JP2970041B2 - Switching power supply - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a self-excited switching power supply.

[0002]

2. Description of the Related Art A switching regulator of a type in which a series circuit of a transformer and a switching transistor is connected to a DC power supply and the switching transistor is driven in a positive feedback manner by a voltage of a driving winding coupled to the transformer is widely used.

There is also known a method of delaying a turn-on operation of a switching transistor by a voltage of a driving winding (feedback winding). That is, when a capacitor is connected in parallel to the switching transistor, or when the switching transistor has a stray capacitance, when the switching transistor starts to turn on before the switching transistor voltage becomes zero, the voltage and current of the switching transistor There is known a method of forcibly delaying the on-start time of the switching transistor to reduce the power loss due to the product of the power loss and the power loss.

[0004]

However, since the on-time width of the conventional switching transistor is performed by bypassing the base current, a power loss is caused. In addition, since the bypass transistor functions as a variable resistance element, it has been difficult to quickly turn on and off the main transistor.
It is also difficult to reliably and easily obtain the effect of turn-on delay.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a switching power supply capable of reliably and easily performing a turn-on delay effect of a switching element.

[0006]

According to the present invention, there is provided a transformer having a main winding of a transformer connected to one end of a DC power supply, first and second main terminals, and a control terminal. A switching element having the first main terminal connected to one end of the DC power supply via the main winding and the second main terminal connected to the other end of the DC power supply; and a switching element connected to the transformer. And a drive winding electromagnetically coupled to the main winding and connected between the control terminal and the second main terminal of the switching element so as to positively drive the switching element. A switching power supply device comprising a line, and a starting circuit connected to the control terminal of the switching element, wherein the switching element is connected between the control terminal of the switching element and the second main terminal, and A control transistor having a polarity that is forward-biased by the voltage of the driving winding during the ON period of the driving winding, a resistor, a first capacitor, and a series circuit connected to one end of the driving winding; A second capacitor connected to the other end of the line, a variable impedance element connected between the series circuit and the second capacitor, and a second capacitor connected between the second capacitor and one end of the drive winding. And a diode having a directionality that is forward-biased by a voltage induced in the drive winding during the off period of the switching element, and the variable impedance for adjusting an output voltage of the rectifying / smoothing circuit. A voltage control circuit for controlling the element, and a base of the control transistor is directly connected to a connection point between the series circuit and the variable impedance element. Those related to the switching power supply apparatus characterized by being connected via a transistor.

[0007] In order to reduce the influence of fluctuations in the input voltage, a trigger capacitor can be connected between the drive winding and the switching element, and a drive signal sustaining circuit can be provided in parallel with the capacitor.

[0008]

The second capacitor is charged with the voltage generated in the drive winding during the off period of the switching element. The charging voltage of the second capacitor turns on the control transistor during the off period of the switching element. Even if the reverse voltage of the drive winding drops during the switching period of the switching element from off to on, and then a forward voltage is generated,
Since the control transistor is on, the switching element does not immediately turn on.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, an RCC (on / off type) switching power supply according to an embodiment of the present invention will be described with reference to FIGS. One terminal 2 and the other terminal 3 of a DC power supply 1 comprising a rectifier and a smoothing circuit connected to an AC power supply
A main transistor 6 as a switching element for conversion is connected between the first and second transformers via a primary winding (main winding) 5 of the transformer 4. That is, the collector (first main terminal) of the main transistor 6 is connected to one power supply terminal 2 via the primary winding 5, and the emitter (second main terminal) is connected to the other power supply terminal 3. ing.

A rectifying / smoothing circuit 10 including a diode 8 and a capacitor 9 is connected to a secondary winding 7 which is an output winding of the transformer 4. A load 11 is connected to the rectifying / smoothing circuit 10.

One end of the tertiary winding of the transformer 4, ie, the drive winding 12, is connected to the base (control terminal) of the main transistor 6 via a resistor 13 and a capacitor 14, and the other end is connected to the emitter of the main transistor 6. Have been. The resistance 1
An ON drive signal sustaining circuit 15 is connected in parallel with the capacitor 3 and the capacitor 14.

