JPH0739349Y2 - Overvoltage protection circuit for self-excited converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention detects an overvoltage induced in a base winding by a Zener diode and makes a thyristor inserted in a primary side circuit of a converter conductive to control a transistor. The present invention relates to an overvoltage protection circuit for a self-excited converter that draws out the base current of a switching transistor to stop oscillation.

[Prior Art] A self-excited converter switches on / off a switching transistor Q ₁ by using an induced voltage in a base winding N ₃ of a transformer T to switch a DC input as shown in FIG. 2, for example. The primary winding N ₁ and the secondary winding N ₂ perform voltage conversion, and the output is rectified and smoothed by the rectifying diode D and the smoothing capacitor C. The output voltage can be stabilized by, for example, providing a control transistor Q ₂ between the base and emitter of the switching transistor Q ₁ and applying a control signal to the base.

In such a self-excited converter, if a trouble occurs in a main electronic component, the control state becomes uncontrolled and the output voltage rises remarkably, and the normal component is damaged, abnormal heat is generated, or the load is damaged. Therefore, if the output is in an overvoltage state, an overvoltage protection circuit is incorporated to detect it and immediately reduce the output voltage.

The conventional overvoltage protection circuit 10 is, for example, a circuit surrounded by a broken line in FIG. Further provided detection winding N ₄ in the transformer T, and peak charge in the capacitor C ₁ with rectifying the output diode D _1, and applies the output to the over-voltage detection Zener diode ZD _1. On the other hand, a resistor R ₁ , a thyristor Th ₁ , and a resistor R _{2 are provided} between the primary side input terminals a and b of the converter.
Insert a series circuit consisting of a switching transistor
To Q ₁ base - connection between the emitter and the control transistor
Connect so that the terminal voltage of resistor R ₂ is applied to the base of Q ₂ . And the Zener diode ZD ₁ for detecting the overvoltage
Is supplied to the gate of the thyristor Th ₁ .

Thyristor Th ₁ remains off while the converter is operating normally. However, if the output goes into an overvoltage state for some reason, an abnormally high voltage is induced in the detection winding N ₄ , the zener diode for overvoltage detection ZD ₁ becomes conductive, and the thyristor Th ₁ is triggered. Therefore, a current flows through the resistor R ₂ to cause a voltage drop, and the control transistor Q ₂ becomes conductive to draw the base current of the switching transistor Q ₁ and stop the oscillation operation.

[Problems to be solved by the invention] However, in such a circuit, the converter main body has a simple structure with a very small number of parts, but an overvoltage protection circuit which is not absolutely necessary when the device is operating normally. Has a large number of parts, and a detection winding must be added to the transformer, which is a serious problem in terms of cost and space. Moreover, since the number of parts is large and the circuit is complicated, assembly is troublesome and reliability is lowered.

The object of the present invention is to eliminate the above-mentioned drawbacks of the conventional technology,
An object of the present invention is to provide an overvoltage protection circuit for a self-excited converter that has a small number of parts, has a simplified circuit, and can be downsized.

[Means for Solving the Problems] The present invention is to connect a primary winding of a converter transformer and a switching transistor in series between input terminals of a primary side, and to form a resistor and a capacitor in parallel with a base winding of the converter transformer. Is a self-exciting converter circuit in which a terminal voltage of the capacitor is provided between the base and emitter of a control transistor connected between the base and emitter of a switching transistor.

In order to achieve the above object, a series circuit of a resistor and a thyristor is provided between the primary side input terminal on one side of the converter and the base of the control transistor, and the gate of the thyristor and the primary side input terminal on the other side are connected. A series circuit of a Zener diode for detecting overvoltage induced in the base winding and a diode is connected between them, and a thyristor that conducts when an overvoltage occurs is used to conduct the control transistor and draw the base current of the switching transistor. .

[Operation] The control transistor performs both overcurrent control and protection during overvoltage detection. When the output voltage is normal, the Zener diode does not conduct and the thyristor remains off.

If the output becomes abnormal for some reason and becomes an overvoltage state, the Zener diode becomes conductive. This triggers the thyristor and causes a current to flow in the base of the control transistor. This makes the control transistor conductive,
It draws the base current of the switching transistor. The switching transistor is turned off, the converter oscillation stops, and the output voltage drops. This provides overvoltage protection.

[Embodiment] FIG. 1 is a circuit diagram showing an example of a self-excited on-off control type converter incorporating an overvoltage protection circuit according to the present invention. The configuration and operation of the self-excited converter itself may be the same as those conventionally known, and corresponding parts are designated by the same reference numerals to simplify the description. Switching transistor using induced voltage of base winding N ₃ of transformer T
Q ₁ is turned on - off, and switching to the primary winding of the DC input
The voltage is converted by N ₁ and the secondary winding N _2, and the output is rectified and smoothed by the rectifying diode D and the smoothing capacitor C.

