JPH05316719A - High voltage protecting circuit - Google Patents

Abstract

PURPOSE:To turn OFF a high voltage output and to protect a high voltage circuit of a TV receiver by turning ON a transistor by a thyristor to be turned ON by a high voltage and removing a gate bias of a P-type MOSFET. CONSTITUTION:When an abnormally high voltage of a high voltage output circuit 13 is detected by a high voltage detector 14, a signal is output to conduct a thyristor 3. A DC voltage 18 is applied to resistors 1, 2, a current flows, a gate of a transistor Tr4 is energized to turn ON the Tr4, and a current flows to a resistor 6. A Zener diode 5 is short-circuited by the Tr4, a potential difference between a gate and a source of a MOSFET is almost eliminated, the MOSFET 7 is turned OFF, and the circuit 13 does not produce an output. Even if an output signal of the detector 14 becomes zero, the circuit 13 continues a stopping state by a holding current of the thyristor 3. Thus, a high voltage circuit of a TV receiver is safely protected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-scanning television receiver having a variable power supply voltage circuit, in which the high voltage protection circuit is a variable power supply voltage circuit.

[0002]

2. Description of the Related Art A conventional power supply voltage variable circuit is described in "Power Control Circuit Design Know-how, Chapter 6, Power Supply Circuits PP.179-1."
81 CQ Publishing Company ", and the base drive circuit among them is a transformer coupling as shown in FIG. 2, the high voltage protection circuit is an N-type MOS-FET which is a power output switching element. The output is cut off by turning off the drive waveform of 15.

[0003]

In contrast to the above-mentioned conventional technique, as shown in FIG. 1, in the power supply voltage variable circuit using the P-type MOS-FET 7 which does not require the drive transformer 16, the Zener diode 5 and the resistor 6 are used. Since the gate of FET 7 was biased, the output could not be cut off even if the drive waveform was turned off.

[0004]

In order to achieve the above object, in order to short-circuit both ends of a gate bias Zener diode 5 of a FET 7 which is a power output switching element, a transistor 4, a resistor 1 and a resistor 2 are provided.
And thyristor 3 were added.

[0005]

When an abnormally high voltage is generated, the output is cut off by removing the gate bias of the FET 7 by the transistor 4, so that power is not supplied to the high voltage circuit 13 and an abnormally high voltage state can be avoided.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. When a DC voltage 18 is applied, the high voltage output circuit 13 generates a high voltage by the DC voltage output from the step-down chopper circuit composed of the FET 7, the diode 10, the choke coil 11 and the capacitor 12. The drive circuit 9 is a high voltage output circuit.
The ON-OFF duty of the drive waveform is controlled so that the DC voltage supplied to 13 becomes normal, and the gate of the FET 7 is driven via the coupling capacitor 8.

Next, when the high voltage rises for some reason, the signal detected by the high voltage detection circuit is applied to the gate of the thyristor 3. This allows the thyristor
3 is in the ON state, and the current flows through the resistors 1 and 2, so that the transistor 4 also flows from the emitter to the base, and the transistor 4 is also turned on.
It becomes a state. Therefore, the Zener diode 5
FET 7 because it is shorted by transistor 4
Only the saturation voltage of the transistor 4 is applied between the gate and the source of the FET 7, and the FET 7 is turned off. As a result, the voltage supply to the high voltage output circuit 13 is cut off, so that high voltage is not generated. After this, the detection signal is in the normal state, but due to the holding current of the thyristor 3, the FET
7 remains off. Therefore, once the abnormal high voltage is generated, the high voltage output circuit 13 is stopped and this state is continued.

In the above example, the transistor 4 is used as a short-circuiting method for the Zener diode 5. However, the same effect can be obtained by using a relay instead of the transistor 4. it can.

[0009]

As described above, according to the present invention, when the abnormal high voltage is detected, the power supply circuit is stopped, so that the generation of the abnormal high voltage can be prevented.

[Brief description of drawings]

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

FIG. 2 is a diagram of a high voltage protection circuit according to a conventional example.

[Explanation of symbols]

 1 ... Resistor, 2 ... Resistor, 3 ... Thyristor, 4 ... Transistor, 5 ... Zener diode, 6 ... Resistor, 7 ... P-type MOS-FET, 8 ... Coupling capacitor, 9 ... Drive circuit, 10 ... Diode, 11 ... Choke coil, 12 ... Smoothing capacitor, 13 ... High-voltage output circuit, 14 ... High-voltage detection circuit, 15 ... N-type MOS-FET, 16 ... Drive transformer, 17 ... Capacitor, 18 ... DC voltage.

Claims (1)

[Claims] 1. A high voltage generating circuit having a variable power supply voltage circuit, wherein a power supply output switching element is a transistor and a thyristor, and a bias of the power supply output switching element is removed to stop the power supply. Protection circuit.