JP2547101Y2 - Overcurrent protection circuit - Google Patents

Description

[Detailed description of the invention]

[0001]

[Industrial applications]

The present invention relates to an overcurrent protection circuit used to prevent damage to a power supply device, and more particularly, to an improved overcurrent protection circuit capable of preventing a malfunction due to an inrush current when power is turned on.

[0002]

[Prior art]

Conventionally, as an overcurrent protection circuit of a power supply device, for example, FIG.
The following are known. The operation of the overcurrent protection circuit will be described below. First, the current value of the input AC current is
, Is converted into a voltage, and then rectified by a rectifier circuit including a diode stack D1 and a resistor R1. After being rectified by this rectifier circuit, the detected signal is
, And R3, and further input to a latch circuit including transistors Q2 and Q3 and a resistor R4.

Here, when the input AC current increases, the detection signal increases, the voltage applied to R3 also increases, and when exceeding a predetermined value, the transistors Q2 and Q3 of the latch circuit are turned on. Then, a low level signal (hereinafter, referred to as an L signal) is output to the PWM control circuit, and the operation of the PWM control circuit and the switching power supply circuit is stopped. Here, since the transistors Q2 and Q3 of the latch circuit form a so-called thyristor connection, once the latch circuit starts to output the L signal, the high level signal (H signal) unless the power is turned off.
Do not return to. In FIG. 3, the capacitor C2 is for preventing malfunction caused by noise, and the capacitor C1 is a smoothing capacitor.

[0004]

[Problems to be solved by the invention]

In the overcurrent protection circuit according to the prior art as described above, the latch circuit is driven low by an inrush current generated when the power is turned on.
A signal may be output, and the malfunction may cause the overcurrent protection circuit to malfunction, thereby causing a problem that the power supply is suddenly shut off. Therefore, an object of the present invention is to provide an overcurrent protection circuit that can reliably prevent the above-described malfunction at power-on in view of the above-described problems in the related art.

[0005]

[Means for Solving the Problems]

That is, in the present invention, in order to achieve the above object, a current detection unit that detects a power supply current flowing on the primary side of the current transformer as a current on the secondary side, and the current on the secondary side of the current transformer has a predetermined value. A delay means for delaying a transient current at the time of power-on due to a transient phenomenon at the time of rising of a voltage on the primary side of the current transformer, wherein And a switching element which is turned on by the delay means for a required time from the time of power-on and releases a current on the secondary side of the current transformer to the ground.

[0006]

[Action]

According to the present invention, the delay means turns on the switching element for a predetermined time immediately after the power is turned on (generation of an inrush current), and releases a signal to be input to the latch circuit to the ground. Does not generate an L signal. For this reason, malfunction of the overcurrent protection circuit due to inrush current is reliably prevented. In addition, while using a current transformer, the overcurrent is detected by the current on the secondary side, while the switching element is turned on by the transient current generated by the transient phenomenon at the rise of the voltage on the primary side of the current transformer. Because it is turned on only for a predetermined time, even if there is no operation signal from the control unit such as a microcomputer, malfunction of the overcurrent protection circuit due to inrush current is reliably prevented, and the overall circuit configuration is simplified. I can do it.

[0007]

【Example】

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a circuit structure of the overcurrent protection circuit according to the present invention. This overcurrent protection circuit, like the conventional overcurrent protection circuit shown in FIG. 3 described above, detects the current value of the input AC current, converts the current value into a voltage, the secondary side of the current transformer CT1, a diode stack D1. And a rectifier circuit composed of a resistor R1 and a current detector 10 composed of a smoothing capacitor C1
have. The smoothing capacitor C1 of the current detection unit 10
Is connected to three voltage-dividing resistors R2, R5, and R3 connected in series, and the voltage-dividing resistor R3 is connected to a capacitor C2 for noise killer. Then, the voltage dividing resistor R5
Is connected to a latch circuit 20 including transistors Q2 and Q3 and a resistor R4.
0, connected to the switching power supply circuit 40.

In the present invention, a switching element having delay means is inserted between the current detection unit 10 and the latch circuit 20. That is, the switching element of this embodiment is a transistor Q1
The delay means comprises a series circuit of C3 and R7 connected to the input of the transistor Q1. Further, a base resistor R6 is further connected between the base terminal of the transistor Q1 and the ground.

