JPH079040U - Transistor switch circuit - Google Patents

Abstract

(57) [Summary] (Modified) [Objective] To provide a circuit having a simple structure for preventing a transistor from being destroyed when an overcurrent flows in a transistor switch circuit for supplying power to a load. [Structure] A first transistor Q1 for controlling power supply to a load 4 is provided between a power supply terminal 1 and a common terminal 2, a control signal is input from a terminal 3, and an on / off operation of a transistor Q1 is performed. Second transistor Q for controlling
2, an overcurrent protection circuit 5 is provided to disable the second transistor Q2 when the load 4 is short-circuited, and an overcurrent exceeding a predetermined value is provided between the power supply terminal 1 and the transistor Q1. Second transistor Q when
An overcurrent detection circuit for disabling No. 2 is provided.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to a transistor switch circuit, and more particularly to a transistor switch circuit using a transistor for controlling on / off of power supply in a communication device or an electric device such as OA and AV.

[0002]

[Prior art]

 Generally, in a communication device or an electric device, a switch circuit including a transistor is used to supply or disconnect a power source to a load. However, in a normal transistor switch circuit, if the load side is short-circuited for some reason while supplying power to the load, or if an overcurrent is supplied to the power supply side for some reason, There is a risk that the semiconductor element used, that is, the switching transistor, will be destroyed and a large current will flow, leading to fire and smoke accidents. Therefore, some measure for preventing the destruction of the transistor is taken.

FIG. 3 shows a conventional transistor switch circuit in which such a destruction prevention measure is adopted. 1 is a power supply terminal (Vin), 2 is a common terminal (GND), and 3 is a control signal input terminal (ON). / OFF), 4 is a load. Q1 is a PNP transistor, Q2 is an NPN transistor, and R1 to R4 are resistors.

The emitter of the transistor Q1 in FIG. 3 is connected to the power supply terminal 1, its collector is connected to the common terminal 2 via the load 4, and the resistor R1 is connected between the base and emitter of the transistor Q1. It On the other hand, the base of the transistor Q2 is connected to the control signal input terminal 3 via the resistor R3, its emitter is connected to the common terminal 2, and the resistor R4 is connected between the base and emitter of the transistor Q2. Further, the base of the transistor Q1 and the collector of the transistor Q2 are coupled via a resistor R2.

In such a conventional transistor switch circuit, in the state where the power supply voltage Va is applied between the power supply terminal 1 and the common terminal 2, the control signal input terminal 3 is fed from a signal source (not shown). When a positive one-shot signal (set pulse) is input, the transistor Q2, which is a switching control element, turns on, and accordingly, the base of the transistor Q1, which is a power supply control element, passes through the gate protection resistor R2. It is configured to have a low potential and be turned on to supply the power supply current If to the load 4 from the power supply terminal 1. Further, when the negative one-shot signal (reset pulse) is input, the transistor Q2 is turned off, so that the transistor Q1 is also turned off and the supply of the power supply current If to the load 4 is cut off. In this way, the power supply control operation corresponding to the purpose is performed by turning on / off the transistors Q1 and Q2 according to the set / reset signal from the signal source to supply or cut off the power supply current. Is.

[0006]

[Problems to be solved by the device]

 However, such a conventional transistor switch circuit has the following problems. That is, in the same transistor switch circuit, the PNP transistor Q1 is driven to be turned on and off to perform the power supply operation. However, while the power is being supplied to the load 4, for example, an abnormal state, that is, the load 4 is indicated by a dotted line A. In the case where the transistor Q2 is in the short-circuited state, the transistor Q2 is in the on-state, and the transistor Q1 is also in the on-state. Therefore, a large current flows through the emitter / collector of the transistor Q1. There was a drawback that it exceeded the limit and eventually led to a major accident such as smoke and ignition. Also, if an overcurrent is supplied for some reason on the power supply side, a similar accident may occur. An object of the present invention is to eliminate the above-mentioned conventional problems and to provide a transistor switch circuit capable of preventing an accident due to a load short circuit or an overcurrent with a simple circuit configuration.

