JPH0378807A - Short-circuit protecting circuit - Google Patents

Abstract

PURPOSE:To simplify the structure of a part which controls a power circuit by setting a short-circuit detecting port of a controller at a high impedance at start up and then in a pull-up state at the normal drive respectively and turning off the power circuit via a port which controls the power circuit when the detecting port detects a low level. CONSTITUTION:A port P2 is set in a high impedance state at start up and then turned into a pull-up state at the normal drive after lapse of the time constant of a capacitor C2. Then the port P2 is set in a stand-by state to detect a short-circuit, if the output of one of power circuits 11 - 13 has a ground fault, one of diodes D1 - D3 connected to the circuits 11 - 13 that corresponds to the ground fault output is grounded at the cathode side of the relevant diode. Under such conditions, the voltage of the port P2 is equal to the forward voltage VF of the diode. Then an L level of a microcomputer 10 can be decided, and a port P1 is set at a high impedance. Thus the ground fault current is cut. As a result, the power circuit can be protected from ground fault via a simplified control part.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a short-circuit protection circuit that detects a short-circuit state of the outputs of a plurality of constant voltage power supply circuits and protects the power supply circuits.

(Prior Art) Recent electronic devices require a plurality of power supply circuits with different regulated voltages because the drive voltages of used parts such as ICs differ. The driving of these multiple constant voltage power supply circuits is controlled by a microcomputer.

FIG. 3 shows a short circuit protection circuit in a conventional multiplexed constant voltage power supply circuit.

In the figure, 1 is a microcomputer, and a port P of this microcomputer 1 is for power control and has a positive logic output. Port P of the microcomputer 1 is at H level when the power is turned on. Port P is resistor RO, diode DO
The base of the constant voltage power supply circuit 2 is grounded to the base of the NPN transistor Q1 which is grounded via the resistor R1.

The emitter of the transistor Q1 is grounded, and the collector is connected to the power supply Vcc via resistors R2 and R3. Resistance R
2. The connection point of R3 is P, which connects the emitter to the power supply Vcc.
It is connected to the base of the NP transistor Q2, and the collector of the transistor Q2 is connected to the resistor R4 and the Zener diode Dz.
It is grounded through. The connection point between resistor R4 and Zener diode Dz is connected to the base of capacitor C through resistor R5.
It is connected to the base of the NPN transistor Q3 which is grounded through O. The collector of the transistor Q3 is connected to the power supply Vcc, the emitter becomes the output of the power supply circuit 2, and a capacitor CI is connected between the output and ground.

The connection point between the resistor RO and the diode DO is connected to the output of the power supply circuit 2 via the diode D1.

Power supply circuit 3.4 has exactly the same configuration as power supply circuit 2, although the output voltage is different, and diodes D2 and D3 and capacitors C2 and C3 also correspond to diode DI and capacitor CI, respectively.

When the power switch is on, port P of the microcomputer 1 is at H level. This turns on transistor Ql, 02 via resistor RO and diode DO. transistor Q
2 turns on, the Zener diode D is connected via the resistor R4.
A current flows into Z, and the base potential of the output transistor Q3 is approximately the Zener voltage VDZ of the Zener diode Dz.
becomes. The output voltage v1 appearing at the emitter of the transistor Q3 becomes VDZ-VBE (Ql), and a constant voltage output circuit can be realized. Below, the output of power supply circuit 3.4 is also v2,
A constant voltage output of v3 can be generated. The values of the constant voltage output voltages V2 and V3 can be arbitrarily selected by setting the Zener voltage of a Zener diode corresponding to the Zener diode Dz of the power supply circuit 2 in the power supply circuit 3.4 to a desired value.

If any of the diodes Dl-D3 has a ground fault for some reason, the diodes D1-D3 connected to the output terminal
Among them, the cathode side of the diode corresponding to the ground fault output is grounded. As a result, the anode side of the diode DO is clamped to the forward voltage VP of the diode DI-D3, thereby turning off the transistors Ql to Q3 and cutting the ground fault current. To turn on the transistor Ql, the anode side of the diode DO +,
: This is because a voltage higher than VBE(Q3)+VF(Do) is required.

In the circuit described above, since there is a load such as a large capacity capacitor C1 on the output of the power supply circuit 2 that outputs a constant voltage, the drive may be insufficient and startup may not be possible. This is because the impedance of port P is low when the power is turned on and port P becomes H level, and capacitors 01 to C
3 is in short circuit condition. At this time, the capacitor C1 is charged via the diode Dl, the output voltage Vl becomes high, the voltage between the diode DO and the resistor RO rises, and the output voltage is determined by the Zener diode Dz. This is because the current flowing to port P of microcomputer 1 is limited.

It is not enough to charge the capacitor C1. Because of this, it cannot be started. This also applies to the power supply circuit 3.4.

In addition, when the power is turned on, the port P inevitably goes to H level, causing problems such as the power being turned on unnecessarily during system initialization.

In order to ensure reliable startup using the circuit shown in Figure 3, the fourth
As shown in the figure, it is necessary to insert a transistor Q4 serving as a current buffer between the power supply Vcc and increase the drive capability.

