JPH11275861A - Switching power source circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the thermal runway of a switching transistor by reducing the load of a switching power source circuit in accordance with a low-level signal. SOLUTION: The oscillation current of a blocking oscillation circuit 20 induces the output voltages in output windings N13 and N14. The output voltage induced in the winding N13 is rectified, detected by means of an output voltage detecting circuit 28, and supplied to a load 32 via an analog switch SW11. In addition, the output voltage induced in the output winding N14 is rectified and detected by means of a low-voltage detecting circuit 30. When the output voltage of a rectifier circuit 26 becomes lower than that of a Zener voltage, a Zener diode DZ3 is turned off and a bias voltage is supplied to a transistor T16. Therefore, a low-level signal is given to a microcomputer 34, and the microcomputer 34 turns off the analog switch SW11.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply circuit, and more particularly to a switching power supply circuit applied to, for example, a stabilized power supply circuit of a television receiver.

[0002]

2. Description of the Related Art As shown in FIG. 4, in a conventional switching power supply circuit 1 of this type, a microcomputer 2 and a load 3 are individually supplied with power, and the power supplied to the microcomputer 2 is kept constant by a regulator 4. Dripping. Further, the power supplied to the load 3 is kept constant by controlling the ON / OFF of the switching transistor T1 by the control circuit 6 provided on the primary side according to the voltage detected by the detection circuit 5. . Note that the detection circuit 5 and the control circuit 6
Insulated by photocoupler 7.

[0003]

In this switching power supply circuit 1, since the power supply voltage of the microcomputer 2 is kept constant by the regulator 4, even if the voltage of the AC power supply 8 fluctuates, the microcomputer 2 cannot detect the fluctuation. Can not. Therefore, even if the voltage from the AC power supply 8 drops, the switch SW1 for switching the load cannot be turned off, so that the switching transistor T1 provided on the primary side is not provided.
One thermal runaway had occurred.

Therefore, a main object of the present invention is to provide a switching power supply circuit capable of avoiding thermal runaway of a switching transistor.

[0005]

SUMMARY OF THE INVENTION The present invention provides a blocking oscillation circuit having a converter transformer and a first rectifier circuit for rectifying a rectangular wave voltage obtained at a first output winding of the converter transformer and outputting a first output. And a control means for controlling the blocking oscillation circuit in accordance with the first output, a second rectifier circuit for obtaining a second output having a polarity different from that of the first output, and a second output corresponding to a reference voltage. A switching power supply circuit, comprising: a voltage detection circuit that outputs a first signal when the switching power supply circuit turns on.

[0006]

When an AC power supply is applied to the switching power supply circuit, the AC voltage is rectified to generate a DC voltage. This DC voltage is applied to the input winding formed on the primary side of the converter transformer, and also applied to the base of the switching transistor via the starting resistor and the driving resistor. Therefore, the switching transistor turns on,
A blocking oscillation circuit including the input winding oscillates at a predetermined frequency. This oscillating current flows through the input winding and 1
An output voltage is generated in proportion to the number of windings on the secondary side and the secondary side. That is, a first output proportional to the ratio of the number of turns of the input winding to the number of turns of the first output winding is obtained from the first output winding formed on the secondary side of the converter transformer.

In the second rectifier circuit, a second output having a polarity different from that of the first output is rectified. Specifically, a second rectifier circuit is connected to the second output winding of the converter transformer, and a diode extracts a negative portion of the rectangular wave voltage obtained in the second output winding. The first output is rectified by the first rectifier circuit, and the control means controls the blocking oscillation circuit according to the first output. The voltage detection circuit outputs the first signal when the second output becomes the reference voltage.

[0008] Switching means provided between the first rectifier circuit and the external load is turned off in response to the first signal.

[0009]

According to the present invention, when the external load is reduced when the first signal is output, thermal runaway of the switching transistor can be prevented. The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description of embodiments with reference to the drawings.

[0010]

Referring to FIG. 1, a switching power supply circuit 10 of this embodiment includes a converter transformer 12, and an AC power supply 14 is connected to a primary side 10a of the converter transformer 12. A rectifier circuit 16 is connected to the AC power supply 14,
One output terminal of the rectifier circuit 16 is connected to one end of the input winding N11. One end of the starting resistor R1 and one end of the resistor R2 are connected to a connection point between the rectifier circuit 16 and the input winding N11. The other end of starting resistor R1 is connected to one end of capacitor C1 via starting resistor R3 and resistor R4. Further, the other end of the resistor R2 is connected to the collector of the switching transistor T11 via the capacitor C2. Capacitor C2 and switching transistor T
11 is connected to the other end of the input winding N11. Note that the other output terminal of the rectifier circuit 16 is connected to the emitter of the switching transistor T11.

