JP3327360B2 - Overcurrent protection device for switching power supply circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overcurrent protection device for a switching power supply circuit.

[0002]

2. Description of the Related Art As shown in FIG. 2, a conventional switching power supply circuit includes a rectifier circuit 1 and a switching transformer 2 connected to each other.
A DC voltage is applied to the drain of the switching FET 30 via the secondary winding 3, the switching FET 30 is turned on and off at a predetermined timing by a control pulse of the oscillator 27, and is excited by a current flowing on the primary side of the switching transformer 2. The electromotive force is taken out by the secondary winding 5, converted into a DC voltage by the rectifier circuit 9, and supplied to the load side via the output terminal 10. Also,
The induced electromotive force generated in the tertiary winding 6 is converted into a DC voltage by the diode 7 and the capacitor 8, supplied as a driving power source for the oscillator 27, and supplied from the rectifier circuit 1 to the oscillator 27 via the starting resistor 4. A DC voltage can be supplied so that the oscillator 27 can be oscillated when the power is turned on.

[0003]

However, in the conventional example, for example, the load circuit connected to the output terminal 33 is short-circuited due to a failure or the like, and an overcurrent flows to the load side, thereby expanding the failure range of the load circuit. Or the switching power supply circuit breaks down due to overcurrent. In addition, modes such as a short-circuit that occurs on the load side have various behaviors depending on the short-circuit state.Overcurrent may flow temporarily, and the current that flows thereafter may decrease.In such a case, the output is continuously output. There has been a demand for the development of an overcurrent protection device for a switching power supply circuit capable of stopping and protecting the circuit. The present invention has been made in view of the above-described problems, and detects an overcurrent on the secondary side, continuously stops output even if the overcurrent is temporary, and protects the circuit. And turn on when power is turned on.
It is an object of the present invention to provide an overcurrent protection device for a switching power supply circuit, which is prevented from becoming abnormal.

[0004]

According to a first aspect of the present invention, an oscillating means is provided on a primary side of a switching transformer, an output oscillated by the oscillating means is taken out from a secondary side, converted into a DC voltage, and output. Switching power supply circuit
An overcurrent detecting means provided on the next side includes: a switching transistor; a first diode having an anode connected to the base of the switching transistor and a cathode connected to the ground via a first resistor; A base control circuit for a switching transistor, comprising a first zener diode having a cathode connected to a DC output circuit on the secondary side of the transformer and having an anode connected to the first resistor; an emitter of the switching transistor; A charge / discharge capacitor connected between the ground, a first thyristor having an anode connected to the voltage dividing point of the DC output circuit and a cathode connected to the emitter, and a DC output circuit on the secondary side of the switching transformer. A second side having a cathode connected and an anode connected to the gate of the first thyristor;
And an output means comprising a photocoupler connected between the collector of the switching transistor and the ground. An overcurrent is detected and fed back to the primary side oscillating means via the output means to stop the oscillation of the oscillating means and to set the Zener voltage of the first Zener diode.
Is lower than the Zener voltage of the second Zener diode.
When the power is turned on, the switching transistor is turned off.
It is characterized in that it does not become in a state .

According to a second aspect of the present invention, the oscillating means comprises a power supply.
Connected to the input circuit via a second starting resistor
And provided in the tertiary winding of the switching transformer.
The oscillator connected to the power supply circuit
Power input circuit via primary winding of switching transformer
, The source side is grounded, and the gate is the oscillator.
The switching FET connected to the
And the cathode is connected to ground,
An anode is connected to the gate side of the switching FET.
And a second thyristor.

[0006]

According to the present invention, the voltage at the voltage dividing point of the DC output circuit, which is normally connected to the anode of the first thyristor, is adjusted to make the voltage at the emitter of the switching transistor higher than the voltage at the base. The switching transistor is turned off by setting it low. When a short circuit or the like occurs on the load side connected to the DC output circuit on the secondary side of the switching transformer and an overcurrent flows, the voltage of the DC output circuit drops and a Zener current flows through the first Zener diode. Disappears, the base voltage of the switching transistor drops, the switching transistor is turned on, current flows through the photocoupler, and the second thyristor connected to the gate of the switching FET is turned on to turn the photocoupler on. In the ON state, the operation of the switching FET is stopped, and the ON / OFF of the switching circuit is stopped,
Since the output of the switching power supply circuit stops, the circuit can be protected. Further, when the required current flows, the thyristor is latched in a state where the current continues to flow, so that the output stop state of the switching power supply circuit is maintained and the circuit can be continuously protected.

