JPH0315421B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to overvoltage protection of a single-stone forward type DC-DC capacitor, and more specifically, when an overvoltage is detected, the transformer is switched off during the non-conducting period of the switching transistor. This invention relates to an overvoltage protection circuit for a single-stone forward converter in which the output of the converter is reduced by short-circuiting the secondary windings of the converter.

[Prior art] A single-stone forward converter is a type of self-excited switching regulator that uses blocking oscillation, has a small number of parts, and can provide more output power than a ringing chain-yoke converter. Because of this advantage, it is used in power supplies with relatively small output capacity.

However, when the power supply input voltage rises abnormally, or when a failure occurs in each main part of the constant voltage power supply and the constant voltage control circuit ceases to function, the output voltage increases. When such an overvoltage condition occurs, problems such as damage to the output smoothing capacitor, abnormal heat generation of the switching transistor, resistor, etc., and damage to the load occur. In particular, large capacity capacitors for output smoothing are filled with a liquid electrolyte material such as etching glycol, so there is a risk that the capacitors may become punctured and burnt out, causing a major accident. Therefore, if an overvoltage occurs, it is necessary to detect it and immediately reduce the output. For this reason, single-stone forward converters also incorporate an overvoltage protection circuit.

A conventional overvoltage protection circuit is configured as shown in FIG. 2, for example. In the same figure, a portion surrounded by a broken line and marked with the reference numeral 11 is a conventional overvoltage protection circuit. That is, the transformer T is provided with a detection winding _N1 ,
The output is rectified by diode D ₃ , and
Capacitor _C1 performs a peak charge, and its output is applied to Zener diode _ZD1 for overvoltage detection. On the other hand, the primary input section of the converter has a resistor R ₂ ,
A series circuit consisting of a thyristor SCR ₁ and a resistor R ₃ is inserted, and the terminal voltage of the resistor R ₃ is connected to an overvoltage protection transistor Q ₂ connected between the base and emitter of an oscillation switching transistor Q ₁ .
applied to the base of The configuration is such that the current flowing through the Zener diode ZD ₁ for overvoltage detection is supplied to the gate of the thyristor SCR ₁ .

While the capacitor is operating normally, both the thyristor SCR ₁ and the overvoltage protection transistor Q ₂ are in an off state. However, if an overvoltage condition occurs due to some reason, an abnormally high voltage will be induced in the detection winding _N1 , causing the overvoltage detection Zener diode _ZD1 to conduct, thereby causing the thyristor to
SCR ₁ will fire and conduct. Therefore, current flows through the resistor _R3 , causing a voltage drop, and the terminal voltage is applied to the base of the overvoltage protection transistor _Q2 . As a result, the overvoltage protection transistor _Q2 becomes conductive, lowering the base potential of the switching transistor _Q1 of the converter and stopping its oscillation operation.

[Problems to be Solved by the Invention] However, with this configuration, the converter main body has a simple configuration with an extremely small number of parts, whereas the overvoltage protection circuit consists of resistors R ₁ to _{R 4} ,
Thyristor SCR ₁ , overvoltage protection transistor
The number of components such as Q ₂ , Zener diode ZD ₁ for overvoltage detection, diode D ₃ , capacitor C ₁ and transformer detection winding N ₁ is significantly increased, which is a major factor in increasing costs. Also, since the number of parts is large and the circuit is complicated, it is difficult to assemble.
There were drawbacks such as reduced reliability.

The object of the present invention is to overcome the above-mentioned drawbacks of the prior art, and to provide an improved one-stone system which has a very small number of components and a simple circuit, which is therefore highly reliable, easy to assemble, and can be miniaturized. An object of the present invention is to provide an overvoltage protection circuit for a forward converter.

[Means for Solving the Problems] The present invention includes a transformer whose primary winding and secondary output winding have the same polarity, a switching transistor connected in series to the primary winding, and a rectifier diode.
It is equipped with a secondary side rectifying and smoothing circuit consisting of D ₁ , D ₂ , a chiyoke coil, and a smoothing capacitor, and the rectifier diode D ₁ is made conductive by the voltage induced in the secondary output winding when the switching transistor is conductive. This is an overvoltage protection circuit for a single-stone forward converter in which the current in the choke coil continues to flow through the rectifier diode _D2 when the switching transistor is non-conducting. In order to achieve the above object, the present invention includes:
A thyristor is inserted between the secondary output windings of the transformer, and an overvoltage detection element is connected between the secondary rectified and smoothed output and the gate of the thyristor,
When overvoltage is detected, the thyristor is fired by the overvoltage detection element, and during the non-conducting period of the switching transistor, the secondary output winding of the transformer is short-circuited by the thyristor, and the conducting period of the switching transistor is short-circuited. It is configured to short circuit and reduce the output voltage.

Here, as the overvoltage detection element connected between the secondary side rectified and smoothed output and the gate of the thyristor, a Zener diode having a predetermined constant voltage characteristic that becomes conductive at the time of overvoltage is usually used. It is also desirable to insert a diode in series with the thyristor to protect the thyristor from reverse voltage.

