JPH08172771A - Overvoltage protecting circuit of switching power supply - Google Patents

Abstract

PURPOSE: To provide the overvoltage protecting circuit of a switching power supply which immediately shuts off the AC line on the primary side after the detection of the overvoltage and prevents the breakdown of a load circuit. CONSTITUTION: A switching power supply circuit has a switching circuit 3 which switches a DC power supply 2 for rectifying the AC from an AC plug 1, a switching transformer 4, and a rectifying circuit 5 which rectifies the output of the transformer. Furthermore, the following circuits are provided. An output- voltage detecting circuit 7 detects the rectified voltage of the rectifying circuit 5. An overvoltage detecting circuit 6 detects the overvoltage of the rectified voltage of the rectifying circuit. A subordinate power supply 10, which in different from the switching power supply, is provided. A relay circuit 8 is driven with the subordinate power supply and turns on/off an AC line inputted into the DC power supply, and a control part 9 controls the relay circuit. The relay circuit 8 is controlled based on the detected signal from the overvoltage detecting circuit 6 and the control signal from the control part 9, and the on/off control of the switching power supply circuit is performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overvoltage protection circuit for a switching power supply and, more particularly, to an on / off control of an AC line.

[0002]

2. Description of the Related Art A conventional overvoltage protection circuit for a switching power supply, as shown in FIG. 4, inputs a detection signal from an overvoltage detection circuit provided on the secondary side of a transformer to a switching control section for controlling the switching circuit, and When this occurs, the transmission was stopped so that switching was not performed. However, the cause of the overvoltage is not limited to the abnormality of the switching circuit.
There are some failures that cannot be cut without cutting the AC line on the primary side, such as transformer insulation failure and failure of the rectifier on the primary side. In such a case, even if the switching oscillator is stopped, the cause of the overvoltage cannot be removed. Therefore, there is a problem that the main load circuit is greatly damaged.

[0003]

SUMMARY OF THE INVENTION The present invention solves the problems described above, and regardless of the cause, immediately after detecting an overvoltage, the AC line on the primary side is disconnected to prevent damage to the main load circuit. It is an object of the present invention to provide an overvoltage protection circuit for a switching power supply that prevents it.

[0004]

To solve the above problems, the present invention provides a switching circuit for switching a DC power supply for rectifying an AC power supply input from an AC line, and a switched signal for receiving an input winding, In a switching power supply circuit including a transformer derived from an output winding and a rectifier circuit that rectifies the output of the transformer, an output voltage detection circuit that detects the rectified voltage of the rectifier circuit, and an overvoltage of the rectified voltage of the rectifier circuit An overvoltage detection circuit for detecting and a sub power supply different from the switching power supply are provided, and a relay circuit driven by the sub power supply to turn on / off an AC line input to the DC power supply, and control for controlling the relay circuit And a relay circuit is controlled by a detection signal from the overvoltage detection circuit and a control signal from the control unit. To the which the switching power supply circuit is turned on / off control.

[0005]

With the above configuration, according to the overvoltage protection circuit for the switching power supply of the present invention, when the power is turned on, the relay circuit is controlled by the control signal from the control unit to supply the power from the AC line. When an overvoltage of the power supply voltage connected to the load circuit is detected, the detection signal immediately controls the relay circuit to cut off the power supply from the AC line, and also feeds back the overvoltage detection signal to the control unit to detect an abnormality. I try to let you know.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An overvoltage protection circuit for a switching power supply according to the present invention will be described in detail below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of an overvoltage protection circuit for a switching power supply according to the present invention. In the figure, 1 is an AC plug connected to an AC line, and 2 is a rectifier for rectifying an AC power source. Reference numeral 3 denotes a switching control unit that generates a switching signal, and the detection voltage from the output voltage detection unit 7
The output voltage is controlled to be constant. A switching transformer 4 insulates the primary side from the secondary side and outputs the switching signal from the switching control unit by boosting or reducing the pressure. Reference numeral 5 is a rectifying circuit, which is composed of a rectifying diode 5a and a smoothing capacitor 5b, and rectifies and smoothes the switching signal to supply a DC power supply to the load circuit. An overvoltage detection circuit 6 monitors the voltage from the rectifying circuit 5 to detect an overvoltage. The output voltage detection circuit 7 detects the voltage supplied to the load circuit and feeds it back to the switching controller. A relay circuit 8 turns on / off the AC line input to the rectifier 2 by a detection signal from the overvoltage detection circuit 6 and a control signal from the control unit 9.
Reference numeral 9 denotes the control unit, which is operated by operating the operation switch 9a or by inputting a detection signal from the overvoltage detection circuit 6.
The relay circuit 8 is controlled and an abnormality or the like is displayed on the display unit 9b. Reference numeral 9b denotes the display unit, which displays a power supply control state from the control unit 9, a power supply abnormality state, and the like. Reference numeral 10 denotes a sub power supply unit that supplies power to the relay circuit 8 and the control unit.