The other end of the capacitor C1 is connected to a feedback winding N12.
Of the capacitor C1 and the feedback winding N12
Is connected to the anode of the diode D1, the cathode of the Zener diode DZ1, and the cathode of the diode D2. The cathode of the diode D1 is connected to the anode of the diode D3, and the connection point between the diodes D1 and D3 is connected to the connection point between the starting resistors R3 and R4. One end of a driving resistor R5 and one end of a resistor R6 are connected to a connection point between the diode D1 and the diode D3. The other end of the driving resistor R5 is connected to the base of the switching transistor T11,
Connected to the collector of transistor T12. Resistance R6
Is connected to the base of the transistor T12, and a connection point between the resistor R6 and the transistor T12 is connected to a resistor R7.
Is connected to the anode of the Zener diode DZ1 via a resistor R8, and to the anode of the diode D2 via a resistor R8. Further, the resistor R6 and the transistor T12
Is connected to one end of a resistor R9 and a capacitor C
3 is connected, and the other end of the resistor R9 and the other end of the capacitor C3 are connected to the other end of the feedback winding N12. The connection point between the resistor R9 and the feedback winding N12 is connected to the emitter of the transistor T12 and the emitter of the switching transistor T11.

The cathode of the diode D3 is connected to the emitter of the transistor T13.
To one end. The cathode of the diode D3 is connected to a connection point between the starting resistor R1 and the starting resistor R3. The collector of the transistor T13 is connected to the base of the transistor T12.
8 is connected to the emitter of the phototransistor TP1. The other end of the resistor R10 is connected to the transistor T1.
3 and the collector of the phototransistor TP1.

In the switching power supply circuit 10, a blocking oscillation circuit 20 is formed on the primary side 10a. The blocking oscillation circuit 20 includes an input winding N11, a feedback winding N12, a starting resistor R1, a starting resistor R3, and a resistor R4. ,
It comprises a resistor R5, a capacitor C1, a diode D1, and a switching transistor T11. The control circuit (control means) 22 includes resistors R6 to R1.
0, a capacitor C3, a diode D2, a Zener diode DZ1, transistors T12 and T14, and a phototransistor TP1.

Further, the secondary side 1 of the converter transformer 12
0b is an output winding N13 as a first output winding, an output winding N14 as a second output winding, a rectifier circuit 24 as a first rectifier circuit, and a second rectifier circuit 2 as a second rectifier circuit.
6, including an output voltage detection circuit 28, a low voltage detection circuit 30 as a voltage detection circuit, and an analog switch SW11;
Load (external load) 3 via analog switch SW11
2 are connected. The secondary side 10 b includes a microcomputer 34, which is connected to the low voltage detection circuit 30.
The load 32 is not included in the switching power supply circuit 10.

More specifically, one end (starting end) of the winding N13
Has a diode D4 and a capacitor C connected in parallel.
4 is connected, and a resistor R11,
One ends of R12 and R13 are connected. Note that the other end of the output winding N13 is connected to a ground plane. A smoothing capacitor C5 is connected to a connection point between the diode D4 and the resistor R11.
And the other end of the smoothing capacitor C5 is connected to the ground plane. The rectifier circuit 24 is formed by the diode D4, the capacitor C4, and the smoothing capacitor C5.

The other end of the resistor R11 is connected to a ground via a variable resistor R14 and a resistor R15.
Connected to the base of transistor T14. The base of the transistor T14 is connected to the variable section P of the variable resistor R14. The other end of the resistor R12 is connected to a Zener diode DZ2.
And the anode of the Zener diode DZ2 is connected to the ground plane. The emitter of the transistor T14 is connected to a connection point between the resistor R12 and the Zener diode DZ2. The other end of the resistor R13 is connected to a resistor R16.
Connected to the ground plane via the
8 is connected to the anode of the photodiode DP1. The cathode of the photodiode DP1 is connected to the collector of the transistor T14. The output voltage detection circuit 28 is formed by the resistors R11 to R13, the variable resistors R14, the resistors R15 and R16, the zener diode DZ2, the photodiode DP1, and the transistor T14.

The cathode of a diode D5 is connected to one end (starting end) of the output winding N14, and the anode of the diode D5 is connected to the anode of a zener diode DZ3 via a resistor R17. One end of the resistor R18 and one end of the capacitor C6 are connected to a connection point between the resistor R17 and the Zener diode DZ3, and the other end of the resistor R18 and the other end of the capacitor C6 are connected to a ground plane. Note that the other end of the output winding N14 is connected to a ground plane. A rectifier circuit 26 is formed by the diode D5, the resistors R17 and R18, and the capacitor C6.

