JPS6339417A - Circuit for preventing overcurrent detector from misoperation - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a malfunction prevention circuit for an overcurrent detection circuit in a switching power supply.

[Prior Art] Conventionally, in a circuit that supplies power to a load using a switching power supply or the like, when an overcurrent flows through the load, this overcurrent is detected and the oscillation of the switching power supply is stopped. , such a switching power supply is the fourth
As shown in the figure, an overcurrent detection circuit is provided on the output side and is configured to detect an overcurrent flowing through the current detection resistor.

Figure 4 shows a multi-output circuit using such a switching power supply, in which 1 is a rectifier for the output of the AC power supply, 2
is a smoothing capacitor thereof, 3 is an output transformer of the switching power supply, 4 is a switching transistor, 5 is a drive circuit thereof, 6 is a first secondary winding of the output transformer 3,
7.8 is a diode connected to the output side of the secondary winding, 9 is an inductor, 10 is a capacitor, 11.12 is a resistor, and the midpoint between these two resistors is connected to the drive circuit 5, and the output Output ■ is output from terminals 13 and 14.

Further, 15 is the second secondary winding of the output transformer 3;
6.17 is a diode connected to the output side of the secondary winding, 18 is an inductor, and 19 is the secondary winding 15.
is a capacitor in parallel with .

20 is a transistor parallel to the secondary winding 15; 21.
22 is a resistor on the collector side, 23 is a capacitor connected to the drive circuit 5, 24 is a current detection resistor connected in series to the secondary winding 15, and 25 is a base resistor of the transistor 20. 20, resistance 21.
22, a capacitor 23, a current detection resistor 24, and a base resistor 25 constitute an overcurrent detection circuit.

26 is an output control transistor connected in series to the secondary winding 15, 27 is its drive circuit, 28 is a switch connected to the base, 29.30 is a resistor, and outputs from output terminals 31.32. ■ is output.

In the circuit as described above, when it is detected that the output (■) on the side of the first secondary winding 6 is completely output, a signal is applied to the base of the output control transistor 26 on the side of the second secondary winding 15. When the connected switch 28 turns ON, an output ■ is output. However, if a fault such as a short circuit occurs in the load of this output ■, an overcurrent flows to the current detection resistor 24, and the overcurrent is detected. The circuit operates and sends an overcurrent detection signal to the drive circuit 5 of the switching transistor 4 of the switching power supply to stop the switching transistor 4.

[Problems to be Solved by the Invention] In the conventional circuit as described above, when the capacitor 33 is connected to the load of the output (■), malfunction may occur due to rush current to the load capacitor 33 when the power is turned on. Therefore, there is a problem in that it is difficult to distinguish between the inrush current of the load capacitor and the load short-circuit current.

That is, when the switch 2B connected to the base of the transistor 26 is ONL and output ■ is output, a rush current flows from the capacitor 19 through the current detection resistor 24 and the transistor 26 to the capacitor 33 connected to the load of the output ■. Therefore, the overcurrent detection circuit malfunctions due to the rush current flowing through the current detection resistor 24, causing the drive circuit 5 of the switching transistor 4 of the switching power supply to malfunction.
A problem arises in that a malfunction signal is sent to the switching transistor 4 and the switching transistor 4 is stopped.

SUMMARY OF THE INVENTION Therefore, the present invention provides a malfunction prevention circuit for an overcurrent detection circuit that prevents the above-mentioned malfunction even if a capacitor is connected to the load of output (2).

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present invention provides a power supply circuit in which an overcurrent detection circuit is provided on the output side of a switching power supply and outputs to a load connected to a capacitor. In,
A capacitor is connected after the current detection element of the overcurrent detection circuit to form an overcurrent detection malfunction prevention circuit.

[Function] Since the capacitor is connected after the current detection element, even if a rush current flows to the load side capacitor when the power is turned on, this current does not flow to the current detection element, so an overcurrent detection signal is not output. Since the overcurrent detection circuit does not operate, malfunctions such as stopping the switching transistor of the switching power supply will not occur as in the conventional case.

[Examples] Examples of the present invention will be described below with reference to the drawings. FIG. 1 shows an example of a multi-output circuit using a switching power supply according to an embodiment of the present invention, and the illustrated example shows an example of two outputs, output I and output (2). In the figure, 1 is the AC power output rectifier, 2
is a smoothing capacitor thereof, 3 is an output transformer of the switching power supply, 4 is a switching transistor, 5 is a drive circuit thereof, 6 is a first secondary winding of the output transformer 3,
7.8 is a diode connected to the output side of the secondary winding, 9 is also an inductor, 10 is also a capacitor, 1
1.12 is a resistor, and the midpoint between these two resistors is connected to the drive circuit 5, and the output (2) is output from the output terminal 13.14.

