JPH066674Y2 - Self-exciting converter overcurrent protection circuit - Google Patents

Description

[Detailed Description of the Invention] [Industrial application] The present invention is to add an auxiliary winding for overcurrent protection operation to a base winding of a transformer, and to provide a resistance between both ends of the base winding and the auxiliary winding. The present invention relates to an overcurrent protection circuit of a self-excited converter in which a series circuit of a capacitor and a capacitor is connected, an overcurrent is detected by a terminal voltage of the capacitor, and a base current of a switching transistor is extracted by a protection transistor.

[Prior Art] An example of a self-exciting converter incorporating an overcurrent protection circuit is shown in FIG. The primary side of the converter is the blocking oscillator itself, the switching transistor Q ₁ is connected in series with the primary winding N ₁ of the transformer T, and the induced voltage of the base winding N ₃ of the transformer T is connected to the base of the switching transistor Q _1. Is applied via a resistor R ₁ and a capacitor C ₁ . The resistor R ₂ is a switching transistor Q
₁ for base bias.

The primary side of the converter oscillates by itself and an output is induced in the secondary winding N ₂ of the transformer T. This output is rectified by the rectifying diode D and smoothed by the smoothing capacitor C. The self-exciting converter itself does not have controllability of output voltage, but it can be made constant voltage by combining with a circuit having other controllability.

Well conventional overcurrent protection circuit 10, an overcurrent detection resistor R ₃ of low resistance values connected to the emitter of the switching transistor Q _1, the switching transistor protection transistor Q ₂ to which is connected to the base circuit for Q ₁ It consists, on the transistor Q ₂ in the terminal voltage of the overcurrent detection resistor R ₃ - was configured to turn off control.
That switching transistor emitter current for Q ₁ is detected as a voltage drop of the resistor R _3, the base of the protection transistor Q ₂ and its terminal voltage - is intended for comparing the emitter voltage V _BE, excessive current flows and the resistance The voltage drop of R ₃ becomes large, the protection transistor Q ₂ becomes conductive, the base current of the switching transistor Q ₁ is drawn, the peak value of the collector current is made constant, and the internal circuit is protected.

[Problems to be Solved by the Invention] However, in such an overcurrent protection circuit, the peak value of the collector current is kept constant by the protection transistor Q ₂ conducting when the load is over and drawing the base current of the transistor Q _1. However, since the output voltage decreases with an overload, the output current increases.

Thus the converter output voltage - Load current characteristics becomes fourth becomes overloaded, as shown in FIG shaped "f" flows twice or more short-circuit current I _s of the maximum output current I when the load short-circuit. As described above, when the load is short-circuited, the excessive current I _s flows and the switching transistor Q ₁ or the rectifying diode D generates heat. Therefore, in the prior art, it is necessary to provide a sufficient margin for electronic parts such as the switching transistor and the rectifying diode. It had to use a large capacity, which was a factor of cost increase.

The object of the present invention is to eliminate the above-mentioned drawbacks of the conventional technology,
It is possible to suppress excessive current when load is short-circuited to less than the maximum output current, and therefore to suppress heat generation of major parts when an abnormality occurs, and also to provide a small and inexpensive self-exciting converter with a small number of parts. It is to provide an overcurrent protection circuit that can be obtained.

[Means for Solving the Problems] The present invention that can achieve the above-mentioned object is to connect a switching transistor in series with the primary winding of a converter transformer, and to connect between the base and the emitter of the switching transistor. In the self-excited converter that connects the base winding of the converter transformer to the self-excited oscillation and induces output in the secondary winding of the converter transformer, the auxiliary winding is wound around the base winding for driving the switching transistor. Add a series circuit of a resistor and a capacitor between both ends of the base winding and the auxiliary winding, install a diode in parallel with the resistor, and arrange a protection transistor between the base of the switching transistor and the auxiliary winding terminal. Connected so that the terminal voltage of the capacitor is applied to the base of the protection transistor. It is an overcurrent protection circuit.

In a more preferable embodiment, a diode is connected between the connection point between the resistor and the capacitor and the base of the protection transistor, and the terminal voltage of the capacitor is applied via the diode.

[Operation] The time constant circuit consisting of a resistor and a capacitor repeats charging and discharging according to the self-oscillation operation of the converter. While the switching transistor is conducting, the terminal voltage of the capacitor gradually rises. The initial value of the charge / discharge of the terminal voltage of the capacitor changes according to the output voltage, and the absolute value of the capacitor terminal voltage decreases as the output voltage decreases. Therefore, the period until the protection transistor becomes conductive (the period during which the switching transistor is conductive) becomes short, and the output current decreases.

The auxiliary winding added to the base winding serves to make the output voltage-load current characteristic a fold-back characteristic. If this auxiliary winding is not provided, even if an overcurrent is detected and the protection transistor conducts, its collector-emitter saturation voltage (about 0.4V) remains, so the switching transistor cannot be turned off instantly, resulting in loss. Occurs and the output current cannot be reduced. However, when the auxiliary winding is provided as in the present invention, a reverse bias can be applied between the base and emitter of the switching transistor, and it can be instantly turned off. For this reason, the load current characteristic has a "F" shape.

