JPH06153382A - Protective circuit for switching power supply - Google Patents

Abstract

PURPOSE:To provide a protective circuit for switching power supply in which protective function is exhibited economically in safety upon occurrence of overcurrent and overvoltage by externally adding a latch off circuit to a low function IC having no latch off function. CONSTITUTION:Latch off function for turning power supply OFF upon occurrence of overcurrent and overvoltage is provide additionally for a control IC circuit IC1 having a control output terminal Iout for controlling a switching element Q1 in a switching power supply 1 and a control input terminal Iin for turning the switching element Q1 OFF upon provision of a voltage higher than a predetermined level from an overcurrent detecting circuit. In other words, overvoltage detecting circuits (R6, DZ1, PCo) are provided together with latch off circuits (R1, R2, SCR, R3, PC1) for delivering a voltage higher than a predetermined level to the control input terminal upon receiving an output from the overvoltage detecting circuit and an output from the overcurrent detecting circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overvoltage and overcurrent protection circuit for a switching power supply, and more particularly, it protects against overvoltage by using a control IC having a pulse-by-pulse overcurrent protection function. The present invention relates to a protection circuit for a switching power supply that enables control.

[0002]

2. Description of the Related Art A switching element that performs a switching operation of a primary side current in a switching power supply is controlled by a switching control circuit. The switching control circuit is provided with a protection function for controlling the operation of the switching element by an input from the overcurrent or overvoltage detection circuit in order to deal with the overcurrent or overvoltage. In the switching power supply circuit 20 shown in FIG. 2, the current detection resistor R _D inserted in series with the switching element Q _1.
The overcurrent is detected by the voltage generated at both ends of the. This detection voltage is input to the control input terminal I _in of the control IC circuit IC ₁ through a low pass filter composed of a resistor R _F and a capacitor C _F for removing a surge voltage. When the detected voltage becomes larger than the set threshold voltage V _th to the control IC circuit IC _1, the control IC circuit IC ₁
The pulse-by-pulse control operation for controlling the switching control pulse output from the control output terminal I _out of 1 by 1 pulse unit is performed. That is, when an overcurrent is detected, the output voltage is reduced by narrowing the pulse width of the switching control output, and a large current is prevented from flowing.

[0003]

When the control IC circuit adopted as the switching control circuit as described above has only the overcurrent protection function by the pulse-by-pulse control,
There was a problem that the load side overheats because the short-circuit current continues to flow. Further, the protection function is not exerted in the control IC circuit against overvoltage. To set up a protection circuit that can cope with overcurrent and overvoltage, a high-performance IC with control input terminals corresponding to overcurrent and overvoltage is adopted, and each overcurrent and overvoltage detection circuit is configured. This can be achieved by performing a latch-off operation in which the power supply output is turned off in response to the input from each detection circuit, but of course the cost increase is inevitable. Therefore,
It is an object of the present invention to provide a protection circuit for a switching power supply that externally adds a latch-off circuit to a low-function IC having no latch-off function to economically and safely perform a protection function when an overcurrent or overvoltage occurs. And

[0004]

In order to achieve the above-mentioned object, the means adopted by the present invention is a control output terminal for controlling a switching element for performing a switching operation of a switching power supply, and a voltage higher than a predetermined voltage from an overcurrent detection circuit. In a protection circuit for a switching power supply, which comprises a control IC having a control input terminal for turning off the switching element when a voltage is input, the switching power supply is provided with an overvoltage detection means, and the overvoltage detection means is provided. A latch-off circuit for inputting a voltage equal to or higher than a predetermined voltage to the control input terminal is added to the control IC when the detection output from the control circuit is input and when the detection output from the overcurrent detection circuit is input. It is configured as a protection circuit for a switching power supply.

[0005]

According to the present invention, a control output terminal for controlling a switching element that performs a switching operation of a switching power supply, and a control input for turning off the switching element when a voltage higher than a predetermined voltage is input from an overcurrent detection circuit. A latch-off function for controlling the power supply output to be turned off when an overcurrent and an overvoltage occur can be added to a control IC having a terminal. That is, an overvoltage detecting means is provided, and when this detection output and the detection output from the overcurrent detection circuit are input, a latchoff circuit is provided which generates a voltage higher than a predetermined voltage and inputs the voltage to the control input terminal. ing. Therefore, when an overvoltage and an overcurrent occur, the latch-off circuit operates and a voltage higher than a predetermined voltage is input to the control input terminal, and the control IC turns off the switching element.