The pnp type control transistor 16 is connected between the base and the emitter of the main transistor 6.
That is, the emitter of the control transistor 16 is connected to the base of the main transistor 6, and the collector is connected to the emitter of the main transistor 6.

A series circuit of a resistor 17 and a first capacitor 18 is connected to one end of the drive winding 12. The other end of the drive winding 12 is connected to a second capacitor 19. A phototransistor 20 is connected between the first capacitor 18 and the second capacitor 19 as a variable impedance element for voltage control. Phototransistor 20
The connection point 21 between the first capacitor 18 and the first capacitor 18 is connected to the base of the control transistor 16 via the base and the emitter of the pnp transistor 22. The collector of the transistor 22 is connected to the second capacitor 1 via the resistor 23.
It is connected to a connection point 24 between the photo transistor 9 and the phototransistor 20. A bias setting resistor 25 is connected in parallel with the phototransistor 20. A diode 26 is connected between the second capacitor 19 and the upper end of the drive winding 12 and has a direction of being forward biased during the off period. Resistance 17
A positive voltage clamping diode 27 is connected between the driving coil 12 and the lower end of the driving winding 12.

A control circuit 28 for controlling the phototransistor 20 includes voltage detecting resistors 29 and 30, an error amplifier 31, a reference voltage source 32, and a light emitting diode 33. Since the detection resistors 29 and 30 are connected to the rectifying / smoothing circuit 10, the error amplifier 31 outputs a voltage corresponding to the difference between the reference voltage and the detection voltage, and the light emitting diode 33 optically coupled to the phototransistor 20 generates an error. Light is emitted according to the output of the amplifier 31.

A capacitor 34 connected between the lower end of the primary winding 5 and the ground, that is, in parallel with the main transistor 6,
It is for partial resonance. Instead of the capacitor 34, a stray capacitance of the main transistor 16 and the like can be used. The resistor 35 connected between the power supply terminal 2 and the base of the main transistor 6 functions as a starting circuit for supplying a starting current to the main transistor 6.

FIG. 2 shows the ON drive sustaining circuit 15 of FIG. 1 in detail. The drive sustaining circuit 15 includes a capacitor 36, diodes 37 and 38, a transistor 39, and a resistor 4
0, 41, and 42. The capacitor 36 is connected to the drive winding 12 via the diodes 37 and 38. Since the diodes 37 and 38 have a direction of being forward-biased during the off-period of the main transistor 6, the off-state is independent of the input voltage fluctuation. The capacitor 36 is charged with the constant voltage during the period. The transistor 39 connected between the capacitor 36 and the base of the main transistor 6 is turned on by the current flowing through the resistor 42 during the ON period of the main transistor 6,
A base current is supplied using the capacitor 36 as a power supply. When the main transistor 6 is turned on, a differential current (trigger current) flows at t1 via the capacitor 14 as shown in FIG. 3A, and the main transistor 6 is turned on. At the same time, the transistor 39 is turned on, and the capacitor 3
6, the base current is continuously supplied as shown in FIG. When the main transistor 6 is turned off at t2, the capacitor 14 is charged in the reverse direction.

[0017]

Next, the operation of the circuit shown in FIG. 1 will be described with reference to FIGS. FIG. 5 is a circuit diagram showing a part of FIG. 1 to explain the state of the main transistor 6 in the off period. FIG. 4 is a waveform diagram showing the currents I1, I2, I3 and the voltages V1, V2, V3 of FIGS.