[0007]

Embodiments will be described below with reference to the drawings. FIG. 1 is a block diagram of an overcurrent protection device for a switching power supply circuit according to an embodiment of the present invention. In the drawing, the same components as those shown in FIG. 2 are denoted by the same reference numerals. First, a circuit such as an oscillating means provided on the primary side will be described. The rectifier circuit 1 converts an AC input into a DC voltage, inputs the DC voltage to one end of a primary winding 3 of a switching transformer 2, connects the other end of the primary winding 3 to a drain of a switching FET 30, Are connected to ground via a current limiting resistor 32.

Further, power is supplied from the rectifier circuit 1 to the oscillator 27 via the starting resistor 4 to start and oscillate the oscillator 27, and an oscillation signal is supplied to the gate of the FET 30 via the input resistor 31. When the input is made, the FET 30 is turned on and off, and a current flows through the primary winding 3 by turning on and off the FET 30 to excite a voltage on the secondary winding 5 and the tertiary winding 6 of the switching transformer 2. The voltage excited in the tertiary winding 6 of the switching transformer 2 is converted into a DC voltage by the diode 7 and the capacitor 8 and supplied to the oscillator 27 as a drive power source. The oscillator 27 is oscillated. The voltage excited in the secondary winding 5 is converted into a DC voltage by a rectifier circuit 9 composed of a diode and a capacitor, and is output via an equivalent resistor 10 of the rectifier circuit 9 via an output terminal 33 to be output. Are supplied as power to the load side via a regulator circuit or the like, for example.

Next, the overcurrent detecting means provided on the secondary side will be described. The DC output circuit from the rectifier circuit 9 is connected to the cathode of the Zener diode 12 via the current limiting resistor 11, and the anode of the Zener diode 12 is connected to the ground via the biasing resistor 13. A Zener current flows through the diode 12 and the resistor 1
3 is applied as a bias voltage to the base of a switching transistor 15 via a diode 14 for preventing a reverse current. The DC output circuit from the rectifier circuit 9 and the ground are connected in series by a resistor 17 and a resistor 18 to divide the DC voltage. The anode of the thyristor 19 is connected to this voltage dividing point, and the cathode of the thyristor 19 is connected. In addition to being connected to the emitter of the transistor 15, a charging / discharging capacitor 22 is connected between the emitter of the transistor 15 and the ground. The DC output circuit from the rectifier circuit 9 is connected to the cathode of the Zener diode 20 via the current limiting resistor 16, the anode of the Zener diode 20 is connected to the gate of the thyristor 19, and the connection between the gate of the thyristor 19 and the cathode And a zener current through a zener diode 20 to the resistor 2.
1 to turn on the thyristor 19, and charge current flows from the voltage dividing point of the resistor 17 and the resistor 18 to the charge / discharge capacitor 22 via the thyristor 19 to charge the charge / discharge capacitor 22.

For example, the Zener voltage of the Zener diode 12 is made lower than the Zener voltage of the Zener diode 20 so that the Zener current flows through the Zener diode 12 faster than the Zener diode 20 when the power is turned on. before the emitter voltage as the base voltage rises, so that the transistor 15 is not turned on. The voltage at the voltage dividing point of the resistor 17 and the resistor 18 is selected so that the voltage charged in the charge / discharge capacitor 22 becomes lower than the voltage applied to the base of the transistor 15, and the transistor 15 is kept in the off state. I can do it.

When an abnormality such as a short circuit occurs on the load side connected to the output terminal 33 and an overcurrent flows through the DC output circuit on the secondary side of the switching transformer 2, the voltage of the DC output circuit becomes And the Zener current stops flowing through the Zener diode 12 and the transistor 15
, The transistor 15 is turned on. The collector of the transistor 15 is a photocoupler 25
Is connected to the ground via a photodiode 23 of
The photodiode 23 emits light by the collector current of the transistor 15 to output a signal. The power supply circuit for the oscillator 27 is connected to the collector of the phototransistor 24 of the photocoupler 25 via the current limiting resistor 26, the emitter of the phototransistor 24 is connected to the gate of the thyristor 28, and the cathode of the thyristor 28 is connected to ground. The thyristor 28 has an anode connected to the output terminal of the oscillation signal of the oscillator 27, and a bias resistor 29 connected between the gate and the cathode of the thyristor 28.