[Function] With this configuration, if constant voltage control becomes impossible for some reason and an excessive voltage occurs at the output, the overvoltage detection element, such as a Zener diode, becomes conductive, which causes a trigger current to flow to the thyristor. current, causing the thyristor to conduct.
As a result, the secondary output windings are short-circuited during the period when the switching transistor is required to conduct, so no voltage is generated on the secondary side during non-conduction, so the conduction period of the switching transistor is shortened. , the output voltage decreases. Such an operation not only protects the load but also prevents damage to the converter itself.

[Example] Hereinafter, the present invention will be explained in more detail based on the drawings. FIG. 1 is a circuit diagram showing an embodiment of a single-stone forward converter incorporating an overvoltage protection circuit according to the present invention. The structure and operation of the single-stone forward converter itself may be exactly the same as those conventionally known. Feedback winding of transformer T
Switching transistor _Q1 is turned on and off using the induced voltage of _P2 to convert direct current to alternating current,
The voltage is converted by the collector winding _P1 and the secondary output winding _S1 , and the output is rectified and smoothed by the rectifier diodes _D1 , _D2 , the chiyoke coil L, and the smoothing capacitor C. To stabilize the output voltage,
This can be achieved by providing a control transistor _Q3 between the base and emitter of the switching transistor _Q1 and applying a control signal to the base of the control transistor _Q3 .

Now, in FIG. 1, the part surrounded by the broken line is the overvoltage protection circuit 1 according to the present invention. As is clear from this circuit diagram, the overvoltage protection circuit 1 according to the invention includes a diode D and a thyristor between both terminals of the secondary output winding _S1 of a single-stone forward converter.
In addition to inserting a series connection with the SCR, the output of the secondary side York coil L and the thyristor SCR
A Zener diode ZD is connected as an overvoltage detection element between the gate terminal of the
This configuration has a resistor R connected between the gate and cathode terminals of the SCR.

This overvoltage protection circuit operates as follows.
Now, suppose that for some reason the constant voltage operation of the single-stone forward converter becomes uncontrollable and the output voltage increases. When the increased output voltage exceeds the voltage value set as overvoltage, the Zener diode ZD becomes conductive, thereby causing the thyristor to
Ignite the SCR to make it conductive. Then, the secondary output winding _S1 of the transformer T is short-circuited through the thyristor SCR and the diode D in their forward direction. As a result, during the non-conducting period of the switching transistor _Q1 , the secondary output winding S is short-circuited, and no voltage is generated on the secondary side during the non-conducting period. A transformer operates so that the amount of change in magnetic flux per cycle is zero (so that the positive and negative areas are equal), so if no voltage is generated during non-conduction, the conduction time will be narrowed, and as a result, The output voltage is reduced, and it is possible to protect the load and prevent damage to the converter itself.

Here, the diode D inserted in series with the thyristor SCR is not necessary in principle, but it protects the thyristor from being subjected to an excessive reverse voltage during normal operation. It is something that fulfills a function. Further, the resistor R inserted between the gate and cathode of the thyristor is a gate parallel resistor, and functions to ensure the withstand voltage of the thyristor and prevent malfunction.

[Effects of the Invention] Since the present invention is an overvoltage protection circuit for a single-stone forward converter configured as described above, the number of parts is extremely small, the circuit configuration is simplified, and therefore reliability is extremely high and assembly is easy. It is also easy to use, and can produce excellent effects such as miniaturization.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of a one-channel forward converter incorporating an overvoltage protection circuit according to the present invention, and FIG. 2 is a circuit diagram of a conventional technology. 1, 11...Overvoltage protection circuit, T...Transformer, _Q1 ...Switching transistor for oscillation,
S ₁ ... Secondary output winding, D ₁ , D ₂ ... Rectifier diode, L ... Chiyoke coil, C ... Smoothing capacitor, SCR ... Thyristor, ZD ... Zener diode for overvoltage detection.

Claims (1)

[Claims] 1. A transformer whose primary winding and secondary output winding are of the same polarity, a switching transistor connected in series to the primary winding, rectifier diodes D ₁ and D ₂ , a chiyoke coil, and a smoothing capacitor. The rectifier diode _D1 is made conductive by the voltage induced in the secondary output winding when the switching transistor is turned on, supplying power to the load through the choke coil, and the switching transistor is turned off. In a single-stone forward converter in which the current in the choke coil continues to flow through the rectifier diode _D2 , a thyristor is inserted between the secondary output windings, and a thyristor is inserted between the secondary rectified smoothed output and the gate of the thyristor. an overvoltage detection element is connected to the overvoltage detection element, the overvoltage detection element fires a thyristor when the overvoltage is detected, and the thyristor shorts between the secondary output windings of the transformer during the non-conduction period of the switching transistor; An overvoltage protection circuit for a single-stone forward converter that reduces the output voltage by shortening the conduction period of the switching transistor.