The cathode of Zener diode DZ3 is connected to the base of transistor T15 via resistor R19, and the collector of transistor T15 is connected to the ground plane. The emitter of the transistor T15 is connected via a resistor R20 to an output terminal 34a provided in the microcomputer 34. One end of a capacitor C7 and one end of a resistor R21 are connected to a connection point between the transistor T15 and the resistor R20, and the other end of the capacitor C7 is connected to a ground plane. The other end of the resistor R21 is connected to the base of the transistor T16, and the emitter of the transistor T16 is connected to the ground. The collector of the transistor T16 is
It is connected to the input terminal 30b of the microcomputer 34. The Zener diode DZ3, the resistors R19, R20, R21,
The low voltage detection circuit 30 is formed by the capacitor C7 and the transistors T15 and T16.

Further, a diode D6 and a capacitor C8 connected in parallel are connected to the start end of the output winding N14, and the other end of the parallel circuit is connected to the microcomputer 34 via the regulator 38. One end of a smoothing capacitor C9 is connected to a connection point between the diode D6 and the regulator 38, and the other end is connected to a ground plane. The diode D6, the capacitor C8 and the smoothing capacitor C
9 forms a rectifier circuit 36.

An AC power supply 14 is connected to the switching power supply circuit 10.
Is applied, the rectifier circuit 16 smoothes and rectifies the AC voltage to obtain a DC voltage. The DC voltage from the rectifier circuit 16 is applied to the base of the transistor T1 via the starting resistors R1, R3 and the driving resistor R5.
It is also provided to the input winding N11. Therefore, the switching transistor T11 conducts, a voltage is induced in the feedback winding N12, and the induced current from the feedback winding N12 is positively fed back to the base of the switching transistor T11 via the capacitor C1, the resistor R4, and the driving resistor R5. You. That is, the switching transistor T11 oscillates, and the blocking oscillation circuit 20 operates. Therefore, the oscillating current flows through the input winding N11, and an output voltage is generated at the output winding N13 and the output winding N14 in proportion to the number of turns of the input winding N11. In this embodiment, the output windings N13 and N14 are wound in the opposite direction to the input winding N11. In the figure
Is provided to indicate the winding start side of the windings N11 to N14.

The output voltage generated at the output winding N13 is rectified by the diode D4 and the smoothing capacitor C5, and the rectified DC voltage is supplied to the output voltage detection circuit 28. In the output voltage detection circuit 28, the transistor T14 is turned on or off by the difference between the voltage of the variable portion P of the variable resistor R14 and the zener voltage of the zener diode DZ2. Note that the capacitor C4 is provided to remove a DC voltage generated in the output winding N13 due to an oscillation current remaining after the switching transistor T11 is turned off and oscillation stops. The DC voltage charged in the capacitor C4 is discharged via the smoothing capacitor C5.

For example, when the DC voltage rises, the voltage of the variable portion P becomes higher than the Zener voltage of the Zener diode DZ2, and when the transistor T14 turns on, the collector current of the transistor T14 increases and the photodiode D
The light emission amount of P1 increases. When the light emission amount of the photodiode DP1 increases, the collector current of the phototransistor TP1 increases. The collector current of the phototransistor TP1 is amplified by the transistor T13, and the amplified current is applied to the base of the transistor T12. The rectangular wave voltage generated in the feedback winding N12 is integrated by the resistor R9 and the capacitor C3, and the integrated sawtooth voltage is superimposed on the amplified current. That is, the voltage value of the sawtooth voltage is increased and applied to the base of the transistor T12. Therefore, the transistor T12 turns on,
Since the base current of the switching transistor T11 is bypassed, the ON period of the switching transistor T11 is shortened, and the DC voltage taken out by the rectifier circuit 24 is reduced.

On the other hand, when the DC voltage taken out by the rectifier circuit 24 decreases, no forward voltage is generated between the base and the emitter of the transistor T14, so that the transistor T14 does not turn on. Therefore, the light emission amount of the photodiode DP1 in the photocoupler 18 decreases,
The collector current of transistor T14 decreases. Therefore, the collector current of the phototransistor TP1 also decreases. This collector current is amplified by the transistor T13 and applied to the base of the transistor T12. Therefore, the ON period of the switching transistor T11 becomes longer, and the voltage value of the DC voltage taken out by the rectifier circuit 24 is increased.