The phototransistor 24 of the photocoupler 25 is turned on by the light emission of the photodiode 23, a current flows through the bias resistor 29, and the thyristor 28 is turned on to bias the thyristor 28. Thyristor 28
Is turned on, the output terminal of the oscillation signal of the oscillator 27 is grounded, the on / off switch operation of the FET 30 is stopped, and the output of the switching power supply circuit is stopped. In addition to preventing the failure range from expanding, the switching power supply circuit can be protected from failure due to overcurrent. Modes such as a short-circuit that occurs on the load side connected to the output terminal 33 have various behaviors depending on the short-circuit state. Overcurrent temporarily flows, and the current that flows thereafter may decrease. When the current flows, the current is latched in a state where the current continues to flow, so that the output stop state of the switching power supply circuit is maintained and the circuit can be continuously protected.

[0013]

As described above, according to the present invention,
The Zener voltage of the Zener diode 12 is
Lower the Zener voltage of Aether 20 and turn on the power
From the Zener diode 20 to the Zener diode 12
The zener current flows faster. Then, an overcurrent of the DC output circuit on the secondary side is detected by an overcurrent detecting means constituted by a switching transistor 15 and the like provided on the secondary side, and the detection signal is transmitted to the photocoupler 25 and the thyristor 28.
The signal is fed back through the FET 3 provided on the primary side.
Oscillation of an oscillating means composed of a zero and an oscillator 27 is stopped . Therefore, the transistor 15
So that the base voltage rises before the
Prevent transistor 15 from turning on when turning on
It is possible to detect the overcurrent on the secondary side and keep the oscillation stopped state by the thyristor 28 even if the overcurrent is temporary. An overcurrent protection device for a switching power supply circuit capable of protecting a circuit can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram of an overcurrent protection device for a switching power supply circuit, showing one embodiment of the present invention.

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

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Rectifier circuit 2 Transformer 3 Primary winding 4 Resistor 5 Secondary winding 6 Tertiary winding 7 Diode 8 Capacitor 9 Rectifier circuit 10 Resistor 11 Resistor 12 Zener diode 13 Resistor 14 Diode 15 Transistor 16 Resistor 17 Resistance Device 18 Resistor 19 Thyristor 20 Zener diode 21 Resistor 22 Capacitor 23 Photodiode 24 Phototransistor 25 Photocoupler 26 Resistor 27 Oscillator 28 Thyristor 29 Resistor 30 FET 31 Resistor 32 Resistor 33 Output terminal

Claims (2)

(57) [Claims] An oscillation means is provided on a primary side of a switching transformer, an output oscillated by the oscillation means is taken out from a secondary side, converted into a DC voltage and output, and provided on a secondary side. The overcurrent detection means includes a switching transistor, a first diode having an anode connected to the base of the switching transistor, and a cathode connected to the ground via a first resistor.
A switching transistor base control circuit having a cathode side connected to a secondary side DC output circuit and a first Zener diode having an anode connected to the first resistor; a switching transistor base control circuit; And a charging / discharging capacitor connected between the ground and an anode connected to a voltage dividing point of the DC output circuit,
A first thyristor having a cathode connected to the emitter; and a second zener diode having a cathode connected to a DC output circuit on the secondary side of the switching transformer and having an anode connected to the gate of the first thyristor. An emitter control circuit for the switching transistor, and an output means composed of a photocoupler connected between the collector of the switching transistor and the ground. The overcurrent detection means detects an overcurrent of the secondary side circuit, and Overcurrent of the switching power supply circuit that feeds back to the primary side oscillating means via the output means and stops oscillation of the oscillating means.
Current protection device, wherein said first Zener diode is
The Zener voltage to the Zener diode
Voltage when the power is turned on.
The switch is not turned on.
Overcurrent protection device for switching power supply circuit. 2. The power supply circuit according to claim 1, wherein
2 is connected through the starting resistor of
Power supply circuit provided in the tertiary winding of the switching transformer
Oscillator connected to
Connected to the power input circuit through the primary winding of the
Switch with the ground connected and the gate connected to the oscillator
And a gate connected to the output side of the photocoupler.
The cathode is connected to ground and the anode is connected to the switch.
Thyristor connected to the gate side of the switching FET
2. The switch of claim 1, wherein the switch comprises:
Overcurrent protection device for power supply circuit.