JPH0832174B2 - Ringing choke converter - Google Patents

Description

TECHNICAL FIELD The present invention relates to a ringing choke converter used in a constant voltage power supply circuit or the like.

(B) Conventional Technology A switching transistor is connected in series to the primary winding of a transformer, and the electromotive voltage of the feedback winding of the transformer is positively fed back to the base of the switching transistor to extract the voltage from the secondary winding of the transformer. The ringing choke converter described above has been used as a switching regulator having a relatively small capacity from the past.

When such a ringing choke converter is used as a DC constant voltage power supply circuit, a circuit that controls the base current of the switching transistor, a load supply voltage detection circuit, and a circuit that performs negative feedback control of the base current of the switching transistor according to the load supply voltage. Is provided.

However, the load supply voltage may rise abnormally when the feedback loop that performs the negative feedback for the above-mentioned constant voltage is opened or due to the failure of other circuit parts.The overvoltage protection circuit is provided to prevent this. Has been.

2 and 3 show circuit examples of a ringing choke converter including a conventional overvoltage protection circuit.

In the example shown in FIG. 2, the switching transistor Q1 is connected to the primary winding N1 of the transformer 1, and one end of the feedback winding Nb of the transformer is switched via the resistor R2.
It is connected to the base of Q1, and the starting resistor R1 is connected between the input power supply line and the base of the switching transistor Q1. Rectifier diode D1 on the secondary winding N2 of the transformer
And the smoothing capacitor C1 is connected. The voltage rectified and smoothed by the diode D1 and the capacitor C1 is supplied to the load. An overvoltage protection circuit including a Zener diode ZD2, resistors R8 and R9, a capacitor C4 and a thyristor Q5 is provided between the load voltage supply lines. This overvoltage protection circuit turns on the Zener diode ZD2 when the voltage between output lines exceeds a certain value (Zener voltage of Zener diode ZD2), such as when the feedback loop (not shown) for constant voltage is opened. , A gate trigger voltage is applied to the gate of thyristor Q5, and thyristor Q5 turns off. As a result, the output lines are short-circuited by the thyristor Q5, and the load is protected from overvoltage.

In the example shown in FIG. 3, the transistor Q6 is connected between the base / emitter of the switching transistor Q1, and the light receiving portion PCa of the photocoupler, the resistor R12, and the capacitor C5 are connected between the connection point of the starting resistors R10 and R11 and the base of the transistor Q6. The circuit consists of a zener diode ZD2 and a photocoupler light emitting part PCl between the load voltage supply lines.
Is provided. Due to this configuration, the load supply voltage is constant (Zener diode
(Zener voltage of ZD2 plus forward drop voltage of light emitting unit PCl of the photocoupler)), the Zener diode ZD2 becomes conductive and the light emitting unit PCl of the photocoupler emits light.
This allows the photosensitive thyristor, which is the light receiving part of the photocoupler.
PCa turns on, turning on transistor Q6. As a result, the base potential of the switching transistor Q1 becomes substantially the ground potential, and the oscillation operation is stopped. In this way the load is protected from overvoltage.

(C) Problem to be Solved by the Invention The ringing choke converter including the conventional overvoltage protection circuit has the following drawbacks. That is, as shown in FIG. 2, in a circuit that performs overvoltage protection by short-circuiting the output lines that output the load supply voltage, the oscillating state continues even when the thyristor Q5 is in the ON state, so if it is left for a long time, Switching transistor
Q1 becomes overheated and finally destroyed.

On the other hand, as shown in FIG. 3, in the circuit in which the secondary side voltage (load supply voltage) is detected and the base current of the switching transistor is controlled via the photocoupler, an insulating element such as a photocoupler is used. In addition to this, a large number of parts are required, which is expensive. Further, in general, the photocoupler has a large variation in electrical characteristics, which causes a problem that the operating point of overvoltage protection varies.

An object of the present invention is to provide a ringing choke converter which solves the above conventional problems by detecting overvoltage supply to a load on the primary side and controlling a base current of a switching transistor to perform overvoltage protection. It is in.