A series circuit of a resistor R ₃ and a capacitor C ₂ is provided in parallel with the base winding N ₃ , and a triangular wave is created by them. The base of the switching transistor Q ₁ - the control transistor Q ₃ which is connected between the emitter terminal voltage V ₁ of the capacitor C ₂ is applied, its base - the control transistor Q ₃ on the comparison between the emitter voltage V _BE - Off,
Performs pulse width control of switching transistor Q ₁ . A resistor R ₄ connected between one primary side input terminal a of the converter and the base of the switching transistor Q ₁ is for base bias.

The portion surrounded by the broken line in FIG. 1 is the overvoltage protection circuit 20 according to the present invention. This overvoltage protection circuit 20 is provided with a series circuit of a resistor R ₅ and a thyristor Th ₂ between one primary side input terminal a of the converter and the base of the control transistor Q ₃ , and the gate of the thyristor Th ₂ and the other primary side of the thyristor Th _2. Input terminal b
A Zener diode ZD ₂ for detecting an overvoltage induced in the base winding N ₃ is provided between and. Here it is connected a Zener diode ZD ₂ so as to face the anode to the gate of the thyristor Th _2, the cathode side of the Zener diode ZD ₂ is connected to the cathode of the diode D _2, the other anode of the diode D ₂ is of the converter Is connected to the primary side input end b.

The operation of the overvoltage protection circuit 20 configured in this way is as follows. In Fig. 1, the terminal voltage V of the capacitor C _2
1 is the control transistor Q when the secondary side output voltage is E _S.
Switching transistor Q _{1 based on} the emitter of ₃
When is off, it is expressed by the following equation.

Then, when the switching transistor Q ₁ becomes conductive, it is gradually charged from this voltage through the resistor R ₃ according to the time constant determined by them. Therefore, the output voltage E _S can be detected by the terminal voltage of the capacitor C ₂ .

When the output voltage rises and reaches an overvoltage state for some reason, the terminal voltage V ₁ of the capacitor C ₂ immediately after the switching transistor Q ₁ is turned off is extremely low (absolute value is large) and exceeds the Zener voltage. Zener diode
ZD ₂ becomes conductive and trigger current flows through the gate of thyristor Th ₂ . As a result, the thyristor Th ₂ becomes conductive, a current is supplied to the base of the control transistor Q ₃ through the resistor R ₅ and the thyristor Th _2, and the control transistor Q ₃ becomes conductive.
As a result, the base current of the switching transistor Q ₁ is extracted, the oscillation of the transistor Q ₁ is stopped, the output voltage is lowered, and the load is protected and the converter itself is prevented from being damaged.

Here, the diode D ₂ inserted in series with the thyristor Th ₂ does not need to be provided in principle, but fulfills the function of eliminating the interaction with the external circuit by adding this circuit. . Further, the resistor and the capacitor inserted between the gate and the cathode of the thyristor ensure the withstand voltage of the thyristor and prevent the malfunction.

[Advantage of the Invention] The present invention controls the operation of the control transistor by making the control transistor also have the protection function against the overvoltage as described above, detecting the overvoltage induced in the base winding by the Zener diode, and turning on the thyristor. Since it is configured like this, the circuit configuration is significantly simplified compared to the conventional technology, the number of required parts can be almost halved, and it is not necessary to provide a dedicated winding for the transformer, which improves reliability and is easy to assemble. Therefore, excellent effects such as downsizing can be obtained.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a self-exciting converter incorporating an overvoltage protection circuit according to the present invention, and FIG. 2 is a circuit showing an example of a conventional technique. 10, 20 ... overvoltage protection circuit, T ... transformer, N ₁ ... primary winding, N ₂ ... secondary windings, N ₃ ... base winding, Q ₁ ... switching transistors, Q _2, Q ₃ ... control transistor, Th ₁ , Th ₂ …
Thyristor, ZD ₁ , ZD ₂ ... Zener diode, R ₃ , R ₅ ...
Resistor, C ₂ ... Capacitor.

Claims (1)

[Scope of utility model registration request] 1. A primary winding of a converter transformer and a switching transistor are connected in series between input terminals of a primary side, and a series circuit of a resistor and a capacitor is provided in parallel with a base winding of the converter transformer. In a converter circuit in which a terminal voltage is applied between the base and emitter of a control transistor connected between the base and emitter of a switching transistor, a resistor and a thyristor are connected between the primary side input terminal on one side of the converter and the base of the control transistor. A series circuit is provided, and a series circuit of a Zener diode for overvoltage detection and a diode is connected between the gate of the thyristor and the other primary side input terminal, and the control transistor is turned on by the thyristor turned on when an overvoltage occurs. Excitation converter overvoltage protection circuit.