When power supply from the DC power supply 1 is started, a base current flows to the main transistor 6 through the starting resistor 35, and this is turned on. As a result, the power supply voltage is applied to the primary winding 5, a positive feedback voltage is obtained in the drive winding 12, and the main transistor 6 is driven by the voltage of the drive winding 12. 1
Since the secondary winding 5 has an inductance, the collector current Ic of the main transistor 6 increases with time. Thereafter, when the control transistor 16 is turned on, the main transistor 6 is turned off. The diode 8 of the rectifying / smoothing circuit 10 has a polarity that the main transistor 6 is turned off during the on-period and turned on during the off-period. I do. When the release of the stored energy is completed, an oscillating voltage for turning on the main transistor 6 is generated in the drive winding 12, and the main transistor 6 is turned on.

Next, the on / off control operation of the main transistor 6 will be described in more detail. During most of the off period (t1 to t4) of the main transistor 6, a voltage corresponding to the voltage of the output smoothing capacitor 9 controlled to a constant value is obtained in the drive winding 12 downward. As a result, the second capacitor 19 is charged in a closed circuit including the drive winding 12, the second capacitor 19, and the diode 26. Further, the first capacitor 18 is charged in a closed circuit including the drive winding 12, the collector and the base of the control transistor 16, the emitter and the base of the transistor 22, the first capacitor 18, and the resistor 17. At the same time, the second capacitor 1
9 and the drive winding 12 as a power source, a current flows between the collector and the base of the control transistor 16, between the emitter and the base of the transistor 22, a circuit including the resistor 25 and the phototransistor 20. On is maintained. In addition, during the off period of the main transistor 6, the base driving capacitor 14 is
As shown in (2), the base side of the main transistor 6 is positively charged.

When the discharge of the energy from the transformer 4 in FIG. 4 is completed, the voltage V2 of the drive winding 12 is inverted from negative to positive. This voltage V2 changes with a slope as shown in FIG. Assuming that the voltage V2 of the drive winding 12 is, for example, -8 V at the time t2, even if the negative amplitude starts to decrease gradually, the collector-base voltage of the transistor 16 operating as a reverse transistor and the voltage of the transistor 22 As long as the voltage V3 corresponding to the sum of the voltage between the emitter and the base does not exceed the sum of the rising voltages of the two transistors 16 and 22 (for example, 1.0 V), the on state of the two transistors 16 and 22 is maintained. Drive winding 1
2 rises from -8V to about -1V, the voltage between the collector and base of the transistor 16,
It becomes impossible to maintain the on state of the two transistors 16 and 22 in the circuit including the phototransistor 20 and the diode 26 between the emitter and the base of the transistor 22, and they are turned off at the time t3 in FIG.

By the way, when the amplitude of the reverse voltage of the drive winding 12 decreases at the time t2, the discharge of the capacitor 14 starts, and this discharge current flows as the emitter current I3 of the transistor 16. At time t3, the transistor 16 is turned off, but the storage-based current continues to flow through the transistor 16. Therefore, the collector current Ic of the main transistor 6 does not immediately flow at time t3. That is, a turn-on delay action of the main transistor 6 occurs. Even if the direction of the voltage V2 of the drive winding 12 becomes positive, the collector current Ic of the main transistor 6 does not rise at this initial stage because the control transistor 16 is turned on in the storage. Thereafter, when the control transistor 16 is completely turned off, the main transistor 6 is turned on, and the collector current Ic rises from time t4. When the main transistor 6 is turned on, the discharge current of the partial resonance capacitor 34 shown in FIG. However, since this current flows after the voltage between the collector and the emitter of the main transistor 6 decreases, the power loss is small.

Since the two transistors 16 and 22 on the control side are off during the ON period of the main transistor 6, the potential at the connection point 21, that is, the voltage V3,
It changes independently of 22. If the voltages of the phototransistor 20 and the resistor 17 are disregarded of Va and Vb, the voltage V3 during the ON period can be expressed by the following equation. V3 = V2 + Vc1 That is, the voltage V3 at the node 21 changes depending on the voltage of the first capacitor 18. When the first capacitor 18 is reversely charged and the voltage Vc1 gradually decreases, the voltage V3 also gradually decreases as shown in FIG. This voltage V3 is shown in FIG.
When the threshold voltage Vth of the two transistors 16 and 22 becomes lower than the threshold voltage Vth at time t5, the two transistors 16 and 22 are turned on again,
The current I3 flows through the control transistor 16. On the other hand, the base current I2 of the main transistor 6 rapidly decreases, cannot maintain saturation, and is turned off at t6.

