JPH0713398Y2 - Power protection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Purpose of Invention] (Field of Industrial Application) The present invention relates to an abnormality protection circuit for a power supply circuit used in a color television receiver or the like.

(Prior Art) In some power supply circuits of television receivers and the like in recent years, a circuit for shorting a rush prevention resistor by using a thyristor, a relay or the like is added during steady operation. The circuit is constructed, for example, as shown in FIG. 1 in FIG.
Is a standby power supply circuit that transforms and rectifies a voltage supplied from an AC power supply (not shown) into a predetermined voltage and supplies the voltage to a microcomputer (not shown). The standby power supply circuit 1 is composed of a transformer 2 and a rectifying circuit composed of a diode 3 and a capacitor 4. Reference numeral 5 is an AC rectifier circuit that rectifies the AC input, and is composed of a diode rectifier 6 and a smoothing capacitor 7. Reference numeral 8 is a main power supply circuit that transforms the output voltage of the AC rectifier circuit 5 into a predetermined voltage, rectifies it, and supplies it as a + B voltage to the microcomputer. The main power supply circuit 8 is composed of a converter transformer 9, a switching transistor 10, a base resistor 11, a diode 12 and a smoothing capacitor 13. A rush prevention resistor 14 and a thyristor 15 having the illustrated polarity are connected in parallel between the negative output terminal of the AC rectifier circuit 5 and the ground. Thyristor 15
Is connected to the positive side of the winding 9 _b of the converter transformer 9. With one end of the AC rectifier circuit 5 on the AC input side
The contact of the power relay 17 is connected to the AC input terminal _16b . One end of the coil of the power supply relay 17 is connected to the output side of the standby power supply circuit 1, and the other end is grounded via the collector and the emitter of the relay drive transistor 18. An ON / OFF control signal is supplied to the base of the relay drive transistor 18 from a microcomputer (not shown).

In the circuit configured as described above, when AC power is supplied from the AC power supply, the standby power supply circuit 1 is driven and DC power is supplied to a microcomputer (not shown). Then, the microcomputer is driven, and the microcomputer supplies the on-base current to the relay drive transistor 18. Therefore, the coil of the power supply relay 17 is excited by the conduction of the relay drive transistor 18, and the contact of the relay 17 is turned on. When the power relay 17 is turned on, the AC rectifier circuit 5 is driven and the charging current flows through the smoothing capacitor 7. At this time, the main power supply circuit 8 does not operate until the charging of the smoothing capacitor 7 is completed, and therefore the thyristor 15 is kept in the off state because the on-gate current is not supplied. Therefore, at startup, the rush current is limited by the rush prevention resistor 14, and an excessive rush current does not flow. Next, when charging of the smoothing capacitor 7 is completed, the main power supply circuit 8 starts to operate. That is, when the switching transistor 10 is turned on, energy is stored in the primary side of the converter transformer 9, and when the switching transistor 10 is turned off, the energy is discharged to the secondary side, and a stable voltage is output by repeating them. Since the operation of the main power supply circuit 8 is on gate current from the winding 9 _b to the thyristor 15 of the converter transformer 9 is supplied, the thyristor 15 is allowed to short-circuit the rush prevention resistor 14 turned on. As a result, the current flows through the thyristor 15 without passing through the rush prevention resistor 14, so that no wasted power is consumed during steady operation and no heat is generated. Moreover, if the resistance value of the rush prevention resistor 14 is increased, the rush current can be significantly suppressed,
You can use the one with small rated power.

(Problems to be solved by the invention) However, in the circuit configured as described above, during normal operation, for example, a failure of the thyristor 15 causes a rush prevention resistance.
If 14 cannot be short-circuited, the anti-rush resistor 14 will heat up, which is very dangerous.

The present invention has been made in view of the above points, and its purpose is to:
An object of the present invention is to provide a power supply protection circuit that can prevent overheating of the rush prevention resistor even if the circuit for short-circuiting the rush prevention resistor fails.

[Configuration of the Invention] (Means for Solving the Problem) The present invention has a switching element in parallel with a rush current prevention resistor, and turns on the switching element after the main power supply circuit is driven to turn on the rush current prevention resistor. In the short-circuited power supply circuit, means for detecting the voltage across the rush current prevention resistor is provided, and the power supply relay is turned off when a voltage higher than a predetermined value is detected after the main power supply circuit is driven. The feature is that a protection circuit is provided.

(Function) The rush current is limited by the rush prevention resistor until the rectifier circuit is activated and the operation of the main power supply circuit is stabilized. When the circuit for short-circuiting the rush prevention resistor fails during steady operation of the main power supply circuit, a current flows through the rush prevention resistor, and a potential difference is generated across the resistance. Therefore, a predetermined potential difference is generated between the reference voltage circuit and the negative potential generation side end of the rush prevention resistor, and the switching element is turned on. Then, the light-emitting element of the photocoupler emits light, the light-receiving element becomes conductive, and the protection circuit operates. As a result, the ON control signal from the microcomputer is invalidated and the operation of the power supply circuit is stopped. Therefore, even if the circuit for short-circuiting the rush prevention resistor fails during the steady operation of the main power supply circuit, the rush prevention resistor does not overheat.

