JP3111616B2 - Input short-circuit protection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an input short-circuit protection circuit for a power supply.

[0002]

2. Description of the Related Art In recent years, when a large load current is driven in an output circuit, a bipolar transistor of an emitter follower type, a collector output type, a push-pull type, or the like is often used. Emphasis is also placed on protection during operation.

The case where a conventional protection diode is used will be described below. FIG. 2 shows a case where a conventional input short-circuit protection diode D1 is used. In FIG. 2, Q1 and Q2 are pre-driver transistors, Q3 is an output transistor,
R1 to R5 are resistors, C1 is an output capacitor, and D1 is an input short-circuit protection diode.

The operation of the input short-circuit protection diode configured as described above will be described below.

FIG. 2 shows an example of a constant voltage circuit. When the input is short-circuited to ground while the output is stabilized,
First, considering a state where there is no input short-circuit protection diode D1, when the emitter of the transistor Q3 is grounded, the charge charged in the capacitor C1 is discharged, and the collector of the transistor Q3 goes from the emitter to the base, that is, from the output to the input. A large current flows, and depending on the capacitance value of the capacitor C1, the transistor Q3 may be broken. In order to prevent this, the transistor Q
3, a large current is bypassed and protected by adding an input short-circuit protection diode D1 having an anode-cathode voltage smaller than the base-emitter voltage to turn on.

[0006]

However, in the above-mentioned conventional configuration, protection when the input is grounded is provided by an external input short-circuit protection diode, and external components are required together with the constant voltage circuit. Further, there is a problem that the output voltage drops due to the discharge of the output capacitor.

SUMMARY OF THE INVENTION The present invention solves the above-mentioned conventional problems and provides an input short-circuit protection circuit which functions as a protection when an input is short-circuited without using an external input short-circuit protection diode and can keep an output voltage. The purpose is to provide.

[0008]

In order to achieve the above object, an input short-circuit protection circuit according to the present invention comprises a ground terminal and a power supply.
Input terminal and output terminal to which a capacitive load is connected
And an emitter connected to the input terminal and a connector connected to the output terminal.
A first PNP transistor to which a collector is connected;
One end of which is connected to the base of the first PNP transistor
A resistor and a collector connected to the other end of the first resistor.
A second PNP transformer having an emitter connected to the input terminal;
A transistor and a base connected to the second PNP transistor.
NPN with a collector connected and a base connected to the output terminal
A transistor; and an emitter of the NPN transistor.
A second resistor inserted and connected between the ground terminals.
When the input terminal and the ground terminal are short-circuited,
It has a configuration for interrupting the operation of the two PNP transistors .

[0009]

With this configuration, when the input is short-circuited to the ground, the NPN transistor is switched to the second PNP transistor.
By injecting current into the base of this transistor
The operation of the first PNP transistor
Cut off the base current flowing to the terminal. By blocking the base current of the first PNP transistor, a large current can be prevented from flowing, and the output voltage can be maintained.

[0010]

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows an input short-circuit protection circuit according to an embodiment of the present invention. In FIG. 1, Q1, Q2
Is a pre-driver transistor, Q3 is an output transistor, R1 to R6 are resistors, and C1 is an output capacitor.

The operation of the input short-circuit protection circuit of the present embodiment configured as described above will be described below.

When the input is short-circuited to ground, the charge stored in the capacitor C1 is discharged while the output is stabilized, and a large current tends to flow from the collector of the transistor Q3 to the emitter and the base. However, in this state, the collector potential of the transistor Q5 becomes lower than the base potential, and current tends to flow into the base of the transistor Q4. Therefore, since the transistor Q4 is turned off, the base current of the transistor Q3 is cut off and a large current does not flow, so that the transistor Q3 can be prevented from being destroyed. Further, there is no path for discharging the charge of the capacitor C1, and the output voltage is maintained.

[0014]

According to the present invention, when the input is short-circuited to the ground,
By providing a circuit for shutting off the base current of the first PNP transistor, a large input-to-output current can be prevented without any external components, and an excellent input short-circuit protection can be obtained in which the output voltage can be maintained. A circuit can be realized.

[Brief description of the drawings]

FIG. 1 is a block diagram including an input short-circuit protection circuit according to an embodiment of the present invention.

FIG. 2 is a block diagram including a conventional input short-circuit protection diode.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Start-up circuit 2 Reference voltage generation circuit 3 Differential amplifier 4 Output circuit 5 Output capacitor 6 Input short-circuit protection circuit (diode)

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-U 61-78313 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) H02H 7/20 H02H 7/12

Claims (1)

(57) [Claims] An input terminal to which a voltage is applied, an output terminal to which a capacitive load is connected, a first PNP transistor having an emitter connected to the input terminal and a collector connected to the output terminal. A first resistor having one end connected to the base of the first PNP transistor; a second PNP transistor having a collector connected to the other end of the first resistor and an emitter connected to the input terminal; comprising two and NPN transistor whose base is connected to the output terminal based on connecting the collector of the PNP transistor, a second resistor and which is inserted and connected between the emitter and the ground terminal of the NPN transistor, the said input terminal When the ground terminal is short-circuited,
The operation of the second PNP transistor is cut off.
Input short-circuit protection circuit to be.