JPS6181175A - Overvoltage protecting circuit of one-element forward converter - Google Patents

Abstract

PURPOSE:To obtain a highly reliable overvoltage protecting circuit by connecting an overvoltage detector between the secondary side rectified and smoothed output and the gate of a thyristor, and shortcircuiting by a thyristor between the secondary side output windings of a transformer, thereby reducing the number of parts. CONSTITUTION:An one-element forward converter associated with an over voltage protecting circuit 1 turns ON and OFF a switching transistor Q1 by utilizing the induced voltage of the feedback winding P2 of a transformer T, converts a DC into an AC, rectifies and smoothes the output of the secondary side output winding S1 and outputs a DC voltage Vout. This circuit 1 has a series circuit of a diode D and a thyristor SCR inserted between the terminals of the secondary side output windings S1 and an overvoltage detector (Zener diode) ZD connected between the gate terminals of the SCR. Thus, when the output voltage becomes an overvoltage, the diode ZD is conducted to fire the SCr to shortcircuit between the windings S1.

Description

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

[Prior art] A single-stone forward converter is a type of automatic switching regulator that uses blocking oscillation.
It has the advantage of having a small number of parts and being able to obtain more output power than a ringing Chicoke converter, and is used for a Ti source with a relatively small output capacity.

However, when the input voltage of the power supply increases 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. If such an overvoltage condition occurs, the output smoothing capacitor may be damaged, switching transistors, resistors, etc. may become abnormally heated, and
Otherwise, problems such as damage to the load may occur. In particular, since large capacity capacitors for output smoothing are filled with liquid electrolyte material such as ethylene glycol, 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.

Therefore, an overvoltage protection circuit is also incorporated in the stone-type forward converter.

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 @N, the output of which is rectified by a diode D3, peak-charged by a capacitor C1, and the output is passed through a Zener diode ZD for overvoltage detection.

to be applied. On the other hand, the primary input section of the converter has a resistor R.
2. A series circuit consisting of a thyristor 5CR1 and a resistor R5 is inserted, and the pressure at the terminal Ma of the resistor R5 is applied to the pace of the overvoltage protection transistor Q2 connected between the pace and emitter of the oscillation switching transistor Q8. Ru. The current flowing through the overvoltage detection Zener diode ZD is supplied to the gate of the thyristor SCR.

While the converter is operating normally, the thyristor SC
R and overvoltage protection transistor Q2 are both in an off state. However, if an overvoltage condition occurs for some reason, an abnormally high voltage will be induced in the detection winding N□.
The Zener diode ZD for overvoltage detection becomes conductive, thereby causing the thyristor SCR to fire and become conductive.

Therefore, a current flows through the resistor R3, a voltage drop occurs, and the terminal voltage is applied to the pin of the overvoltage protection transistor Q2. Accordingly, the overvoltage protection transistor Q
2 is conductive and the switching transistor Q of the converter
The pace potential of , is lowered, and its oscillation operation is stopped.

[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;
The overvoltage protection circuit is a resistor R.

~R4, thyristor SCR, overvoltage protection transistor Q2. Overvoltage detection Zener diode ZD, diode D5. Capacitor C and transformer detection winding 41
The number of parts such as 1 IN was significantly increased, which was a major factor in increasing costs. It also had the disadvantage of having a large number of parts (the circuit became more complex, making it more difficult to assemble, and lowering reliability).

The object of the present invention is to eliminate the above-mentioned drawbacks of the prior art, and to provide an improved single-stone circuit that has an extremely small number of parts and a single-layer circuit, which is therefore highly reliable, easy to assemble, and can be miniaturized. The purpose of this invention is to provide overvoltage protection for a forward converter.

[Means for solving the problems] The present invention, which can achieve the above objects, has the following features:
A thyristor is inserted between the secondary output windings of the stone-type forward converter, and an overvoltage detection element is connected between the secondary rectification and smoothing output and the gate of the thyristor. igniting a thyristor to short-circuit the secondary output windings of the transformer with the thyristor during a non-conducting period of the switching transistor;
This is an overvoltage protection circuit for a single-stone forward converter that reduces the output voltage by shortening the conduction period of the switching transistor.

Here, as the overvoltage detection element connected between the secondary side rectified 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 the configuration shown in B, 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 through the thyristor. The thyristor is made conductive. As a result, the secondary output winding is short-circuited during the non-conducting period of the switching transistor, so the voltage during the non-conducting period is not generated on the secondary side, so the conducting period of the switching transistor is shortened, and the output voltage decreases. Such an operation not only protects the load but also prevents damage to the converter itself.

[Example] The present invention will now be described 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 configuration and operation of the single-stone forward converter itself may be exactly the same as those conventionally known. Using the induced voltage in the feedback winding P2 of the transformer T, the switching transistor Q is turned on and off to convert direct current to alternating current, the voltage is converted by the collector winding P1 and the secondary output winding Sl, and the The output is rectified and smoothed by rectifier diodes D, D2, check film L, and smoothing capacitor C. The output voltage can be stabilized by providing a control transistor Q3 between the base and emitter of the switching transistor Q 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 that covers the unfired tpl. As is clear from this circuit diagram, the overvoltage protection circuit ll11 according to the present invention is -5
A series connection of a diode D and a thyristor SCR is inserted between both terminals of the secondary output winding S of the forward converter, and a series connection of a diode D and a thyristor SCR is inserted between the output of the secondary side check coil L and the gate terminal of the thyristor SCR. A Zener diode ZD is connected to the thyristor SCR as an overvoltage detection element, and a resistor R is connected between the gate and cathode terminals of the thyristor SCR.

This overvoltage protection circuit operates as follows.

Suppose now that the constant voltage operation of the single-stone forward converter is no longer controlled due to some reason, and the output voltage increases. When the increased output voltage exceeds a voltage value set as an overvoltage, the Zener diode ZD becomes conductive, thereby firing the thyristor SCR and making it conductive.

Then, the secondary output winding @S of the transformer T is the thyristor S.
They will be forward short-circuited through CR and diode D. As a result, during the non-conducting period of the switching transistor Q, the secondary output winding! sS1 is short-circuited, and no voltage is generated on the secondary side when non-conducting. 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 is used to protect the thyristor so that an excessive reverse voltage is not applied to the thyristor 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 Ford converter configured as described above, the number of parts is extremely small, the circuit configuration is simplified, and the reliability is extremely high. Moreover, it is easy to assemble, and can have excellent effects such as miniaturization.

[Brief explanation of the drawing]

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, and FIG.
The figure is a circuit diagram of the prior art. 1.11...Overvoltage protection circuit, T...transformer, Q,
Switching transistor for output oscillation, S... Secondary output winding, D, D2... Rectifier diode, L-
・・Chirik coil, C・・smoothing capacitor, SCR
...Thyristor, ZD...Zener diode for overvoltage detection.

Claims (1)

[Claims] 1. A thyristor is inserted between the secondary output windings of the single-stone forward converter, and an overvoltage detection element is connected between the secondary rectified and smoothed output and the gate of the thyristor, and when an overvoltage is detected, the overvoltage is detected. The thyristor is ignited by the element, and the thyristor short-circuits the secondary output winding of the transformer during the non-conducting period of the switching transistor, thereby shortening the conducting period of the switching transistor and lowering the output voltage. Overvoltage protection circuit for single-stone forward converter.