Embodiment An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, the same parts as those in FIG. 2 are designated by the same reference numerals and the description thereof will be omitted. When the AC rectifier circuit 5 operates, if the thyristor 15 is in the off state, a rush prevention resistor
A current flows through 14 and a potential difference is generated across the current. That is, the cathode connection side end A of the thyristor 15 has a voltage of minus several volts to + several volts, and the emitter of the NPN transistor 21 is connected to this connection point A. A reference voltage circuit formed by connecting a capacitor 22 and a diode 23 having the illustrated polarity in series between both ends of the winding 9b of the converter transformer 9.
24 are connected. In the reference voltage circuit 24, the potential at the intermediate connection point B is higher than the potential at the connection point A by a predetermined voltage V _f.
Design so that it is higher than the above. The transistor 21
Is connected to the connection point B, and the collector is grounded through the light emitting diode of the photocoupler 25 and the resistor 26. The collector of the phototransistor of the photocoupler 25 is connected to the point C on the output side of the main power supply circuit 8 via the resistor 27. The base of the relay drive transistor 18 is grounded via the collector and emitter of the transistor 29 of the protection circuit 28. The base of the transistor 29 is connected to the output side of the standby power supply circuit 1 through the cathode and anode of the thyristor 30 and the resistor 31 in this order. The gate of the thyristor 30 is connected to the emitter of the phototransistor of the photocoupler 25.

In the circuit configured as described above, when the standby power supply circuit 1, the power supply relay 17, the AC rectifier circuit 5, and the main power supply circuit 8 are all driven and in a steady operation state, if the thyristor 15 is normally ON. The potential at the point A of the rush prevention resistor 14 is substantially equal to the ground level. However, if the thyristor 15 or the like fails and the rush prevention resistor 14 is not short-circuited, a potential of minus several volts to + several volts is generated at the point A. Then, the transistor 21 is biased and becomes conductive.
For this reason, the light emitted from the photocoupler 25 has a resistance of 2
6, a current flows through the transistor 21 and the anti-rush resistor 14, and the diode emits light. As a result, the phototransistor of the photocoupler 25 becomes conductive, and an on-gate current is supplied from the output side (point C) of the main power supply circuit 8 to the thyristor 30 via the resistor 27 and the phototransistor.
The thyristor turns on. Then, the base current is supplied from the output side of the standby power supply circuit 1 to the transistor 29 via the resistor 31 and the thyristor 30, and the transistor 29 is turned on. As a result, the ON control signal supplied from the microcomputer is grounded via the transistor 29, and the relay drive transistor 18 is turned off. Therefore, the power supply relay 17 is turned off, the operation of the power supply circuit is stopped, and the rush prevention resistor 14 is prevented from overheating.

Further, at the time of AC input, the main power supply circuit 8 does not operate until the charging of the smoothing capacitor 7 of the AC rectifier circuit 5 is completed, and therefore the thyristor 15 is kept in the off state because no on-gate current is supplied. . Also in this case, the potential at the point A becomes minus several volts to plus several volts, the transistor 21 is turned on, and a current flows through the light emitting diode of the photocoupler 25. However, at this time, since the main power supply circuit 8 is inoperative, power is not supplied to the phototransistor and the protection circuit 28 does not operate. Therefore, the power relay 17 is not accidentally turned off at the time of AC input (at the time of startup).

As described above, according to the present invention, when the circuit that short-circuits the rush prevention stop during normal operation does not operate normally, the voltage across the resistor is detected and the protection circuit stops the drive of the power supply circuit. As a result, overheating of the rush prevention resistor can be prevented in advance, and the safety of the circuit can be improved. Also, the protection circuit is configured to perform the protection operation only when the photocoupler is driven during the operation of the main power supply circuit, so it is not possible to accidentally stop the circuit when the AC input is not operating. Absent.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing an example of a conventional power supply circuit. 1 ... Standby power supply circuit, 5 ... AC rectifier circuit 7 ... Smoothing capacitor, 8 ... Main power supply circuit 14 ... Rush prevention resistor, 15,30 ... Thyristor 17 ... Power relay 18 ... Relay drive transistor 21 ... Transistor, 24 ... Reference voltage circuit 25 ... Photo coupler, 28 ... Protection circuit

Claims (1)

[Scope of utility model registration request] 1. A standby power supply for rectifying an AC input to supply a DC voltage to a first load circuit, and an operation of the first load circuit provided between the AC input source and the rectifier circuit. At times, it is turned on by a control signal supplied from this load circuit, and a power supply relay that supplies an AC input to the rectifier circuit, and a DC voltage from the rectifier circuit is controlled to supply a stabilized DC voltage to the second load circuit. A main power supply circuit, a rush prevention resistor for limiting a current flowing to the DC side of the rectifier circuit when the power supply relay is turned on, and a rush prevention resistor connected in parallel to drive the main power supply circuit. After that, a switching element that short-circuits the rush prevention resistor and a means for detecting a voltage across the rush prevention resistor are included, and the voltage across the rush prevention resistor has a predetermined value after the main power supply circuit is driven. When it exceeds, the power supply protection circuit comprising the protection circuit to turn off the power relay.