[0007]

[Means for Solving the Problems]

 To achieve this object, the present invention provides a load and a first transistor circuit for controlling power supply to the load between a power supply terminal and a common terminal, and a control signal input terminal for controlling the load. A transistor switch circuit having an input terminal connected thereto and a second transistor circuit for controlling on / off operation of the first transistor circuit, comprising: a connection point between the load and the first transistor circuit. A transistor switch circuit provided between the control input terminal of the second transistor circuit and an overcurrent protection circuit for disabling the second transistor circuit when the load is short-circuited; To disable the second transistor when an overcurrent exceeding a predetermined supply current is detected between the power supply terminal and the first transistor We propose a transistor switch circuit with an overcurrent detection circuit.

[0008]

【Example】

 Hereinafter, the embodiment of the present invention will be described in detail with reference to FIGS. 1 is a circuit configuration diagram of a transistor switch circuit according to a first embodiment of the present invention. The same reference numerals as those in FIG. 3 denote the same components, C1 is a capacitor, D1 is a discharging diode, and R5 is a resistor. , Which form the overcurrent protection circuit 5. The emitter of the transistor Q1 in FIG. 1 is connected to the power supply terminal 1, its collector is connected to the common terminal 2 via the load 4, and the resistor R1 is connected between the base and emitter of the transistor Q1. On the other hand, the base of the transistor Q2 is connected to the control signal input terminal 3 via the resistor R3 and the capacitor C1, its emitter is connected to the common terminal 2, and the resistor R4 is connected between the base and emitter of the transistor Q2. Has been done. Further, the base of the transistor Q1 and the collector of the transistor Q2 are coupled via a resistor R2. Further, a diode D1 having a cathode on the side of the control signal input terminal 3 and an anode on the side of the resistor R3 is connected in parallel with the capacitor C1, and the anode of the diode D1 is connected to the collector of the transistor Q1 via the resistor R5. Has been.

Therefore, in the transistor switch circuit of FIG. 1, with the power supply voltage Va applied between the power supply terminal 1 and the common terminal 2, the capacitor C1 is connected to the control signal input terminal 3 from a signal source (not shown). When a one-shot signal (set pulse) is input via the transistor Q2, which is a switching control element, the transistor Q2, which is a switching control element, is turned on, and accordingly, the base of the transistor Q1 which is a power supply control element is at a low potential via the gate protection resistor R2. Then, the power supply terminal 1 is turned on, and the power supply current If is supplied from the power supply terminal 1 to the load 4. At the same time, since the power supply current is supplied from the collector of the transistor Q1 to the base of the transistor Q2 via the resistor R5, the transistor Q1 continuously maintains the ON state. When a negative one-shot signal (reset pulse) is input, the transistor Q2 turns off, so the transistor Q1 also turns off and the supply of the power supply current If to the load 4 is cut off. In this way, the power supply control operation corresponding to the purpose and function is performed by turning on / off the transistors Q1 and Q2 in accordance with the set / reset signal from the signal source to supply or cut off the power supply current. Done.

Next, to describe the overcurrent protection operation, even in the transistor switch circuit of FIG. 1, the PNP transistor Q1 is driven on / off to perform the power supply operation, but the power is supplied to the load 4. If, for example, an abnormal state, that is, the load 4 is short-circuited (short-circuited) as shown by the dotted line A, the normal state, that is, the transistor Q2 is kept on and the transistor Q1 is kept on. At the same time that the power supply current is drawn into the common terminal 2 due to the short-circuiting of the load 5, the transistor is routed through the route of the overcurrent protection circuit 5 including the resistor R5 and the parallel circuit of the capacitor C1 and the discharging diode D1. The base of Q2 becomes low potential. As a result, there is no current source that keeps the transistor Q2 on, so the transistor Q2 turns off and at the same time the transistor Q1 turns off. Therefore, a large current does not flow in the emitter / collector of the transistor Q1, and it is possible to prevent a major accident such as smoking or ignition due to destruction due to the power supply current exceeding the rated power of the transistor Q1.

