JPH1118291A - Overvoltage protection device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent failure of an output to work properly, when an output transistor is protected against surge voltage and overvoltage produced by noise lasts for a short time. SOLUTION: A timer circuit 300 which, when a detection signal from an overvoltage detection circuit 100 continues for a specified time, outputs a detection signal to an output circuit 200, is placed between the overvoltage detection circuit 100 and the output circuit 200. If overvoltage produced due to noise continues for a short time, the timer circuit 300 enables avoidance of bringing an output transitor T5 in continuity to prevent so as the output control from being executed. Therefore, malfunctions of the circuits can be prevented in such a case.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overvoltage protection device for protecting an output transistor for controlling voltage supply to loads such as various lamps mounted on a vehicle or the like from an overvoltage (surge voltage). The present invention relates to a device having an output transistor for receiving an overvoltage generated in a battery for use in a vehicle.

[0002]

2. Description of the Related Art Conventionally, in a control device using a semiconductor device, there is an overvoltage protection device for preventing a semiconductor device connected to a power supply from being destroyed when a surge voltage due to noise is generated in the power supply. For example, there is a method of providing a resistor and a Zener diode between the collector and the base of the output transistor in order to prevent a surge voltage from being applied to an NPN type output transistor in an output circuit connected to a power supply.

[0003] That is, when a surge voltage is applied to the output transistor, the collector potential rises and the Zener diode turns on. With this, a current is injected into the base of the output transistor to turn on the output transistor and the overvoltage is output. This is to protect the voltage from being applied to the transistor. However, according to the above method, there is a problem that the output transistor oscillates and is destroyed. Specifically, at the moment when the output transistor is turned on, the zener diode is turned off and the base current is cut, so that the output transistor is turned off. When the output transistor is turned off, the collector potential rises again and the output transistor is turned on. Oscillation occurs that repeats this, and the output transistor is overheated and destroyed.

In order to prevent this, an overvoltage detection circuit (surge detection circuit) is provided in the overvoltage protection device to detect a surge voltage. When the surge voltage is detected, the output transistor is turned on without fail to reduce the surge voltage. Some absorb (see Japanese Patent Application Laid-Open No. 7-154224).

[0005]

However, even when the overvoltage generated by noise is of a short time, the output transistor is turned on, causing a problem that a circuit malfunction occurs. The present invention has been made in view of the above points, and has as its object to prevent an output malfunction when an overvoltage generated by noise is short-time when protecting an output transistor with a surge voltage. .

[0006]

In order to achieve the above object, the present invention employs the following technical means. According to the first aspect of the present invention, a delay means (300) for outputting a detection signal to the output means (200) when the detection signal continues for a predetermined time between the overvoltage detection means (100) and the output means (200). ).

As described above, by providing the delay means (300), when the overvoltage generated by noise is of a short time, the output transistor (T5) is not turned on and the output control is not executed. Therefore, a circuit malfunction in such a case can be prevented. According to the invention described in claim 2, when the detection signal is output from the delay means (300),
A means (R2, R3, T3, T4) for changing the first predetermined voltage to a second predetermined voltage lower than the first predetermined voltage is provided. In this way, the delay means (300) can be applied to a device in which the on / off voltage of the output transistor (T5) is provided with a so-called hysteresis, and the same effect as in claim 1 can be obtained.

[0008] The reference numerals in parentheses of the above means indicate the correspondence with the specific means described in the embodiment described later.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram including a timer circuit (delay means) 300 for preventing malfunction due to noise between an overvoltage detection circuit 100 and an output circuit 200 when an overvoltage protection device is applied to a circuit for driving a tail lamp of a vehicle. Is shown. Hereinafter, the configuration of the circuit will be described with reference to FIG.

[0010] The overvoltage protection device is connected to a battery 1, and a coil 5 serving as a load and an output transistor T5 constituting an output circuit 200 are arranged so as to connect between both electrodes of the battery 1. First, the overvoltage detection circuit 100 will be described. Overvoltage detection circuit 100
Are four Zener diodes Z that determine the threshold voltage
D1, ZD2, ZD3, ZD4, a resistor R1, and an NP for outputting an on / off signal of an output transistor T5.
N-type transistors T1, T2 and constant current sources I1, I2
It has.