(Problem to be Solved by the Invention) The conventional short-circuit protection circuit described above has a very large load on each output terminal, so it is necessary to increase the drive capacity at startup, and therefore an additional circuit such as a current buffer is provided. There was a need.

The present invention aims to eliminate the above-mentioned problems and provide a short-circuit protection circuit in which the configuration of the portion that controls the power supply circuit is simplified.

[Structure of the Invention] (Means for Solving the Problems) The short circuit protection circuit of the present invention includes a plurality of power supply circuits that each generate a constant voltage, and a first port that generates a control signal to start the power supply circuits. and a controller having a second port for detecting a short-circuited state of the output of the power supply circuit; the second port is set to a high impedance state during power supply and startup, and the second port is set to a high impedance state during normal driving.
and means for setting the port in a pull-up state, and controlling the first port to turn off the power supply circuit when the second port detects a short circuit.

(Function) By the means described above, the short-circuit detection port of the controller is set to high impedance when the power is turned off and started, the short-circuit detection port is set to a pull-up state during normal operation, and the power supply circuit is turned off when the detection port detects a low level. The power circuit is turned off from the port being controlled.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

In Figure 1, 10 is a power control port P1 which becomes high impedance (oven) when the power is turned off and goes to L level when the power is turned on. Up (H level)
This is a microcomputer consisting of a port P2 for short circuit detection. 11 is a constant voltage power supply circuit. The port Pi is connected to the power supply Vcc via the resistor R1 of the power supply circuit 11, and is also connected to the base of a PNP transistor Ql whose emitter is connected to the power supply VCC. The collector of the transistor Q1 is grounded via a resistor R2 and a Zener diode Dz. The connection point between the resistor R2 and the Zener diode Dz is connected via a resistor R3 to the base of a transistor Q2 whose base is grounded via a capacitor CO. The emitter of transistor Q2, which serves as the output of power supply circuit 11, is grounded via capacitor C2.

Port P1 is also connected to a portion corresponding to the base of transistor Q1 in power supply circuit 11 of power supply circuit 12 and power supply circuit 13 having the same configuration as power supply circuit 11 whose outputs are grounded through capacitors C2 and C3, respectively. The power supply circuits 12 and 13 have Zener diodes Dz with different Zener voltages, and are set to different constant voltage power supply circuits.

P2 is connected to the connection point between the output of the power supply circuit 11 and the capacitor C1 via a diode Di. Similarly, port P2 is connected to a connection point between the output of power supply circuit 12 and capacitor C2 and a connection point between output of power supply circuit 13 and capacitor C3 via diodes D2 and D3, respectively.

When the power is off, the port PL is in high impedance, so the transistor Ql is turned off, and since no current flows through the Zener diode Dz, the transistor Q2 is also turned off. At this time, a voltage equal to the power supply Vcc is applied to the port Pl, so a high voltage port is used. Furthermore, when the power is turned off, P2 is set to a high impedance so that a voltage - does not appear on the emitter side of the transistor Q2 via the diode Di-D3.

When the power is turned on, the port PL is set to a level, the transistor Ql is driven by a low-level output current, and a voltage of Vl-VDZ+VBE (Q2) can be obtained at the output of the power supply circuit 11. At this time, a large capacitive load is connected to the output of the power supply circuit 11.
Since the output section of is composed of an emitter-follower transistor Ql, it can easily rise. The above also applies to the power supply circuits 12 and 13.

After the time constant of the capacitor C2 elapses, the port P2 changes from high impedance to pull-up (H level) and enters a standby state for detecting a short circuit. If any of the outputs of the power supply circuits 11 to 13 is grounded due to some abnormality, the cathode side of the diode corresponding to the ground fault output among the diodes D1 to D3 connected thereto becomes grounded.

At this time, the voltage at port P2 becomes equal to the forward voltage VF of the diode, and the microcomputer 10 can determine the L level. Based on this determination result, the port PI can be set to high impedance, the ground fault current can be cut off, and the power supply circuit whose output has a ground fault can be protected.

FIG. 2 is a simple flowchart showing the operation described above.

In the above embodiment, three power supply circuits 11 to 13 are used, but the present invention is not limited to this, and the present invention can be applied to four or more power supply circuits.

[Effects of the Invention] As described above, according to the short circuit protection circuit of the present invention,
The power supply circuit can be protected from ground faults with a small number of parts, and the power supply circuit can be controlled with an easy-to-handle low active.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a schematic flowchart showing the operation of FIG. 1, and FIGS. 3 and 4 are conventional circuit diagrams, respectively. 10...Microcontroller 11-13...Power circuit Pl, P2...Port

Claims (1)

[Claims] A controller having a plurality of power supply circuits each generating a constant voltage, a first port generating a control signal for activating the power supply circuits, and a second port detecting a short circuit state of the output of the power supply circuit; The second port is in a high impedance state during off and startup, the second port is in a pull-up state during normal driving, and when the second port detects a short circuit, the second port is in a high impedance state.
and means for controlling a port of the power supply circuit to turn off the power supply circuit.