[0006]

Embodiments of the present invention will be described below with reference to the accompanying drawings for the understanding of the present invention. The following embodiments are examples of embodying the present invention and do not limit the technical scope of the present invention. FIG. 1 is a circuit diagram showing the configuration of a switching power supply protection circuit according to an embodiment of the present invention. In the figure, elements common to the conventional configuration are given the same reference numerals. In FIG. 1, a switching power supply 1 is configured as a switching power supply that performs switching control of a switching element Q ₁ inserted in series on the primary side of a switching transformer T by a control IC circuit IC ₁ to convert voltage. The power input V _in input to the primary side of the switching transformer T is
An alternating current is output to the secondary side by the switching operation of the switching element Q ₁ , and a power supply output V is output by a rectifying circuit composed of diodes D ₁ and D _{2, an} inductor L _{1 and a} capacitor C _2.
It is output as _out. This power supply output side has a resistor R ₆
And constant voltage diode DZ ₁ and photocoupler light emitting side PC
An overvoltage detection circuit in which _o and are connected in series is provided.
Further, an overcurrent detecting circuit is constituted by a resistor R ₄ for detecting an overcurrent, a resistor R ₅ for eliminating a surge, and a capacitor C ₁ which are inserted in series with the switching element Q ₁ . Further, the power supply voltage V _CC of the control IC circuit IC ₁ is set to the resistance R
_A thyristor SCR is inserted at a position divided by ₁ and a resistor R _2, and a resistor R ₃ is connected in series between the anode and the gate of this thyristor SCR and the light receiving side P of the photocoupler.
C _i is connected to the gate of the thyristor SCR, and the overcurrent detection circuit is connected to the gate of the thyristor SCR to form a latch-off circuit. The control IC circuit IC ₁ has a control output terminal I for controlling switching of the switching element Q _{1.
It has an Out} terminal and a control input terminal I _in that controls the switching output to be turned off when there is a voltage input of a predetermined voltage or more. The control input terminal I _in is originally provided to limit overcurrent by pulse-by-pulse control, but in the present embodiment, the switching control output can be latched off in response to overvoltage and overcurrent. The latch-off circuit is provided as an external additional circuit to input a control voltage. That is, when the cathode of the thyristor SCR is connected to the control input terminal I _in and the thyristor SCR becomes conductive, the voltage divided by the resistors R ₁ and R ₂ is input. This voltage is the value of the resistor R _1, R ₂ is set to be larger than the set in the control IC circuit IC ₁ threshold voltage V _th, when the thyristor SCR is conductive, the control IC circuit IC ₁ is Latch-off control of switching control output. In the above structure, when an overvoltage is generated, it is detected by the overvoltage detection circuit and the photocoupler PC operates, so that an ignition gate current flows through the gate of the thyristor SCR and the thyristor SCR becomes conductive. Further, when an overcurrent occurs, is detected by the overcurrent detection circuit, a voltage generated in the resistor R ₄ is because flows applied ignition gate current to the gate of the thyristor SCR, thyristor SCR is conductive. Therefore, when an overvoltage or an overcurrent occurs, the thyristor SCR is operated by the detection output from each detection circuit, and the latch-off operation for turning off the switching control output of the control IC circuit IC ₁ is performed. Can be protected. Although the overvoltage detection circuit is provided on the secondary side of the switching transformer T in the above embodiment, it may be provided on the primary side.

[0007]

As described above, according to the present invention, an inexpensive control IC having no latch-off function is used to externally add a latch-off circuit corresponding to overcurrent and overvoltage to prevent overcurrent and overvoltage. The latch-off circuit is operated by the detection output from each detection circuit, and the control voltage is input to the control input terminal to perform the latch-off operation. Therefore, an economical and safe protection function is configured by using an inexpensive control IC, and a protection circuit for a switching power supply can be provided at low cost without using a high-performance IC.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of a switching power supply protection circuit according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a configuration of a protection circuit for a switching power supply according to a conventional example.

[Explanation of symbols]

₁ --Switching power supply IC ₁ --Control IC circuit (control IC) I _out --Control output terminal I _in --Control input terminal Q ₁ --Switching element R ₄ , R ₅ , C ₁ --Overcurrent detection Circuit R ₆ , DZ, PC _o ── Overvoltage detection circuit R ₁ , R ₂ , R ₃ , PC _i , SCR ── Latch-off circuit

Claims (1)

[Claims] 1. A control output terminal for controlling a switching element that performs a switching operation of a switching power supply; and a control input terminal for turning off the switching element when there is a voltage input of a predetermined voltage or more from an overcurrent detection circuit. In a protection circuit for a switching power supply including the control IC, an overvoltage detecting means is provided in the switching power supply, and when a detection output from the overvoltage detecting means is input, A protection circuit for a switching power supply, wherein a latch-off circuit for inputting a voltage equal to or higher than a predetermined voltage to the control input terminal when a detection output is input is added to the control IC.