JPH07163044A - Over-voltage protective circuit in switching power supply - Google Patents

Abstract

PURPOSE:To make it possible to reduce a voltage for a load to a safe level, by lightening a luminous element and stopping the power for the load when a voltage regulating element is charged with electricity. CONSTITUTION:When a voltage applied to a load rises over a given level, the power for the load is stopped while a voltage regulating element ZD is energized to lighten a luminous element PCO. Then, a switching element QSW receives a trigger signal, and a capacitor Co is short-circuited by the switching element QSW. Immediately, a remaining charge in the capacitor Co is discharged through resistors R1 and R2 that are connected in series with the switching element QSW. Consequently, the voltage to the load can be promptly reduced to a given safe level.

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.

[0002]

2. Description of the Related Art FIG. 2 is a circuit diagram showing a configuration of a conventional switching power supply. In the conventional switching power supply, the AC power supply AC _IN is rectified and this is rectified by the controller CN.
The primary side of the transformer T is switched by switching at T, the voltage of this primary side is transformed into the secondary side power source, and the DC output voltage of V _OUT is obtained from this secondary side power source, and this is supplied to the load. I have to.

Then, an overvoltage is applied to the load side.
In order to prevent this, a resistor R, a Zener diode ZD,
And overvoltage detection consisting of light emitting device PCO of photocoupler
Switching control according to the circuit 11 and the detection of overvoltage
Overvoltage protection circuit consisting of control circuit 12 to stop
Is provided. In addition, D₀₁Is a rectifier circuit, C _IN
And C_OIs a capacitor, Q_SWIs a switching element, PC
I is the light receiving element of the photo coupler, D₁And D₂Is Daio
C and L are coils.

In this switching power supply circuit, in a normal operation, the DC output voltage V _OUT on the secondary side is controlled by the primary side and kept at a predetermined voltage. However, abnormality, for example, at the time of short-circuit of the diode D ₁ and the coil L, the DC output voltage V _{OU T} rose to several times the voltage,
It becomes overvoltage.

At this time, in the overvoltage detection circuit 11, V
_{Under the condition of OUT} > V _ZD + V _F (V _ZD is the Zener voltage of the Zener diode ZD, V _F is the forward voltage drop of the light emitting element PCO of the photocoupler), it is assumed that the DC output voltage V _OUT becomes an overvoltage. Is detected, a forward current flows through the light emitting element PCO of the photocoupler. This signal is the light receiving element P of the photo coupler on the controller CNT side.
Informed to CI.

Upon receiving this signal, the controller CNT stops the on / off control to the switching element Q _SW (stops the oscillation), and thereby protects the secondary side from the abnormal voltage continuously. The movement is designed to work.

[0007]

[Problems to be Solved by the Invention]
Such a conventional switching power supply circuit has several hundred watts to several watts.
When using a large-capacity power supply of kilowatts,
Densa C_OSince the capacity of the
At the time of occurrence, even if the controller CNT stops oscillating,
Although the output voltage drops to a safe voltage, the discharge time is timed.
Number C_O× R_L(R _LIs the time proportional to the internal resistance of the load)
Is required.

FIG. 3 is a circuit diagram showing the voltage V _O due to the capacitor C _O when the controller CNT stops oscillating. As shown in this figure, the voltage V _O is V _O = V _PEAK (1-e ↑ (−1 / (C _O × R _L ))).

In order to avoid such a situation, there is a crowbar circuit in which an output is short-circuited by using a thyristor when an overvoltage occurs, but when this circuit is used, a large capacity thyristor is required.

The present invention has been made in consideration of such circumstances, and the voltage of the output terminal can be immediately lowered to a safe voltage range even if the overvoltage protection function works in a state close to no load. An overvoltage protection circuit for a switching power supply is provided.

