects an overcurrent. Resistor 4 and capacitor 5
And a current is supplied through the resistor 6 so that the base potential of the transistor 1 is DC-shifted by a constant potential (about 0.4 to 0.5 V) when the charging voltage of the capacitor 5 becomes a constant value. The second transistor 7 is provided. The diode 8 is arranged to prevent the charging circuit from being charged during normal operation. The charging circuit for the resistor 4 and the capacitor 5 is provided for the purpose of preventing the protection circuit from operating in the event of an instantaneous overcurrent, preventing malfunctions and ensuring a smooth rise of the load voltage at the time of startup. In the overcurrent protection operation of this circuit, when the overcurrent is once detected and the transistor 2 is turned on for a certain period of time, the transistor 7 is turned on and the transistor 2 is DC biased, so that the base potential is raised accordingly. Since the main switch current is about 1/3 to 1/4 of the detected current value at the start of the protection operation, the output voltage and current characteristics are drooping characteristics as shown in FIG. 4 point b), an excessive output current does not flow even if the output voltage drops. It should be noted that during the delay time in which the transistor 2 is turned on and the transistor 7 is turned off, the same curve of output voltage / current characteristics as in the conventional example is followed (shown by the dotted line in FIG. 4), but this is a very short time. Therefore, it is not necessary to consider the current capacity of the secondary side rectifying diode.

As described above, according to the present invention, the cost rating of the expensive secondary side output diode can be lowered by adding inexpensive parts, and at the same time, the output current flowing at the time of load abnormality can be reduced to 1/3 of that of the conventional circuit.
It was possible to reduce the power supply to about 1/4, and it was possible to improve the reliability of the switching power supply itself and the reliability of the equipment configured using it.

[Brief description of drawings]

1 and 2 are a conventional circuit diagram and its output characteristic diagram, and FIG.
FIG. 4 is a circuit diagram of an embodiment of the present invention and its output characteristic diagram. In the figure, V ₁ is an input power supply, V ₀ is an output voltage, Q ₁ is a main switch, T is a voltage conversion transformer, 1 is a current detection resistor,
2, 7 are transistors, 3, 4, 6 are resistors, and 5 is a capacitor.

Claims (1)

(57) [Scope of utility model registration request] 1. A main switch connected to a primary winding of a voltage conversion transformer, a load connected to a secondary winding side of the transformer via a rectifying diode, and a current flowing through the main switch is detected. In an overcurrent protection circuit for a switching power supply including an overcurrent protection circuit for controlling the main switch, the overcurrent protection circuit includes a current detection resistor connected in series with the main switch, and a first end across the current detection resistor. A first transistor having a base and an emitter connected through a resistor, a series circuit of a second resistor and a capacitor connected between the collector of the first transistor and an auxiliary power supply, and an emitter and a base at both ends of the capacitor. A second transistor having a collector connected to the base of the first transistor through a third resistor, Overcurrent protection circuit of the switching power supply being characterized in that so as to control the main switch utilized.