In the above description, the phototransistor 20 has a constant resistance. In this embodiment, since the phototransistor 20 is used for voltage control, the resistance value changes according to the change of the output voltage. Assuming that the output voltage has become higher than the reference value, the resistance value of the phototransistor 20 decreases, the time constant of reverse charging of the first capacitor 18 decreases, and as shown in FIG. The decrease is sharp, and the on-time width of the main transistor 6 is reduced.

This embodiment has the following effects. (A) Since the control transistor 16 functions not as a variable resistance element but as a switching element, a base current bypass operation during the ON period does not occur continuously, and the power loss is reduced. (B) Since the control transistor 16 is in the on state during the off period, a current flows through the control transistor 16 when the main transistor 6 is turned on, so that a delay effect at the time of turning on the main transistor 6 occurs. Turn-on loss can be reduced. (C) Since the transistors 16 and 22 are switched based on the voltage V3 and the main transistor 6 is controlled,
The switching speed of the main transistor 6 can be increased.

[0025]

[Modifications] The present invention is not limited to the above-described embodiment, and for example, the following modifications are possible.

A configuration in which the transistor 22 is omitted can be employed.

The phototransistor 20 can be replaced by a voltage or current control impedance element (such as a transistor).

The base current supply circuit of the main transistor 6 can be variously modified. For example, the continuous drive circuit 15
A diode can be connected in parallel with the capacitor 14.

The output voltage may be detected by the tertiary winding 12 or an independent detection winding.

[0030]

As is apparent from the above description, according to the present invention, a turn-on delay effect can be easily and satisfactorily obtained.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a switching power supply according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a part of FIG. 1 in detail.

FIG. 3 is a waveform chart for explaining the operation of the circuit of FIG. 2;

FIG. 4 shows I1, I2, I3, Ic, V1, V2,
FIG. 6 is a waveform diagram showing V3.

FIG. 5 is a circuit diagram equivalently showing a voltage relationship of each part in FIG. 1 during an off period of a main transistor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 DC power supply 6 Main transistor 12 Drive winding 16 Control transistor 18 First capacitor 19 Second capacitor 20 Phototransistor

Claims (2)

(57) [Claims] 1. A main winding of a transformer connected to one end of a DC power supply, first and second main terminals, and a control terminal,
The first main terminal is connected to one end of the DC power supply via the main winding, and the second main terminal is connected to a switching element connected to the other end of the DC power supply, and to the transformer. A rectifying / smoothing circuit, a driving winding electromagnetically coupled to the main winding, and connected between the control terminal and the second main terminal of the switching element so as to perform positive feedback driving of the switching element. And a start-up circuit connected to the control terminal of the switching element, wherein the switching power supply is connected between the control terminal of the switching element and the second main terminal, and A control transistor having a polarity that is forward-biased by a voltage of the drive winding during an ON period; a resistor connected to one end of the drive winding and a first capacitor; A second capacitor connected to the other end of the drive winding; a variable impedance element connected between the series circuit and the second capacitor; and a second capacitor connected to the second capacitor. And a diode connected between the driving winding and one end of the driving winding, and having a directionality that is forward-biased by a voltage induced in the driving winding during an OFF period of the switching element; and A voltage control circuit that controls the variable impedance element to adjust the output voltage, and the base of the control transistor is connected directly or via a transistor to a connection point between the series circuit and the variable impedance element. A switching power supply characterized in that the switching power supply is connected. 2. The apparatus according to claim 1, further comprising a capacitor connected between one end of the drive winding and a control terminal of the switching element, and a drive signal sustaining circuit connected in parallel to the capacitor. The switching power supply device according to claim 1.