The operation of the overcurrent protection circuit will be described. In the above circuit configuration, the input AC current is
After being detected by the CT1 and converted into a voltage, and then rectified by the diode stack D1 and the resistor R1, the detected and rectified signal is divided by the resistors R2, R5 and R3 and input to the latch circuit 20 described above. That is, referring to the waveform diagram of FIG. 2, (a) of FIG. 2 shows a voltage waveform on the primary side of the current transformer CT1 when the power is turned on as t0, and (b) of FIG. The voltage waveform of the detection signal is shown on the secondary side of the current transformer CT1. This detection signal is rectified by the diode stack D1 and the resistor R1, and becomes as shown in FIG.

Here, when the input AC current increases, the detection signal increases, the voltage appearing at R3 (shown in FIG. 2 (c)) also increases, and when this exceeds a predetermined value, the latch circuit
The 20 transistors Q2 and Q3 are turned on, and the PWM control circuit 3
The L signal is output to 0, thereby stopping the operation of the PWM control circuit 30 and the switching power supply circuit 40. Here, since the transistors Q2 and Q3 of the latch circuit 20 form a so-called thyristor connection, once the latch circuit 20 starts to output the L signal, the transistor Q2 and Q3 remain high until the power is turned off.
It does not return to the signal.

Next, when the power is turned on, the primary side voltage is also applied to the series circuit of the capacitor C3 and the resistor R7 constituting the delay means. At this time, due to a transient phenomenon, the series circuit The current i (t) flows according to the following equation: i (t) = (V / R3)） {1-exp (−t / C3R7)}. However, in the above equation, V is the value of the input DC voltage.

[0012] The current i (t) generates a voltage at the base terminal of the transistor Q1, that is, across the base resistor R6, and this voltage rapidly increases at t0 as shown in FIG. It increases, gradually decreases, and returns to 0 again after a certain period of time. At this time, while reaching a value exceeding the threshold value Vth of the transistor Q1 (the voltage value between the base and the emitter when the transistor is turned from the OFF state to the ON state, usually 0.60 V to 0.65 V), as shown in FIG. Thus, the transistor Q1 is turned on (the waveform in the figure indicates the collector voltage). And
During this time, as shown in FIG. 2 (f), a signal (represented by a dashed line) input to the latch circuit 20 is guided to the ground through the ON-state transistor Q1. Is not led, and the output of the latch circuit 20 always becomes the H signal during a predetermined period T after the power is turned on (t0).
That is, the latch circuit 20 outputs an L signal due to a malfunction during this period T and stops the PWM control circuit 30 and the switching power supply circuit 40 during the period T due to the action of the delay means formed by the series circuit of the capacitor C3 and the resistor R7. Thus, it is possible to reliably prevent the overcurrent protection circuit from malfunctioning due to an inrush current generated when the power is turned on.

[0013]

[Effect of the invention]

As is clear from the above description, according to the present invention, it is possible to obtain an excellent overcurrent protection circuit that does not malfunction due to an inrush current generated when power is turned on.

[Brief description of the drawings]

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

FIG. 2 is a signal waveform diagram showing signal waveforms at various parts of the circuit for explaining the operation of the overcurrent protection circuit.

FIG. 3 is a circuit diagram showing an example of a conventional overcurrent protection circuit.

[Explanation of symbols]

 CT1… Current transformer D1… Diode stack R1 ～ R7… Resistance C1 ～ C3… Capacitor Q1 ～ Q3… Transistor 10… Current detector 20… Latch circuit 30… PWM control circuit 40… Switching power supply circuit

Claims (1)

(57) [Scope of request for utility model registration] A current detecting section for detecting a power supply current flowing through a primary side of a current transformer on a secondary side as a current; and stopping a power supply circuit when a current on a secondary side of the current transformer exceeds a predetermined value. A delay circuit for delaying a transient current at the time of power-on due to a transient phenomenon at the time of a rise of a voltage on the primary side of the current transformer, and a delay circuit at the time of power-on due to the delay means. And a switching element that is turned on only for a required time from the second time to release the current on the secondary side of the current transformer to the ground.