[0011]

SUMMARY OF THE INVENTION According to the present invention, a power supply means for supplying power to a load, a capacitor connected in parallel with the load, a capacitor connected in parallel with the capacitor, and a trigger signal being applied thereto are short-circuited. Switching element, a resistor that is connected in series with the switching element, limits the current flowing through the switching element, and conducts when the voltage applied to the load exceeds a certain voltage,
A constant voltage element that gives a trigger signal to the switching element, a light emitting element that is connected in series with the constant voltage element and emits light when the constant voltage element is turned on, and control that stops the power supply to the load by the light emission of the light emitting element It is an overvoltage protection circuit for a switching power supply comprising means.

The switching element in the present invention may be any element that can short-circuit the capacitor when a trigger signal is applied, such as a thyristor, triac,
An SSS (silicon symmetrical switch), a Shockray diode, or the like can be used, but a thyristor is mainly preferably used.

As the constant voltage element, any element can be used as long as it can conduct when the voltage applied to the load exceeds a certain voltage and give a trigger signal to the switching element, and a Zener diode, a diac or the like is used. However, a Zener diode is usually used.

Various phototransistors can be used as the light emitting element, but in order to reliably transmit and receive light, a photocoupler type light emitting element in which the light emitting side and the light receiving side are incorporated together is used. It is preferable.

The control means may be any means capable of stopping the power supply to the load by the light emission of the light emitting element, and includes various controllers such as a microprocessor and various switches for disconnecting the power circuit. A combination is used.

[0016]

According to the present invention, when the voltage applied to the load exceeds a certain voltage, the constant voltage element conducts and the light emitting element emits light, the power supply to the load is stopped, and at the same time, the switching is performed. A trigger signal is given to the element. The switching element shorts the capacitor when the trigger signal is applied. As a result, the electric charge stored in the capacitor is discharged through the resistor connected in series with the switching element. Therefore, even if the voltage applied to the load exceeds a certain voltage and the power supply to the load is stopped, the residual charge of the capacitor can be immediately discharged, so the voltage applied to the load can be immediately safe. Voltage can be reduced to a certain level.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the embodiments shown in the drawings. The present invention is not limited to this.

FIG. 1 is a circuit diagram showing the construction of an embodiment of the present invention.
It is a road map. In this figure, AC _INPowers the load
AC power supply, D₀₁Is a rectifier circuit, C_INIs a rectified electric
The primary side capacitor, PCI connected in parallel with the
Light receiving element of photo coupler, Q_SWIs a switching element, CN
T is a switching element Q_SWControl to control
It's Lara.

The controller CNT normally performs switching control (ON / OFF control) of the DC power supply via the switching element Q _SW , and when the overvoltage occurs, the ON / OFF control to the switching element Q _SW is stopped (oscillation). To stop the power supply to the load. T is a transformer, D
₁ and D ₂ are diodes, L is a coil, and C _O is a secondary side capacitor connected in parallel with a load.

The SCR is a thyristor, and this thyristor SCR is connected in parallel with the load and short-circuits the load by applying a trigger signal. R ₂ is a resistor that is connected in series with the thyristor SCR and limits the current flowing through the thyristor SCR.

ZD is a Zener diode, which conducts when the voltage applied to the load exceeds a certain voltage, that is, when it becomes an overvoltage, and gives a trigger signal to the gate of the thyristor SCR. R ₁ is a resistor connected in series with the Zener diode ZD.

PCO is a light emitting element of a photocoupler,
The light emitting element PCO is connected in series with the Zener diode ZD and emits light when the Zener diode ZD is turned on.

The resistor R ₂ and the thyristor SCR,
The resistor R ₁ , the Zener diode ZD and the light emitting element of the photocoupler constitute the overvoltage detection circuit 1, and the light receiving element PCI of the photocoupler, the controller CNT and the switching element Q _SW constitute the control circuit 2. The overvoltage detection circuit 1 and the control circuit 2 constitute an overvoltage protection circuit.