[Embodiment] FIG. 1 is a circuit diagram showing one embodiment of a self-excited on-off converter incorporating an overcurrent protection circuit according to the present invention. Since the converter itself may be basically the same as the prior art shown in FIG. 3, corresponding parts are designated by the same reference numerals, and description thereof will be omitted.

Now, the overcurrent protection circuit 20 according to the present invention is a portion surrounded by a broken line. An auxiliary winding N ₄ is further added to the base winding N ₃ of the converter transformer, and a series circuit of a resistor R ₄ and a capacitor C ₂ is provided between both terminals a and b of the base winding N ₃ and the auxiliary winding N _4. And the first diode D ₁ is connected in parallel to the resistor R ₄ such that its castor side faces the terminal a. In addition, the protection transistor Q _{3 is provided} between the base of the switching transistor Q ₁ and the terminal b of the auxiliary winding N _4.
Is provided and connected so that the terminal voltage of the capacitor C ₂ is applied to the base of the protection transistor Q ₃ . In this embodiment, they are connected via the second diode D ₂ .

During normal operation, the switching transistor Q ₁ oscillates on the basis of the principle of blocking oscillation, and thereby an output is induced in the secondary winding N ₂ of the transformer T. This output is rectified by the rectifying diode D and smoothed by the smoothing capacitor C.

Terminal voltage of capacitor C ₂ (resistor R ₄ and capacitor C ₂
When the output voltage is E _s , the voltage at the connection point A with V _A is, when the switching transistor Q ₁ is off with respect to the emitter of the protection transistor Q ₃ , Becomes When the switching transistor Q ₁ becomes conductive,
The terminal voltage _{V A} of the capacitor _{C 2} is charged according to the time constant of the capacitor _{C 2} and a resistor _{R 4.} In this way, the initial value of charging, that is, the terminal voltage of the capacitor C ₂ immediately after the switching transistor Q ₁ is turned on, changes according to the output voltage. Output voltage E _s due to load short-circuit accident
The lower the value of, the smaller the absolute value of the initial value of charging.

When the charging of the capacitor progresses and its terminal voltage exceeds the sum of the base-emitter voltage V _BE of the protection transistor Q ₃ and the forward voltage drop V _{F2 of} the second diode D ₂ , the protection transistor Q ₃ becomes conductive. . As a result, the base current of the switching transistor Q ₁ is drawn and the output is drooped. Generally, the collector saturation voltage of a transistor is about 0.4V, and the base-emitter voltage is 0.7.
It is about V. Auxiliary winding N added to base winding N _3
4 serve to bias the emitter potential of the protective transistor Q ₃ with respect to the emitter potential of the switching transistor Q ₁ on the negative side. Therefore, when the protection transistor Q ₃ is conductive, the collector voltage is about 0.4 V with respect to its emitter as a reference, but the emitter of the switching transistor Q ₁ is high and is in a reverse bias state, and is completely turned off to block the collector current. It is possible. Therefore, the load current characteristic with respect to the output voltage has a "F" shape as shown in FIG. Therefore, the present invention is particularly effective for a large current capacity converter.

The auxiliary winding N ₄ is a major factor in determining the slope of the foldback characteristic. Actually, a sufficient effect is obtained with the number of turns of about 1 to 2 turns.

The second diode D ₂ has a function of setting a minimum on-time and a function of preventing application of an excessive reverse bias between the base and the emitter of the protection transistor Q ₃ .
The second diode D ₂ can be omitted in some cases, and a Zener diode can be used when it is desired to lengthen the minimum on-time.

[Advantage of the Invention] The present invention adds the auxiliary winding to the base winding as described above, and thereby reverse bias is applied between the base and the emitter of the switching transistor when the protection transistor is turned on. There is an effect that it can be made smaller than the output current and a fold-back type load current characteristic can be realized.

Therefore, according to the present invention, the problem of heat generation of the switching transistor and the rectifying diode, which are the main parts of the converter, is solved, and not only the device can be downsized, but also the parts of small capacity can be used to achieve downsizing and cost reduction. it can.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a self-exciting converter incorporating an overcurrent protection circuit according to the present invention, FIG. 2 is its output voltage-load current characteristic diagram, and FIG. 3 is an example of prior art. FIG. 4 is a circuit diagram showing the above, and FIG. 4 is its output voltage-load current characteristic diagram. 10, 20 ... Overcurrent protection circuit, T ... Transformer, N ₁
…… Primary winding, N ₂ …… Secondary winding, N ₃ …… Base winding, N ₄ …… Auxiliary winding, Q ₁ …… Switching transistor, Q ₃ …… Protection transistor, D ₁ …… No. 1 diode, D ₂ ... second diode, C ₂ ... capacitor, R ₄ ... resistance.

Claims (1)

[Scope of utility model registration request] 1. A switching transistor is connected in series with a primary winding of a converter transformer, and a base winding of the converter transformer is connected between a base and an emitter of the switching transistor to cause self-excited oscillation. In a self-excited converter that induces output in the next winding, an auxiliary winding is added to the base winding, and a series circuit of a resistor and a capacitor is connected between both ends of the base winding and the auxiliary winding and Of a self-exciting converter in which a diode is provided in parallel, a protection transistor is arranged between the base of the switching transistor and the auxiliary winding terminal, and the terminal voltage of the capacitor is applied to the base of the protection transistor. Overcurrent protection circuit.