JPH05236644A - Overcurrent protecting circuit - Google Patents

Abstract

PURPOSE:To eliminate the necessity of increasing power capacity of a resistor, to reduce a component mounting area and to reduce a cost by connecting a collector of a switching transistor to a base of a control transistor, an emitter to the other side of a power source and a base to an emitter of the control transistor through a resistor. CONSTITUTION:If a normal current flows to a main transistor 1, values of a resistor 4 and a base resistor 8 of a switching transistor 7 are set so as to turn OFF the transistor 7. Accordingly, when an overcurrent flows to an emitter of the transistor 1, a base current of the transistor 1 is increased. Thus, the transistor 7 is turned ON, a control transistor 3 is turned OFF, the base current of the transistor 1 is cut OFF, and an overcurrent from the transistor 1 to a load 9 is cut OFF. Thus, a component mounting area can be reduced without increasing power capacity of the resistor 4, and its cost can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overcurrent protection circuit used in power supply devices for various electronic devices.

[0002]

2. Description of the Related Art A conventional overcurrent protection circuit is constructed as shown in FIG. That is, the collector of the control transistor 23 that controls the base current of the transistor 21 with a constant current is connected to the base of the main transistor 21 that controls the output load current, and the emitter of this transistor 23 is used to determine the constant current value. A bias resistor 2 is connected to the base of the control transistor 23 while connecting the resistor 24.
5, 26 are connected, and the load 9 is connected to the output side. The resistor 22 is the control transistor 2
This is for making the base voltage of the main transistor 21 equal to the emitter voltage and keeping the main transistor 21 completely off in a state where 3 is not operating.

[0003]

In the above conventional structure, the base current of the transistor 21 increases due to the overcurrent flowing into the emitter of the transistor 21, and the base current flows into the collector of the control transistor 23. Therefore, it is necessary to increase the power capacity of the control transistor 23. Further, when the current capacity of the control transistor 23 increases, the overcurrent also flows through the emitter resistor 24, so the power capacity of the resistor 24 must be increased. Therefore, there is a problem that the mounting area of the printed circuit board and the like increases and the cost of the parts becomes expensive.

[0004]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention connects a collector of a first transistor to the base of a main PNP transistor which controls an output load current, and an emitter to the emitter of the first transistor. A resistor is connected, the base of the first transistor is connected to the collector of the second transistor, and the base resistor of the second transistor is connected to the emitter of the first transistor.

[0005]

With the above-described structure, when an overcurrent flows through the emitter of the main transistor, the base current of the main transistor increases, which turns on the second transistor and turns off the first transistor. Is cut off, and the overcurrent from the main transistor to the load is cut off.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an overcurrent protection circuit according to the present invention will be described in detail below with reference to the drawings. FIG. 1 is a circuit diagram of an overcurrent protection circuit according to the present invention. In the figure, 1, 3 and 7 are transistors, and transistor 1 is a main PNP transistor that supplies a current to a load. The emitter is connected to the input side, the collector is connected to the output terminal 10, and the base is connected to the base current. It is connected to the collector of the control transistor 3 which is controlled by a constant current. A resistor 4 that determines a constant current and a switching transistor 7 that turns on and off the control transistor 3 are connected to the emitter of the control transistor 3, and the operating point thereof is a bias resistor 5.
A base resistor 8 for determining an on / off operating point is connected to the base of the transistor 7, which is determined by 6, and a load 9 is connected to the output terminals 10 and 11.

With such a configuration, the value of the resistor 4 and the transistor 7 are set so that the transistor 7 is turned off when a normal current flows through the main PNP transistor.
Determine the value of base resistance 8 of. In this case, the value of the load current immediately before the overcurrent starts to flow is Ic. Ic / (hf
Since the current e) flows into the collector of the transistor 3, setting the DC operating point of the transistor 3 determines the value of the resistor 4. Next, assuming that the value of the load current when the overcurrent starts to flow is Id, the current of Id / (hfe) flows into the collector of the transistor 3 and the emitter voltage of the transistor 3 rises. Therefore, the base voltage of the transistor 7 increases. Rises, the transistor 7 turns on, the transistor 3 turns off, current is prevented from flowing to the base of the main transistor 1, and a protection operation is performed to prevent an overcurrent from flowing to the load.

[0008]

As described above, according to the present invention, the protection circuit can be constructed only by connecting the collector of the switching transistor to the base of the control transistor, and it is not necessary to increase the power capacity of the resistor and the component mounting area can be reduced. It is possible and advantageous in terms of cost, and has great practical value.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of an overcurrent protection circuit according to the present invention.

FIG. 2 is a circuit diagram showing an example of a conventional overcurrent protection circuit.

[Explanation of symbols]

 1 Main Transistor 2 Bias Resistor 3 Control Transistor 4 Emitter Resistor 5 Bias Resistor 6 Bias Resistor 7 Switching Transistor 8 Base Resistor 9 Load 10 Output Terminal 11 Output Terminal 21 Main Transistor 22 Bias Resistor 23 Control Transistor 24 Emitter Resistor 25 Bias Resistor 26 Bias resistance

Claims (1)

[Claims] 1. A collector is connected to the base of a power transistor for controlling a load current in which one of a collector and an emitter is connected to the power supply side and the other is connected to the load side, and the emitter is connected to the other side of the power supply through a resistor. In a power supply protection circuit comprising a control transistor having a bias circuit connected to the base, a collector is connected to the base of the control transistor, an emitter is connected to the other side of the power supply, and a base is connected to the emitter of the control transistor through a resistor. Overcurrent protection circuit composed of a transistor.