JPH03171594A - Short-circuit protection circuit - Google Patents

Abstract

PURPOSE:To integrate a circuit easily by detecting short-circuit by a method wherein a collector voltage of an output transistor is divided and the partial voltage is compared with voltage of a pulse generating power source and controlling the output transistor for protection by a switching transistor. CONSTITUTION:In circuit operation in normal time, a lamp 6 is turned on when the voltage of a pulse generating power source 3 changes from low to high and is turned off when the voltage changes from high to low. When both ends of the lamp 6 are short-circuited owing to an irregularity, the collector voltage of an output transistor(TR) 5 is that of a second electric power source 2 and despite of high voltage of the pulse generating power source 3, a TR 17 is turned off by a method wherein voltage is divided by a first resistor 12 and a second resistor 13 so as to make base voltage of the switching TR 17 higher than emitter voltage. As a result, voltage of the pulse generating power source 3 changes from low to high, TR 17 and TR 5 are turned on by supplying base current to a triger TR 16 for a prescribed period according to capacitor 14 for triger and except these conditions, TR 5 is turned off for protection.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a short-circuit protection circuit in a device that pulse-drives a lighting device such as a lamp load using a transistor.

BACKGROUND OF THE INVENTION In recent years, pulse drive circuits such as pulse width modulation have been used to increase efficiency in driving lamp loads such as automobile dashboard dimming. A short circuit protection circuit is also required to prevent damage to the output transistor.

A conventional short protection circuit will be explained below.

FIG. 2 shows an example of a conventional protection circuit. In FIG. 2, 1 is a first power supply, 2 is a second power supply, 3 is a pulse generation source such as a pulse width modulation circuit, 4 is a bias current limiting resistor, and 5 is a lamp that responds to the pulse generation source 3. 6 is a lamp to be dimmed, 7 is a current detection resistor, 8 is a reference voltage source, 9 is a voltage comparison comparator, 10 is a latch circuit, and l1 is a switching transistor.

The operation of the short circuit protection circuit configured as described above will be explained below.

First, in normal circuit operation, when the voltage of the pulse generation source 3 changes from low to high, the output transistor 5
is turned on, current flows through the lamp 6, and the lamp 6 is turned on. At this time, a voltage is generated in the detection resistor 7, but the resistance value of the detection resistor 7 is set so as to be smaller than that of the reference voltage source 8. The output of the comparator 9 is then turned off. Next, when the voltage of the pulse generation source 3 changes from high to low, the output transistor 5 turns off,
No current is supplied to the lamp 6, and the lamp 6 is turned off. At this time, no voltage is generated in the detection resistor 7, and the comparator 9
The switching transistor 11 is turned off.

Next, when both ends of the lamp 6 are short-circuited due to some abnormality, and the voltage of the pulse generation source 3 changes from low to high, the output transistor 5 is turned on, causing a large current to flow through the lamp, and the detection resistor 7 is turned on. When the voltage exceeds the reference voltage, the switching transistor 11 of the comparator 9 is turned on, the output of the pulse generation source 3 is turned off, and the output transistor 5 is turned off. The state of the switching transistor 5 of the converter 9 is held by a latch circuit IO until released in a suitable manner to protect the output transistor 5 from a short circuit across the lamp 6.

Problems to be Solved by the Invention However, with the above conventional configuration, it is necessary to use metal wiring to make the detection resistor 7 a low resistance of several mΩ to 1Ω, which makes it difficult to control manufacturing tolerances, and It has the disadvantage that it is difficult to reduce costs and cannot be made monolithic.

The present invention solves the above-mentioned conventional problems, and aims to provide a short protection circuit that can be easily constructed with a small number of active elements and passive elements, and that can be easily integrated into an integrated circuit.

Means for Solving the Problems In order to solve the above-mentioned conventional problems, the present invention does not detect short-circuit conditions using the voltage generated in the detection resistor.
A short circuit is detected by comparing the voltage obtained by dividing the collector voltage of the output transistor by the first resistor and the second resistor with the voltage of the pulse generation source. This is achieved by controlling using switching transistors.

Operation: With this structure, when a short circuit occurs in the lamp, the voltage between the base and emitter of the switching transistor is reverse biased and the output transistor is cut off, so that it can be easily constructed with a small number of active and passive elements, and,
The first resistor and the second resistor have an absolute value of several kilohms to several hundred kilohms, and by ensuring relative accuracy, a short protection circuit that can be easily integrated into an integrated circuit is realized.

EXAMPLES Below, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram of a short circuit protection circuit according to an embodiment of the present invention. In FIG. 1, 12 is the first resistor, 13
is a second resistor, 14 is a trigger capacitor, 15 is a pull-down resistor, 16 is a trigger transistor, 17 is a switching transistor, and 18 is a trigger circuit.