(D) Means for Solving the Problem The present invention connects one side of an input power supply line to one end of a primary winding N1 of a transformer, and connects the other end of the primary winding N1 to the collector of a switching transistor Q1 for switching. The emitter of the transistor Q1 is connected to the other of the input power supply lines, and a startup resistance circuit (R4, R5, R6) consisting of multiple resistors connected in series is connected between the one input power supply line and the base of the switching transistor Q1. , Transformer secondary winding N2
A ringing choke converter in which a rectifying / smoothing circuit is connected to, one end of the feedback winding Nb of the transformer is connected to the base of the switching transistor Q1 via a resistor R2, and the other end of the feedback winding Nb is connected to the emitter of Q1. , One end of the resistor R3 is connected to the emitter side of the switching transistor Q1, and the series circuit of this resistor R3, the backflow prevention diode D2 and the Zener diode ZD1 is connected to both ends of the feedback winding Nb. , And a triac Q4 between the emitter of the switching transistor Q1 and the gate of the triac Q4 is connected to the other end of the resistor R3. Potential switching transistor
It is characterized in that a base potential control circuit for reducing the potential to a potential substantially equal to the emitter potential of Q1 is connected.

(E) Action In the ringing choke converter of the present invention, the series circuit including the resistor R3, the backflow prevention diode D2, and the zener diode ZD1 has a constant voltage in the feedback winding of the transformer generated when the switching transistor is in the off state. Conducts when exceeding. The voltage Vb generated in the feedback winding of the transformer when the switching transistor is off is expressed by the following equation.

Vb = V2 × Nb / N2 where V2 is the voltage (flyback voltage) generated in the secondary winding of the transformer, Nb is the number of turns of the feedback winding, and N2 is the number of turns of the secondary winding.

Therefore, when the switching transistor is in the OFF state, the voltage generated in the feedback winding of the transformer is proportional to the voltage generated in the secondary winding, and in the above series circuit, the load supply voltage exceeded a certain value due to the voltage of the feedback winding. To detect.

When the load supply voltage exceeds a certain value, the Zener diode of the series circuit becomes conductive. This allows the triac
Q4 is turned on, and the base potential control circuit acts to lower the base potential of the switching transistor Q1 to approximately the emitter potential, keeping Q1 off. Therefore, when the load supply voltage exceeds a certain value, the switching transistor is kept in the off state and the switching operation is stopped.

(F) Embodiment FIG. 1 shows a circuit diagram of a ringing choke converter which is an embodiment of the present invention.

As shown in FIG. 1, the switching transistor Q1 is connected in series to the primary winding N1 of the transformer 1, and the voltage generated in the feedback winding Nb of the transformer is fed back to the base of the switching transistor Q1 through the resistor R2. A rectifying diode D1 and a smoothing capacitor C1 are connected to the secondary winding N2 of the transformer 1. In addition, the transformer feedback winding Nb
Between output line of diode D2, Zener diode ZD
A constant voltage detection circuit 2 including 1 and a resistor R3 is provided, and a switching transistor control circuit 3 that controls the switching transistor Q1 according to the state of the constant voltage detection circuit 2 is provided between the input power supply lines. This switching transistor control circuit 3 includes a triac Q4 which turns on under a certain condition described later, a malfunction preventing capacitor C2 connected to its gate due to a surge or the like, and a triac Q4.
Resistors that determine the base voltage of transistors Q3 and Q2 and transistor Q3 that are turned on by turning on the transistor to prevent malfunction
R7 and capacitor C3, and resistors R4, R5, and R6 that supply a starting current to the base of the switching transistor Q1 and a predetermined voltage / current to the transistor Q3 and the triac Q4.

 The operation of the circuit shown in FIG. 1 is as follows.

i Normal state First, the resistors R4, R5, R6 act as starting resistors, and the base current flows through the switching transistor Q1 through these resistors. As a result, the switching transistor Q1 becomes conductive, a voltage is applied to the primary winding N1 of the transformer, and at the same time, a voltage is also generated in the feedback winding Nb. This voltage becomes a positive feedback voltage that makes the switching transistor Q1 more conductive, and the switching transistor turns on rapidly. At this time, since the voltage of the secondary winding N2 is applied in the opposite direction to the diode D1, no current flows in the secondary winding N2,
The current flowing through the primary winding N1 is only the exciting current of the transformer. Therefore, the current flowing through the switching transistor Q1 increases in proportion to time, but when the base current cannot keep the switching transistor Q1 in a saturated state, the switching transistor Q1 goes out of saturation and the collector-emitter voltage rises. Will increase. As a result, the voltage of the primary winding N1 of the transformer drops and the voltage of the feedback winding Nb also drops. Since this change is positively fed back, the switching transistor Q1 turns off rapidly. When the primary winding current of the transformer is turned off, the magnetic field of the transformer is kept the same, that is, the current flows through the secondary winding N2 in the direction in which the diode D1 conducts. When all the energy stored in the transformer is transferred to the output side, the current of the diode D1 becomes 0 and the diode turns off. Then resist again
A starting current flows through the base of the switching transistor Q1 through R4, R5, and R6, and the above operation is repeated.