The overvoltage detection circuit 100 includes resistors R2, R3,
An NPN transistor T3 and a PNP transistor T4 are provided. The output terminal of the timer circuit is connected to the base of the transistor T3 so that the transistor T3 is turned on / off in accordance with the output from the timer circuit.

Next, the timer circuit will be described. The timer circuit includes a comparator 10, a capacitor C, a constant voltage source V, and a NOT circuit 20. The signal line L4 connects between the constant current source I2 and the transistor T2 and the negative terminal of the comparator 10, and connects the signal line L4 and the GND terminal via the capacitor C. In addition, the + of the comparator 10 via the constant voltage source V
The terminal is connected to the negative terminal of the battery 1.

The output terminal of the comparator 10 is connected to an output circuit 200 via a NOT circuit 20. The base of the transistor T3 is connected to a NOT circuit 20.
The comparator 10 outputs a low level when the potential difference Vc between both electrodes of the capacitor C is smaller than the voltage of the constant voltage source V, and outputs a high level when the potential difference Vc is larger than the voltage of the constant voltage source V.

Next, the output circuit 200 will be described.
The output circuit 200 includes an OR circuit 30 and a transistor T5. The output signal from the comparator 10 is output from the OR circuit 30 via the NOT circuit 20. The output circuit is connected to an output control unit 40 that outputs a signal based on the on / off state of the light switch. Based on the output from the output control unit 40, the transistor T5 is turned on / off.
The tail lamp turns on and off.

The operation of the overvoltage protection device having the above configuration will be described. When the light switch is turned on (during load driving), a high-level signal is output to the output circuit. In this case, a high-level signal is output to the transistor T5 via the OR circuit, so that the transistor T5 is turned on. Therefore, the surge voltage is absorbed by the coil 5.

When the light switch is off (when the load is not driven), the operation is as follows. First, a case where no overvoltage due to noise has occurred will be described. In this case, the transistor T1 is off and the transistor T4 is on, and a current flows from the constant current source I2, so that the potential difference Vc between both electrodes of the capacitor does not increase. Therefore, the comparator 10 outputs a high-level signal. Then, the NOT circuit 20 outputs a low level,
A low level signal is output to the transistor T5 via the R circuit 30. Therefore, the transistor T5 is turned off, and the output voltage Vout becomes equal to the Vcc terminal voltage.
At this time, a low-level signal is output to the transistor T3, so that the transistor T3 is turned off.

Next, a case where an overvoltage occurs due to noise will be described. In this case, the Vcc terminal voltage is increased to a predetermined threshold voltage Vth (= V
ZD × 4 + Vf: when the sum exceeds a predetermined sum VZD × 4 of breakdown voltage VZD in Zener diodes ZD1 to ZD4 and a base-emitter voltage Vf required for turning on transistor T1, transistor T1 turns on. . As a result, the transistor T2 is turned off, and the current from the constant current source I2 flows to the capacitor C, and the potential difference Vc between both electrodes of the capacitor C increases.

When a predetermined time Td elapses after the Vcc terminal voltage becomes higher than Vth and the potential difference Vc becomes higher than the voltage at the constant voltage source V, the comparator 10 outputs a low-level signal. Is inverted to a high-level signal through the NOT circuit 20, and a high-level signal is output to the transistor T5 via the OR circuit 30. Therefore, the transistor T5 is turned on, and at this moment, the output voltage Vout decreases to the predetermined voltage Vsat. The delay time Td when the potential difference Vc becomes larger than the voltage at the constant voltage source V is represented by Td = CV / I2, and can be arbitrarily set according to the capacitance of the capacitor C and the magnitude of the constant voltage V. Therefore, it can be set according to the magnitude of the noise.