FIG. 2 is a circuit diagram showing an embodiment of an overvoltage detection circuit 6 of an overvoltage protection circuit for a switching power supply according to the present invention. In the figure, 61 is a resistor (R1) and 62 is a zener diode (Dz1). , 63 is a resistor (R2),
64 is a Zener diode (Dz2), 65 is a PNP transistor (Tr1), and 66 is a resistor (R3). Also,
Reference numeral 67 in FIG. 2B is a photocoupler (PC).

FIG. 3 is a circuit diagram showing an embodiment of a relay circuit of an overvoltage protection circuit for a switching power supply according to the present invention. In the figure, 81 is a relay for turning on / off the AC line, which is composed of a switch portion 81a and an electromagnetic coil 81b. Reference numeral 82 denotes a diode (D2) which prevents a back electromotive force from the electromagnetic coil 81b of the relay 81. A NPN transistor (Tr2) 83 drives the relay 81. Reference numeral 84 is a resistor (R4), which is a base resistance of the transistor (Tr2) 83. Reference numeral 85 is an AND circuit. Reference numeral 86 is a holding circuit, 86a in FIG. 3-a is a flip-flop circuit, and FIG. 3-b is a one-shot circuit. 87 is a resistor (R5), which supplies power to the receiving portion of the photocoupler 67. 88 is a photo coupler 67
The detection signal of the photo coupler 67 is received by the receiving section of the.

The operation of the above arrangement will be described below. To turn on the normal switching power supply, the operation switch 9a provided in the control unit 9 is turned on, and the control unit 9 inputs a Hi-level control signal to one input terminal of the AND circuit 85 of the relay circuit 8 to perform AND operation. The holding circuit 86 connected to the other terminal of the circuit 85 is reset to set the output of the AND circuit to the Hi level, the transistor (Tr2) 83 of the relay circuit 8 is turned on, and the relay 81 is turned on.
AC power is supplied to the rectifier 2 from the C line. The rectifier 2 rectifies an AC power source, converts it into a direct current, and inputs it to the switching control unit 3. In the switching control unit 3,
Generates a switching signal to switch the DC power supply,
The pressure is reduced by the switching transformer 4. The reduced pressure signal from the switching transformer 4 is rectified by the rectifier circuit 5 and used as a DC power source to supply power to the load circuit while being input to the overvoltage detection circuit 6 and the output voltage detection circuit 7, and the output voltage detection circuit 7 The voltage detection signal to be detected is fed back to the switching controller 3 so that the output voltage supplied to the load circuit is controlled to be constant. The overvoltage detection circuit 6 does not output a detection signal when the overvoltage is not generated.

Next, the operation of the overvoltage detection circuit 6 will be described with reference to FIG.
An example of the overvoltage detection circuit a will be described. When the output voltage is normal, the Zener diode (D
z1) a predetermined current flows through 62, and the Zener diode (D
z1) 62 has a constant voltage (Ez1) with respect to GND, while the emitter voltage of the transistor (Tr1) 65 is a constant voltage (Ez2) with respect to the output voltage (B) due to the Zener diode (Dz2) 64. Is kept at.
Since the sum of the Zener voltages (Ez1) and (Ez2) is set higher than the output voltage (B), a reverse bias is applied between the base and the emitter of the transistor (Tr1) 65, and the emitter and the collector are connected. Current does not flow, and the collector voltage is at Lo level in the off state. Now
When an overvoltage (B +) is generated for some reason in the output voltage and the overvoltage (B +) becomes higher than the sum of the Zener voltage (Ez1) and (Ez2), forward bias is applied between the base and emitter of the transistor (Tr1) 65. , A current flows between the emitter and the collector, the collector voltage becomes Hi level, and it is output as an overvoltage detection signal.