FIG. 2 shows a transistor switch circuit according to a second embodiment of the present invention, and the same parts as those in FIGS. 1 and 3 are designated by the same reference numerals. Reference numeral 6 is an overcurrent detection circuit, which includes a PNP transistor Q3, an NPN transistor Q4, a capacitor C2, and resistors R6, R7, and R8. The difference from the embodiment of FIG. 1 is that an overcurrent detection circuit 6 is added. That is, in this embodiment, a parallel circuit of the overcurrent detection resistor R6 and the capacitor C2 is inserted and connected between the power supply terminal 1 and the emitter of the transistor Q1, and one end of the resistor R6 on the power supply terminal 1 side is connected. The emitter of the transistor Q3 is connected, and the other end thereof is connected to the base via the resistor R7. The collector of the transistor Q3 is connected to the base of the transistor Q4 via the resistor R8, the emitter is connected to the common terminal 2, and the collector is connected to the connection point between the capacitor C1 and the resistor R4.

Therefore, in the transistor switch circuit according to the second embodiment, when a current that exceeds a predetermined current flows through the resistor R6, the transistor Q3 detects and outputs a detection output. Since the output is supplied to the base of the transistor Q4, the transistor Q4 turns on, the base potential of the transistor Q2 becomes low, the transistor Q2 turns off, and the transistor Q1 also turns off. To be prevented. In addition, in the embodiment of FIG. 1, when the load 4 is short-circuited, the transistor Q2 is turned off to prevent the transistor Q1 from being destroyed. However, in the embodiment of FIG. When an overcurrent is supplied to the input side of Q1, that is, the power supply terminal 1 side, the overcurrent detection circuit 6 detects it and prevents the transistor Q1 from being destroyed.

[0013]

[Effect of device]

 As is clear from the above description, the present invention has a simple configuration in which the overcurrent protection circuit is arranged between the collector of the first transistor and the base of the second transistor, and the load is short-circuited. Even in this case, the transistor for supplying the power supply current can be prevented from being destroyed, which has a practical advantage of preventing accidents such as smoking and ignition due to the destruction. In addition, by providing an overcurrent detection circuit that detects an overcurrent on the power supply side, it is possible to prevent the breakdown of the transistor in advance, so the practical advantage becomes even greater.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram of a transistor switch circuit according to a first embodiment of the present invention.

FIG. 2 is a circuit configuration diagram of a transistor switch circuit according to a second embodiment of the present invention.

FIG. 3 is a circuit configuration diagram of a conventional transistor switch circuit.

[Explanation of symbols]

 1 power supply terminal (Vin) 2 common terminal (GND) 3 control signal input terminal (ON / OFF) 4 load (Vs) 5 overcurrent protection circuit 6 overcurrent detection circuit Q1, Q3 PNP transistor Q2, Q4 NPN transistor R1 R8 resistance D1 diode C1, C2 capacitor

Claims (2)

[Scope of utility model registration request] 1. A load and a first transistor circuit for controlling power supply to the load are provided between a power supply terminal and a common terminal, and a control input terminal is connected to a control signal input terminal, A transistor switch circuit comprising a second transistor circuit for controlling on / off operation of the first transistor circuit, the connection point between the load and the first transistor circuit, and the second transistor circuit. And a control input terminal of the transistor switch circuit, wherein an overcurrent protection circuit is provided for disabling the second transistor circuit when the load is short-circuited. 2. A load and a first transistor circuit for controlling power supply to the load are provided between the power supply terminal and the common terminal, and a control input terminal is connected to the control signal input terminal. And a second transistor circuit for controlling on / off operation of the first transistor circuit, comprising: a connection point between the load and the first transistor circuit; and the second transistor. An overcurrent protection circuit is provided between the control input terminal of the circuit and the control input terminal to disable the second transistor circuit when the load is short-circuited, and a power supply terminal and the first transistor are connected. Between,
A transistor switch circuit comprising an overcurrent detection circuit for disabling the second transistor when an overcurrent exceeding a predetermined supply current is detected.