Since a high level signal is output to the transistor T3 via the NOT circuit 20, the transistor T3 is turned on, and the transistor T4 is turned on accordingly. As a result, the threshold voltage at which the transistor T1 is turned off becomes Vtl (= VZD × 2 + Vf: the breakdown voltage of the Zener diodes ZD3 and ZD4 and the transistor T1).
Required to turn on the base-emitter voltage Vf
).

When the Vcc terminal voltage is equal to the threshold voltage V
When it becomes tl or less, the transistor T1 is turned off and the transistor T2 is turned on. As a result, the output voltage Vout returns to be equal to the Vcc terminal voltage. FIG. 2 is a time chart showing the output voltage Vout in the overvoltage protection device.
When the overvoltage generated by the noise is short as shown in the period A in FIG. 2, the time Ta in which the Vcc terminal voltage exceeds the predetermined voltage Vth is short, and the time Ta <the delay time Td is satisfied. Therefore, the transistor T5 does not turn on.

On the other hand, as shown in the period B, when the overvoltage generated by noise is a long time, Vc
The time Ta in which the voltage at the terminal c exceeds the predetermined voltage Vth is long, and the time Tb> the delay time Td, so that the transistor T5 is turned on. That is, the coil 5 is driven to turn on the tail lamp, thereby absorbing the surge voltage.

As described above, by providing the timer circuit 300, it is possible to prevent the transistor T5 from being turned on and executing the output control when the overvoltage generated by the noise is of a short time. In such a case, it is possible to prevent a circuit malfunction such as an instantaneous lighting of the tail lamp. In the present embodiment, the threshold voltage of the overvoltage detection circuit 100 has hysteresis. However, as shown in FIG. 3, the threshold voltage may not have hysteresis. That is, the threshold value (= VZD × 4 + Vf) may be kept constant by eliminating the resistors R2 and R3 and the transistors T3 and T4 in the overvoltage detection circuit shown in FIG.

FIG. 4 is a time chart showing the output voltage Vout in the overvoltage protection device in this case. As described above, since the threshold value at which the transistor T1 is turned off increases, the time during which the output transistor T5 is turned on is shortened.
Therefore, in such a case, the number of circuits can be reduced, but it can be said that heat generation in the output transistor T5 increases.

Although FIG. 1 and FIG. 3 show four Zener diodes (ZD1 to ZD4), this number can be changed according to the requirements of the system and may be four or less, or four or more. It may be. Further, the connection destination of the connector of the transistor T4 can be changed depending on the required system.

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram of an overvoltage protection device according to an embodiment of the present invention.

FIG. 2 is an output voltage V of the overvoltage protection circuit 100 shown in FIG.
It is a time chart showing out.

FIG. 3 is a circuit configuration diagram of an overvoltage protection device according to another embodiment.

4 is an output voltage V of the overvoltage protection circuit 100 shown in FIG.
It is a time chart showing out.

[Explanation of symbols]

1 ... battery, 5 ... coil, 10 ... comparator,
20 NOT circuit, 30 OR circuit, 100 overvoltage protection circuit, 200 output circuit, 300 timer circuit, C
... Capacitor, V ... Constant voltage source, I1, I2 constant current source, Z
D1, ZD2, ZD3, ZD4 ... Zener diodes.

Continued on the front page (51) Int.Cl. ⁶ Identification code FI H03K 17/16 H03K 17/16 C

Claims (2)

[Claims] A power supply voltage from a power supply (1) is applied to a load (5).
An overvoltage protection device for protecting an output transistor (T5) for controlling supply of power to an overvoltage, detecting that a power supply voltage of the power supply (1) has become equal to or higher than a first predetermined voltage and outputting a detection signal. Means (100), and an output means (20) for controlling the output transistor (T5) to a conductive state based on the detection signal to protect the output transistor (T5) from overvoltage.
0), the overvoltage detecting means (100) and the output means (20).
0), and a delay means (300) for outputting the detection signal to the output means (200) when the detection signal continues for a predetermined time. 2. The output means according to claim 2, wherein said detection signal is output from said output means.
Means (R) for changing the first predetermined voltage to a second predetermined voltage lower than the first predetermined voltage.
2. The overvoltage protection device according to claim 1, further comprising (2, R3, T3, T4).