In the example of the overvoltage detection circuit shown in FIG.
Zener voltages (Ez1) and (Ez2) as in the case of -a.
The sum of the voltage is higher than the normal output voltage (B),
No current is flowing through the photo coupler (PC) 67,
When the overvoltage (B +) is generated, a current flows through the resistor (R3) 63 to the photocoupler (PC) 67 and outputs it as an overvoltage detection signal.

The operation of the relay circuit 8 will be described below with reference to the example of FIG. The overvoltage detection signal detected by the overvoltage detection circuit as described above is input to the relay circuit 8 and the control unit 9 in FIG. The overvoltage detection signal input to the relay circuit 8 is a flip-flop (FF) 86a.
Is inverted to change the output from Hi level to Lo level A
The output of the ND circuit is set to Lo level, and the transistor (Tr
2) By cutting off 83, the current flowing through the electromagnetic coil 81b of the relay 81 is cut off, and the switch unit 8
By opening 1a, the AC line is opened and the power supply to the rectifier 2 is cut off. On the other hand, the control unit 9
When an overvoltage detection signal is input to, a power-off control signal is output to the relay circuit so that the off state is maintained, and a message indicating that an abnormality has occurred is sent to the display unit 9b to display the message. are doing.

The example of FIG. 3-b will be described below. In the case of FIG. 3B, the overvoltage detection signal is received by the receiving unit 88 of the photocoupler, and a current is passed through the resistor (R5) to obtain Lo.
Level, the one-shot circuit 86b is inverted for a certain period of time, the output is changed to the Lo level and is input to the control unit 9 and the AND circuit 85, and the output of the AND circuit 85 is kept for the certain period of time Lo.
As a level, set the base voltage of the transistor (Tr2) to L
By turning off the transistor (Tr2) 83, cutting off the current flowing through the electromagnetic coil 81b of the relay 81, and opening the switch 81a,
The line is opened to cut off the power supply to the rectifier 2 for a certain period of time. On the other hand, the signal from the one-shot circuit 86b returns to the Hi level after a certain period of time, but the signal from the one-shot circuit 86b is input to the control unit 9 and the control unit 9 inputs the signal as described above. Then, the power-off control signal is input to the AND circuit 85, and the AND circuit 85
Holds the output of Lo at the Lo level, and the transistor (Tr2)
The base voltage of the transistor is held at Lo level, and the transistor (T
r2) Holds the cut-off state of 83, holds the interruption of the current flowing through the electromagnetic coil 81b of the relay 81, opens the switch section 81a, opens the AC line, and cuts off the power supply to the rectifier 2 continuously. I am trying to do it. Further, the control unit 9 sends a message indicating that an abnormality has occurred on the display unit 9b and displays the abnormality message on the display unit 9b.

[0014]

As described above, according to the overvoltage protection circuit for the switching power supply of the present invention, when the power is turned on, the relay circuit is controlled by the control signal from the control unit to supply the power from the AC line. The switching power supply is operated to supply power to the load circuit, and when an overvoltage of the output voltage of the switching power supply is detected, the detection signal immediately controls the relay circuit to cut off the power supply from the AC line and to the control section. Since the overvoltage detection signal is fed back to notify the abnormality, regardless of the cause, if an overvoltage is detected, the AC line on the primary side is immediately disconnected to prevent damage to the main load circuit. A circuit can be provided.

[Brief description of drawings]

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

FIG. 2 is a circuit diagram showing an embodiment of an overvoltage detection circuit of an overvoltage protection circuit for a switching power supply according to the present invention.

FIG. 3 is a circuit diagram showing an embodiment of a relay circuit of an overvoltage protection circuit for a switching power supply according to the present invention.