In the switching power supply circuit having such a configuration, the rectified DC power supply is switched by the controller CNT to be the primary side of the transformer T, and the voltage of this primary side is transformed to the secondary power supply, A DC output voltage of V _OUT is obtained from this secondary power source and is supplied to the load.

In a normal operation of this circuit, the DC output voltage V _OUT on the secondary side is controlled by the primary side and kept at a predetermined voltage. However, when an abnormality occurs, for example, when the diode D ₁ or the coil L is short-circuited, the DC output voltage V
_OUT rises to several times the voltage and becomes an overvoltage.

At this time, in the overvoltage detection circuit 1, the DC output voltage V _OUT rises and V _OUT > V _ZD + V _F (V _ZD is the zener voltage of the zener diode ZD, and V _F is the light emitting element PCO of the photocoupler. Forward voltage drop),
The DC output voltage V _OUT is detected as an overvoltage, and a forward current flows through the light emitting element PCO of the photocoupler. This signal is transmitted to the light receiving element PCI of the photo coupler on the controller CNT side.

Upon receiving this signal, the controller CNT suspends the on / off control to the switching element Q _SW , so that the protection operation works so that the abnormal voltage is not continuously applied to the secondary side. Has become.

At the same time, a trigger current flows from the light emitting element PCO of the photocoupler to the gate of the thyristor SCR, and the thyristor SCR is turned on. When the thyristor SCR is turned on, the electric charge accumulated in the secondary side capacitor C _O is discharged through the resistor R ₂ connected in series with the thyristor SCR. Therefore, the DC output voltage V _OUT immediately drops to a safe voltage between the DC output voltage terminal and the ground terminal even if the load is almost unloaded (the internal resistance of the load R _L = ∞).

In this way, when the secondary side capacitor C _{O has} a capacitance of tens of thousands of μF, even when the overvoltage protection function operates in a state of almost no load, the voltage V
_OUT can be immediately _pulled down to a safe voltage range. As a result, it is possible to prevent the residual charge of the secondary side capacitor C _O from affecting the load side at the time of overvoltage. For example, there is an integrated circuit or the like that is easily destroyed by a high voltage on the load side. However, the destruction can be prevented.

[0030]

According to the present invention, when the voltage applied to the load becomes equal to or higher than a certain voltage, the voltage applied to the load is immediately safe even if the protection function operates in a state close to no load. It can be reduced to a wide voltage range. Further, since the resistor is connected in series with the switching element, the current flowing through the switching element can be limited, and it is not necessary to use a large capacity switching element. As a result, since the switching element is downsized, the power supply device can be downsized and the cost can be reduced. Furthermore, since the light emitting element is connected in series with the constant voltage element, the voltage control can be stopped and the discharge can be performed at the same time.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a configuration of a conventional switching power supply.

FIG. 3 is a circuit explanatory diagram showing a voltage when a controller stops oscillation.

[Explanation of symbols]

AC _IN AC power supply C _IN Primary side capacitor CNT controller C _O Secondary side capacitor D ₀₁ Rectifier circuit D ₁ , D ₂ Diode L coil Coil photo detector light receiving element PCO Photo coupler light emitting element Q _SW switching element R ₁ , R ₂ resistance SCR thyristor T transformer ZD Zener diode

Claims (1)

[Claims] 1. A power supply means for supplying power to a load, a capacitor connected in parallel with the load, a switching element connected in parallel with the capacitor and short-circuiting the capacitor when a trigger signal is applied, and a switching element. A resistor that is connected in series and that limits the current that flows in the switching element, a constant voltage element that conducts when the voltage applied to the load exceeds a certain voltage, and that provides a trigger signal to the switching element, and a constant voltage element. An overvoltage protection circuit for a switching power supply comprising a light emitting element that is connected in series and emits light when a constant voltage element is turned on, and a control unit that stops power supply to a load by the light emission of the light emitting element.