JPH0519877A - Power source circuit - Google Patents

Abstract

PURPOSE:To realize a protection circuit which prevents a component from being damaged with an overcurrent in the case of a load short-circuit and a simple control circuit which is combined with the protection circuit to cut off the negative voltage power source circuit of the power source circuit used for various electronic equipments. CONSTITUTION:A resistance 16 is connected to +Vcc2 and the base of a transistor(TR) 21, a resistance is connected to -Vccl and the base of a TR 21, and a capacitor 18 is connected between the base of the TR 21 and the ground. The emitter of the TR 21 is grounded and the collector is connected to the -Vccl and the base of a TR 13, whose emitter is connected to the +Vcc2 while the collector is connected to the base of a TR 6 to constitute the protection circuit for the load short-circuit of the negative voltage power source circuit. Further, the anode of a diode is connected to the collector of the TR 21 to constitute the control circuit which cuts off the negative voltage power source circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply circuit for electronic equipment.

[0002]

2. Description of the Related Art As a method of stabilizing the voltage of a negative voltage power supply circuit of an electronic device, a method using a stabilized output voltage of a positive voltage power supply circuit as a reference voltage is used.

The conventional negative voltage power supply circuit and the protection circuit thereof will be described below. FIG. 2 shows a conventional negative voltage power supply circuit. In FIG. 2, 1 is a transformer,
3 is a rectifying diode, 4, 10 and 11 are smoothing capacitors, 8 and 9 are resistors for setting an output negative voltage value, 6 is an output transistor, 7 is a voltage control transistor, 5 is a resistor for base bias of 6, and 2 is an excessive resistor. The current protection resistor, + Vcc2, is a reference positive voltage. The operation of the negative voltage power supply circuit configured as above will be described below. The secondary side AC voltage of 1 is rectified to DC at 3 and a negative voltage is applied to the collector of 6. 6 is turned on by the bias voltage supplied from 5, and the output voltage -Vcc1 is controlled by 7 and its values are resistors 8, 9, Vcc2, and Vbe of the transistor 7.
It is calculated by including each value of (base-emitter voltage) in the following formula. Vcc1 = Vcc2-{(Vcc2 + Vbe) (resistor 8
+ Resistance 9) / resistance 9} Further, the protective resistance 2 is disconnected when an overcurrent flows due to a load short circuit, and prevents abnormal heat generation of 1 and 6 and damage to these parts due to it.

[0004]

However, in the above-mentioned conventional structure, the protective resistor 2 may not be disconnected when the load is short-circuited due to conditions such as the winding resistance of 1 and the current amplification factor of 6. In this case, Not only damage but also safety hazards of the entire equipment may occur.

Further, in order to operate an apparatus using the above-mentioned negative voltage power supply circuit, it may be necessary to cut off only the negative voltage output, but the conventional negative voltage power supply circuit does not have such a structure.

The present invention solves the above-mentioned conventional problems. The negative voltage is controlled by a protection circuit and a control signal for preventing damage to parts due to a load short circuit of a negative voltage power supply circuit and a resulting safety risk of equipment. An object is to provide a circuit that easily shuts off the output of a power supply circuit.

[0007]

In order to achieve this object, a power supply circuit of the present invention connects another end of a first resistor having one end connected to a positive voltage power supply circuit and one end to a negative voltage power supply circuit. The other ends of the second resistors are connected together to the base of the first transistor, and when the load circuit of the negative voltage power supply circuit is short-circuited, the first transistor becomes conductive and is driven by this transistor. By connecting the anode of the diode to the protection circuit in which the second transistor cuts off the output transistor of the negative voltage power supply circuit and the collector circuit of the first transistor of the protection circuit, and setting the potential of the cathode of the diode to a low level. It has a configuration of a control circuit that shuts off the output transistor of the negative voltage power supply circuit.

[0008]

With this configuration, when a load short circuit occurs in the negative voltage power supply circuit, a change in the output voltage is detected, the output transistor of the power supply circuit is cut off to protect the circuit components, and a low level control signal. The circuit can also be interrupted by.

[0009]

【Example】

(Embodiment 1) An embodiment of the present invention will be described below with reference to the drawings.

In FIG. 1, 1 is a transformer, 3, 12,
19 is a diode, 14 is a positive voltage power supply stabilizing integrated circuit,
6 is a negative voltage power supply output transistor, 7 is an output voltage control transistor, 21 is a load circuit short circuit detection transistor, 13 is a negative voltage output cutoff transistor driven by 21, 4, 10, 11, 13, 15, 18 Is a smoothing capacitor, 5 is a base bias resistor of 6, 8 and 9 are resistors for setting an output negative voltage value, 16 and 17 are resistors for detecting a load short circuit, and 14 and 20 are base bias resistors of 13.

Among the above components, 13, 14, 16, 17, 18, 19, 20, and 21 shown in broken lines in FIG. 1 constitute the protection circuit and control circuit of the present invention.

The operation of the protection circuit and the control circuit configured as described above will be described. When the voltage at the connection point of 16 and 17 is Vb, Vb becomes a value calculated by the following formula when 21 is not connected. Vb = Vcc2- (Vcc2-Vcc1) resistance 16 /
(Resistor 16 + resistor 17) Therefore, if the values of resistors 16 and 17 are properly selected, when -Vcc1 is a normal voltage, Q4 is cut off (that is, Vb <about 0.7V), and -Vcc is 0V (ground potential) due to load short circuit. Then, the operating point of 21 can be set so that 21 becomes conductive (that is, Vb> about 0.7 V), and an example of the value is shown in FIG. In this circuit, if 21 becomes conductive, 13 whose base is connected to the collector of 21 also becomes conductive, and as a result, the voltage of + Vcc2 is applied to the base of the output transistor 6 via 13 as a reverse bias voltage. 6 is cut off so that no overcurrent flows through the circuit, and the circuit components are protected. That is, the above circuit operates as a protection circuit.

Further, by applying a low-level control signal (about 4 V or less) to the cathode of the diode 19 whose anode is connected to the collector of 21, 13 can be turned on and 6 can be cut off. That is, the circuit composed of the diode 19, the transistor 13, and the resistors 14 and 20 operates as a control circuit that shuts off the negative voltage output circuit. The potential value of the low-level control signal capable of conducting 13 can be reduced by increasing the number of 19 diodes.

[0014]

As described above, according to the present invention, by providing the resistors connected to the positive voltage power supply circuit and the negative voltage power supply circuit and the transistors connected to the resistors, the circuit is short-circuited when the load circuit of the negative voltage power supply circuit is short-circuited. (EN) A protection circuit for preventing an overcurrent from flowing through a component, and a control circuit capable of easily interrupting a negative voltage power supply circuit by adding a diode to the transistor circuit.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a power supply circuit provided with a protection circuit and a control circuit according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram of a conventional power supply circuit.

Claims (2)

[Claims] 1. A first transistor in which another end of a resistor whose one end is connected to a positive voltage power supply circuit and another end of a resistor whose one end is connected to a negative voltage power supply circuit are both connected to a base, and a negative voltage When the load circuit of the power supply circuit is short-circuited, the above first
The power supply circuit having a protection circuit in which the transistor of (1) becomes conductive and the second transistor driven by this transistor cuts off the output transistor of the negative voltage power supply circuit. 2. A power supply circuit which cuts off an output transistor of a negative voltage power supply circuit by connecting an anode of a diode to a collector circuit of a first transistor of the protection circuit and setting a potential of a cathode of the diode to a low level.