Further, 15 is the second secondary winding of the output transformer 3;
6.17 is a diode connected to the output side of the secondary winding, and 18 is also an inductor.

Ql is a transistor connected in parallel to the output side of the second secondary winding 15, and RO is a current detection element connected in series to the secondary winding 15. In the illustrated example, a resistor is connected to this element. This is an example using R1 and R2 are resistors on the collector side, C
o is a capacitor connected to the drive circuit 5, and R
3 is the base resistance of the transistor Q1. This transistor Q1, current detection element RO1 resistance R1, R2,
R3 and capacitor CO constitute overcurrent detection circuit A.

Q2 is an output control transistor connected in series to the output side of the secondary winding 15, Dr is its drive circuit, pot is a switch connected to its base, and R4 and R5 are resistors.

A and b are output terminals from which an output ■ is output.

C2 is a load-side capacitor connected to the load B.

C1 is a capacitor connected in parallel to the output side of the second secondary winding 15 after the current detection element RO of the overcurrent detection circuit A.

In the circuit configured as described above, the first secondary winding 6
When it is detected that the side crab has been completely output, the second
When the switch SW connected to the base of the output control transistor Q2 on the side of the secondary winding 15 is turned ON, an output ■ is output, and current is supplied to the load. If a fault occurs in which an overcurrent flows in the load of this output H, the current detection element RO
When an overcurrent flows, the overcurrent detection circuit A operates, and sends an overcurrent detection signal to the drive circuit 5 of the switching transistor 4 of the switching power supply to stop the switching transistor 4.

In such a multi-output circuit, as shown in the figure, a capacitor C is connected to the load at one of the outputs, for example, output (2).
In the circuit to which 2 is connected, when current flows through the load of this output 2, the current supplied to this load is the current from the capacitor C1 connected after the current detection element RO, is a current that flows without passing through the current detection element RO. Therefore, even if a rush current flows to the load side capacitor C2 of the output (■) when the power is turned on, this current does not flow to the current detection element RO and the overcurrent detection signal is is not served. Therefore, since the overcurrent detection circuit A does not operate, a malfunction such as stopping the switching transistor 4 as in the conventional case does not occur.

Figure 2 shows the waveform of the current flowing through the current detecting element RO, and (A) shows the waveform of the current flowing through the current detecting element RO due to the charging current of the capacitor C1 at tl when the power switch is turned on. is the waveform of the current flowing through the current detection element RO due to the charging current of the load-side capacitor C2 at t2 when the switch needle is turned ON. This capacitor C
1 and the value of capacitor C2 on the load side -8-゛The relationship between the values of capacitor C2 and capacitor C2 is that if capacitor C1 is small, the peak current in (a) decreases and the peak current in capacitor C2 increases, and if capacitor C2 on the load side is large, the peak current in (a) increases. The peak current at the mouth increases and the peak current at the mouth decreases. Therefore, the capacitor C1 is approximately equal to the load side capacitor C2.
It is best to select a value that is as low as possible. FIG. 3 shows that in the conventional circuit shown in FIG. 4, the peak current waveform of the current detection resistor 24 due to the charging current of the capacitor 33 on the load side is extremely large at time t2 when the switch 28 is turned ON.

[Effects of the Invention] As described above, in the present invention, since the capacitor is connected after the current detection element, even if a rush current flows to the load side capacitor when the power is turned on, this current does not flow to the current detection element. Therefore, since the overcurrent detection circuit does not operate, a malfunction such as stopping the switching transistor 4 as in the conventional case does not occur.

Furthermore, in the past, it was necessary to raise the overcurrent detection level in order to prevent the above-mentioned malfunction, and this had the disadvantage of placing an excessive burden on the transistor if the load was short-circuited. However, according to the present invention, this can be avoided. There is no need to raise the overcurrent detection level.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of one embodiment of the present invention, FIG.
FIG. 3 is a current waveform diagram of the current detection element, and FIG. 4 is a circuit diagram of a conventional example. △: Overcurrent detection circuit. RO: Current detection element. C1: Capacitor. C2: Load side capacitor.

Claims (1)

[Claims] The power supply circuit has an overcurrent detection circuit on the output side of a switching power supply, and the output is supplied to a capacitor on the load side. Current detection malfunction prevention circuit.