The operation of the short circuit protection circuit configured as described above will be explained below.

First, during normal circuit operation, when the voltage of the pulse generation source 3 changes from low to high, the trigger capacitor 1
4 and a pull-down resistor 15, a base current is supplied to the trigger transistor 16 for a predetermined period of time. Then, the base current of the switching transistor 17 is supplied to turn on the switching transistor 17. Accordingly, the output transistor 5 is also turned on, causing current to flow through the lamp 6 and lighting the lamp 6. At this time, the collector voltage of the output transistor 5 is approximately equal to the voltage of the first power source 1 due to the voltage drop across the resistance of the lamp 6 when the lamp is lit. The collector voltage of the output transistor 5 is divided by the first resistor 12 and the second resistor 13 to lower the base voltage of the switching transistor 17. Therefore, even if the trigger capacitor 14 is charged by the pull-down resistor 15 and the trigger transistor 6 is turned off after the predetermined time, the switching transistor 17 remains on and the lamp 6 is kept lit. Next, when the voltage of the pulse generation source 3 changes from high to low as shown in FIG. , the output transistor 5 is also turned off, no current is supplied to the lamp 6, and the lamp 6 is turned off.At this time, the collector voltage of the output transistor 5 becomes the voltage of the second power supply 2.The second power supply 2
The voltage is divided by the first resistor 12 and the second resistor 13 to increase the base voltage of the switching transistor 17 and keep the switching transistor 17 in an off state. Next, when both ends of the lamp 6 are short-circuited due to some abnormality, the voltage at the collector of the output transistor 5 is the voltage of the second power supply 2, and the voltage is divided by the first resistor 12 and the second resistor 13. By making the base voltage of the switching transistor 17 higher than the emitter voltage, the switching transistor 17 is turned off regardless of the voltage of the pulse generation source 3. Therefore, Fig. 2 (bl
As shown in the figure, when the voltage of the pulse generation source 3 changes from low to high, the trigger capacitor 14 supplies the base current to the trigger transistor 16 for a predetermined period of time, thereby supplying the base current of the switching transistor 17. The switching transistor 17 is turned on and the output transistor 5 is turned on, but the output transistor 5 is turned off during other periods. By setting the predetermined time to be sufficiently shorter than the period in which the output transistor 5 is off, the output transistor 5 is protected from the outputs at both ends of the lamp 6.

As described above, according to the present invention, it is possible to construct a short protection circuit that can be easily configured with a small number of active elements and passive elements and that can be easily integrated into an integrated circuit.

In addition, in the embodiment, the switching transistor 17
Although no element is constructed in the path from the base of the resistor 12 to the connection point between the first resistor 12 and the second resistor 13, it goes without saying that a voltage level shift element or a circuit may be constructed.

Furthermore, according to the present invention, the absolute values of the first resistor and the second resistor can be designed to be from several KΩ to several hundreds of KΩ, and if relative accuracy can be ensured, the accuracy of the absolute values is poor. Since the purpose is also achieved, it is possible to realize a short protection circuit suitable for integrated circuit implementation.

Effects of the Invention As described above, the present invention does not require low resistance such as metal wiring as a detection resistor, so manufacturing control is easy.
Moreover, it can be easily constructed with a small number of active and passive elements.
This makes it possible to realize a short circuit protection circuit that can be easily integrated into an integrated circuit.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of a short circuit protection circuit according to an embodiment of the present invention, Fig. 2 fat is an operating waveform diagram during normal operation of the present invention, and Fig. 2 (bl is an operating waveform when the lamp load of the present invention is short-circuited). Figure 3 is a circuit diagram of a conventional power protection circuit. 1...First power supply, 2...Second power supply, 3...Pulse generation Source, 4...Bias current limiting resistor, 5...Output transistor, 6
...Lamp, 7...Current detection resistor, 8
...Reference voltage source, 9...Voltage comparison converter, 10...Latch circuit, 1l...
...Switching transistor, 12...First resistor, 13...Second resistor, 14...
...Trigger capacitor, 15...Pull-down resistor, 16...Trigger transistor, 17...
・・・・Swiss switching transistor, 18・・・・・・
trigger circuit.

Claims (1)

[Claims] The emitter of a switching transistor of a first conductivity type is connected to a pulse generation source, the collector of the switching transistor is connected directly or via a resistor to the base of an output transistor of a second conductivity type, and the emitter of the output transistor is connected to a base of an output transistor of a second conductivity type. The collector of the output transistor is connected to a lamp load having one end connected to a second power supply, and the collector of the output transistor and the first power supply are connected to the first and second power supplies. A short circuit protection circuit, characterized in that a trigger circuit for supplying a current for a predetermined period of time in synchronization with a division point divided by a resistor and the pulse generation source is connected to the base of the switching transistor.