As described above, the control circuit 22 and the output voltage detection circuit 28 operate so that the DC voltage taken out by the rectifier circuit 24 is kept constant, and a constant voltage is applied to the load 32 via the analog switch SW11. Given. It should be noted that the rectifier circuit 26 and the low-voltage detection circuit 30 formed on the secondary side of the converter transformer 12 use the DC voltage extracted by the rectifier circuit 24 to control the control range of the control circuit 22. Is detected. Based on the detection result, the microcomputer 34 sets the load 32
Is turned on or off. The voltage induced in the output winding N14 is supplied to the rectifier circuit 26 and the rectifier circuit 36
And to the microcomputer 34 via the regulator 38. That is, similarly to the conventional switching power supply circuit 1 shown in FIG. 4, the rectified DC voltage is adjusted, and the adjusted DC voltage (constant voltage) is supplied to the microcomputer 34 as a power supply. Further, the capacitor C8 is provided to remove a DC voltage generated in the output winding N14 due to an oscillation current remaining after the switching transistor T11 is turned off and oscillation stops. The DC voltage charged in the capacitor C8 is discharged via the smoothing capacitor C9.

On the other hand, when the DC voltage extracted by the rectifier circuit is not within the control range of the control circuit 22, the switching transistor T11 runs out of heat. That is, when the AC voltage decreases, the peak value of the rectangular wave voltage generated in the feedback winding N12 decreases, and the base current that is positively fed back and supplied to the switching transistor T11 decreases. And
The transistor T1 is longer than the ON period of the transistor T11.
If the ON period of No. 2 is longer, the output voltage will not be constant even if the collector current flows up to the maximum value (base current × h _fe ). Therefore, no signal is output from the phototransistor 18. This state is an uncontrollable state. Further, in the control circuit 22, the switching transistor T11 cannot be controlled, and the switching loss becomes extremely large. Therefore, heat is generated in the switching transistor T11.

When the control becomes impossible, that is, when the collector current of the switching transistor T11 reaches the maximum value, the switching transistor T11 is turned off. That is, the switching transistor T11 is saturated. When the switching transistor T11 turns off, the collector current starts to decrease. In the feedback winding N12 of the converter transformer 12, a voltage is generated in a direction opposite to that when the switching transistor T11 is on, and the base voltage further decreases. Therefore, the feedback winding N1
2, the switching transistor T1
1 suddenly enters a cutoff state. At this time, the input winding N11
Generates a large induced electromotive force, and the collector voltage of the switching transistor T11 increases. The product of the collector current and the collector voltage at this time is a loss, and the heat generation increases. Therefore, due to the synergistic interaction between the loss and the heat generation, the switching transistor T11 runs out of heat.

However, in the switching power supply circuit 10 of this embodiment, when the DC voltage (first output) extracted by the rectifier circuit 24 is an AC voltage input that can be controlled by the control circuit 22, the Zener diode DZ3 is turned on. And the transistor T15 is also turned on. Therefore, output terminal 3
The constant voltage (bias voltage) output from 4a is a resistor R2
0 to the emitter of transistor T15. At this time, since the transistor T15 is on,
No constant voltage is applied to the base of the transistor T16 from the output terminal 34a. That is, the transistor T16 does not turn on, and no signal (first signal) is supplied to the input terminal 34b. When a signal is not supplied to the input terminal 34b, the microcomputer 34 determines that the DC voltage extracted by the rectifier circuit 24 is within the control range, and turns on the analog switch SW11. Note that the analog switch SW
If 11 is already on, the on state is maintained.

On the other hand, when the DC voltage taken out of the rectifier circuit 24 falls below the control range, that is, when an AC voltage outside the control range is input, the diode D5 is turned off and the Zener diode DZ3 is turned off. Is done.
That is, the output voltage of the output winding N14 becomes the reference voltage,
Therefore, the transistor T15 is turned off and the output terminal 34a
Is applied to the base of transistor T16 via resistors R20 and R21. Therefore, the transistor T16 is turned on, and a low-level signal is supplied to the input terminal 34b. When detecting the low level signal, the microcomputer 34 determines that the AC voltage of the AC power supply 14 is out of the control range, and turns off the analog switch SW11. That is, by turning off the load 32,
Reduce external load. Therefore, thermal runaway of the switching transistor T11 can be prevented.
Note that a fuse (or circuit breaker) is provided between the AC power supply 14 and the rectifier circuit 16, and when the AC current rises abnormally, the AC power supply is cut off by the fuse.