Although the constant voltage circuit is omitted in FIG. 1, for example, the load supply voltage is detected, and the base current of the switching transistor Q1 is controlled according to the detected voltage, thereby controlling the on-duty ratio and thereby the output voltage. Can be stabilized. In such a normal state, the Zener diode ZD1 in the constant voltage detection circuit 2 does not conduct, and the triac Q4 does not connect to the transistors Q3 and Q2.
Also keeps all off.

ii Overvoltage protection state When the load supply voltage exceeds a certain voltage for some reason, the voltage generated in the feedback winding Nb of the transformer also rises when the switching transistor is off. When this voltage becomes larger than the sum of the Zener voltage of the Zener diode ZD1 and the forward drop voltage of the diode D2, the Zener diode ZD1 becomes conductive. The diode D2 prevents the voltage of the feedback winding generated when the switching transistor is turned on from flowing to the Zener diode ZD1 and the resistor R3.

Due to the conduction of the Zener diode ZD1, the resistance R
A voltage drop (negative voltage) of 3 is applied to the gate of TRIAC Q4, and TRIAC Q4 turns on. When the triac Q4 turns on, a base current flows through the transistor Q3 and the transistor Q3 turns on. As a result, a base current flows through the transistor Q2 and the transistor Q2 also turns on.

After that, even if all the stored energy of the transformer is released and the respective winding voltages become 0, the base current of the switching transistor Q1 is almost bypassed to the transistor Q2 side and the switching transistor Q1 is not turned on. Since a holding current flows through the resistors R4 and R5 in the triac Q4, the transistors Q3 and Q2 also remain in the ON state, and the oscillation operation of the switching transistor Q1 remains stopped.

(G) Effect of the Invention According to the present invention, since the switching transistor is kept in the off state during overvoltage protection, the switching transistor is not overheated and can be left in the overvoltage protection state. Further, since the load supply voltage is detected on the primary side, an insulating element such as a photocoupler for giving an overvoltage detection signal is not required, and the entire circuit can be inexpensive and small-sized. Furthermore, the problem due to the variation in the electrical characteristics of the photocoupler is solved.

Moreover, according to the present invention, the output voltage of the feedback winding, which is proportional to the voltage generated in the secondary winding of the transformer, is directly detected from the voltage across the resistor R3. The response speed can be significantly improved, and the load can be reliably protected from overvoltage, as compared with the case where charging is performed and the protective operation is performed when the charging voltage reaches a constant voltage. That is,
A voltage proportional to the flyback voltage generated in the secondary winding of the transformer is applied to the series circuit of the resistor R3, the backflow prevention diode D2, and the zener diode ZD1 without a delay element, and the voltage application turns on the triac Q4. Then, since the switching transistor Q1 continues to be in the off state, the next flyback voltage does not occur in the secondary winding of the transformer. Since the secondary winding is provided with a rectifying / smoothing circuit, the load supply voltage hardly actually increases, and the load is protected from the overvoltage that should occur thereafter.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a ringing choke converter which is an embodiment of the present invention. 2 and 3 are circuit diagrams of conventional ringing choke converters. 1 ... Transformer, N1 ... Primary winding, N2 ... Secondary winding, Nb
...... Feedback winding, 2 …… Constant voltage detection circuit, 3 …… Switching transistor control circuit, Q1 …… Switching transistor.

Claims (1)

[Claims] 1. An input power supply line is connected to one end of a primary winding N1 of a transformer, the other end of the primary winding N1 is connected to a collector of a switching transistor Q1, and an emitter of the switching transistor Q1 is connected to an input power supply line. A startup resistance circuit (R4, R5, R6), which is connected to the other and has a plurality of resistors connected in series between the one input power line and the base of the switching transistor Q1, is connected, and the secondary winding N2 of the transformer is connected.
A ringing choke converter in which a rectifying / smoothing circuit is connected to, one end of the feedback winding Nb of the transformer is connected to the base of the switching transistor Q1 via a resistor R2, and the other end of the feedback winding Nb is connected to the emitter of Q1. , One end of the resistor R3 is connected to the emitter side of the switching transistor Q1, and the series circuit of this resistor R3, the backflow prevention diode D2 and the Zener diode ZD1 is connected to both ends of the feedback winding Nb. , And a triac Q4 between the emitter of the switching transistor Q1 and the gate of the triac Q4 is connected to the other end of the resistor R3. Potential switching transistor
A ringing choke converter to which a base potential control circuit for reducing the potential to be substantially equal to the emitter potential of Q1 is connected.