FIG. 4 is a block diagram showing a conventional overvoltage protection circuit for a switching power supply.

[Explanation of symbols]

 1 AC plug 2 Rectifier 3 Switching control unit 4 Switching transformer 5 Rectifier circuit 6 Overvoltage detection circuit 7 Output voltage detection circuit 8 Relay circuit 9 Control unit 9a Operation unit 9b Display unit 10 Sub power supply unit 61 Resistor 62 Zener diode 63 Resistor 64 Zener diode 65 NPN transistor 66 Resistor 67 Light emitting part of photocoupler 81 Relay 81a Switch part 81b Electromagnetic coil 82 Diode 83 PNP transistor 84 Resistor 85 AND circuit 86a Flip-flop 86b One-shot circuit 87 Resistor 88 Photocoupler reception Department

Claims (7)

[Claims] 1. A switching circuit for switching a DC power source for rectifying an AC power source input from an AC line,
An output voltage detection circuit for detecting a rectified voltage of the rectifier circuit, in a switching power supply circuit including a transformer that receives a switched signal in an input winding and derives from the output winding, and a rectifier circuit that rectifies an output of the transformer An overvoltage detection circuit for detecting an overvoltage of the rectified voltage of the rectification circuit, and a sub-power supply different from the switching power supply, and a relay driven by the sub-power supply to turn on / off an AC line input to the direct-current power supply Circuit and a control section for controlling the relay circuit are provided, and the relay circuit is controlled by the detection signal from the overvoltage detection circuit and the control signal from the control section to control ON / OFF of the switching power supply circuit. An overvoltage protection circuit for a switching power supply. 2. The relay circuit turns on / off an AC line.
A relay that is turned off, a drive circuit that drives the relay, a latch circuit that holds a detection signal from the overvoltage detection circuit, a latch signal from the latch circuit, and a control signal from the control unit are ANDed, and An overvoltage protection circuit for a switching power supply according to claim 1, comprising an AND circuit for inputting an output to the base of the transistor. 3. The relay circuit turns on / off an AC line.
A relay that turns off, a drive circuit that drives the relay, a one-shot circuit that holds the detection signal from the overvoltage detection circuit for a certain period of time, and an AND of the signal from the one-shot circuit and the control signal from the control unit. And an AND circuit for inputting the output to the base of the transistor, and the detection signal from the overvoltage detection circuit or the signal from the one-shot circuit is input to the control unit. 1. An overvoltage protection circuit for a switching power supply according to 1. 4. The overvoltage detection circuit comprises a resistor R1 and a constant voltage diode (Dz1) arranged in series and connected between output terminals of the rectifier circuit, and a constant voltage diode (Dz1).
z2) is connected to the emitter of the transistor (Tr1) and the collector of the transistor (Tr1) is connected to a resistor (R3) between the output terminals of the rectifying circuit, and the resistor (R1) and the constant voltage diode ( 2. A switching power supply according to claim 1, wherein a resistor (R2) is connected to the connection terminal of Dz1) and the base of the transistor (Tr1), and a detection signal is output from the collector of the transistor (Tr1). Overvoltage protection circuit. 5. The overvoltage detection circuit comprises a constant voltage diode (Dz1) and a resistor (R1) arranged in series and connected between the output terminals of the rectifier circuit, and a constant voltage diode (Dz2) and a resistor. (R3) are arranged in series and connected between the output terminals of the rectifier circuit, and the constant voltage diode (D
z1) and a resistor (R1) connecting terminal, the resistor (R3) and the constant voltage diode (Dz2) connecting terminal, a resistor (R2) and an input terminal of a photocoupler (PC) are connected in series, and 2. An overvoltage protection circuit for a switching power supply according to claim 1, wherein a detection signal is output from a coupler (PC). 6. The overvoltage protection circuit for a switching power supply according to claim 1, wherein the control unit is provided with a display unit, and when an overvoltage is detected, it is displayed that an abnormality has occurred. 7. The drive circuit includes a transistor for driving a winding of the relay, a back electromotive force protection diode, and
An overvoltage protection circuit for a switching power supply according to claim 2, wherein the overvoltage protection circuit comprises a base resistor.