In this embodiment, a DC voltage (second output) having the opposite polarity (negative) to that of the rectifier circuit 24 is extracted from the output winding N14 by the diode D5 of the rectifier circuit 26. That is, as shown in FIG. 2, the positive portion of the rectangular wave voltage is rectified by the diode D4 and can be controlled by the control circuit 22, so that the control circuit 22 controls the voltage to a substantially constant value. On the other hand, the negative portion of the rectangular wave voltage cannot be rectified by the diode D4 and cannot be detected by the photocoupler 18. The negative part of this rectangular wave voltage has a magnitude that directly reflects the AC voltage, and the negative part is the rectifier circuit 2.
The low-voltage detection circuit 30 detects whether the current is within the control range of the control circuit 22 or not.

As shown in FIG. 3, the switching power supply circuit 10 of another embodiment further includes a low voltage detection circuit 40,
An input terminal 34c for receiving a signal from the low voltage detection circuit is provided in the microcomputer 34, and a load (external load) 4 is connected to the output stage of the output voltage detection circuit 28 via the analog switch SW12.
1 except that the switching power supply circuit 1 of FIG.
Since it is the same as 0, duplicate description will be omitted. In addition,
The load 42 is not included in the switching power supply circuit 10.
1, a DC voltage is applied to the microcomputer 34 from the output winding N14 through the rectifier circuit 36 and the regulator 38. For convenience of explanation, the rectifier circuit 36 and the regulator 38 are not shown in the drawing. Omitted.

In the switching power supply circuit 10, the zener voltage is set high by connecting the zener diode DZ4, and the transistor T15 is turned off when the AC voltage is lower than when the transistor T17 is turned off. That is, when the transistor T15 is turned off, the transistor T17 is already turned off. Therefore, low level signals are input from transistors T16 and T18 to input terminals 34b and 34c.
When a low-level signal is given to the input terminals 34b and 34c, the microcomputer 34 switches the analog switch SW
11 and SW12 are turned off, and the load 32 and the load 42
However, when a low-level signal is given only to the input terminal 34c, only the analog switch SW12 is turned off. For example, if the load 32 is a cathode of a CRT of a television receiver and a control circuit adjusts the load 38 (cathode), the analog switch SW11 (cathode) is turned off when the AC voltage drops abnormally. That is, power is applied only to the microcomputer 34, and the television receiver is set to the standby state. On the other hand, when the AC voltage slightly drops below the control range, only the analog switch SW12 is turned off, that is, the cathode control circuit is turned off. Therefore, for example, the amount of the electron beam emitted from the cathode is reduced, and the screen output to the CRT becomes dark.

According to these embodiments, the fluctuation of the AC voltage is accurately determined, and the load is reduced when the AC voltage decreases. Therefore, the thermal runaway of the switching transistor T11 can be prevented. In these examples,
Although the output windings N13 and N14 are provided individually, only the output winding N13 may be used. At this time, the input terminal of the rectifier circuit 26 is connected to the connection point between the output winding N13 and the capacitor C4. Therefore, the number of taps provided in converter transformer 12 can be reduced.

In these embodiments, the output voltage is detected as a negative fluctuation. However, the output winding N14 is wound in the same direction as the input winding N11, and the output voltage is applied to a parallel circuit of a diode and a capacitor. If the rectification is performed, it can be detected as a positive fluctuation.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of the present invention.

FIG. 2 is an illustrative view showing a DC voltage extracted from an output winding shown in the embodiment of FIG. 1;

FIG. 3 is a circuit diagram showing another embodiment of the present invention.

FIG. 4 is a circuit diagram showing a conventional switching power supply circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Switching power supply circuit 12 ... Converter transformer 16, 24, 26 ... Rectifier circuit 18 ... Photocoupler 20 ... Blocking oscillation circuit 22 ... Control circuit 28 ... Output voltage detection circuit 30, 36 ... Low voltage detection circuit 32, 38 ... Load 34 ... Microcomputer

Claims (3)

[Claims] 1. A blocking oscillation circuit having a converter transformer, and a first oscillator for rectifying a rectangular wave voltage obtained in a first output winding of the converter transformer and outputting a first output.
A switching power supply circuit including a rectifier circuit and control means for controlling the blocking oscillation circuit in accordance with the first output; a second rectifier circuit for obtaining a second output having a polarity different from the first output; 2 When the output reaches the reference voltage,
A switching power supply circuit comprising a voltage detection circuit that outputs a signal. 2. The switching power supply circuit according to claim 1, wherein said converter transformer includes a second output winding, and said second rectifier circuit receives an output of said second output winding. 3. The switching power supply circuit according to claim 1, further comprising switching means provided between said first rectifier circuit and an external load, said switching means being turned off when said first signal is output.