JPH0974755A - Self-oscillation type switching power supply - Google Patents

Abstract

(57) [Abstract] [Purpose] To realize a self-excited oscillation type switching power supply device capable of controlling the output voltage without being influenced by the output voltage when there is no load due to the component variations, especially the CTR variation of the photocoupler. [Configuration] A circuit for charging and discharging the capacitor 7 by the phototransistor of the photocoupler 9 and the resistor 10,
In the circuit in which the first switching element 3 is controlled by the second switching element 6 which makes the pulse width variable according to the charging time constant, the zener diode 13 and the capacitor 7 are connected in parallel to the capacitor 7.
And a resistor 1 between the cathode of the Zener diode 13 and
4 is connected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a self-excited oscillation type switching power supply device suitable for a power supply for electronic equipment, for example, a power supply for relatively inexpensive electronic equipment such as a personal computer.

[0002]

2. Description of the Related Art In recent years, miniaturization and price reduction of various electronic devices have been promoted, and miniaturization and price reduction of switching type power supply devices used in these electronic devices are also required.
The self-excited oscillation type switching power supply device is most advantageous as a circuit that can be manufactured at the lowest cost.

Hereinafter, a conventional self-oscillation type switching power supply will be described.
The device will be described. Figure 2 shows the conventional self-oscillation type switch.
1 illustrates a configuration of a power supply device for a power supply. Figure 2 Smell
, 1 is a rectifying and smoothing circuit, which receives an AC input voltage
Convert to DC input voltage. 2 is a switching transformer
Then, the primary winding N₁, Drive winding N_DSecondary winding N ₂Have
You. Primary winding N₁Is connected to the positive side of the rectifying and smoothing circuit 1.
Connected to the electrodes and the other end through the first switching element 3.
And is connected to the negative electrode of the rectifying and smoothing circuit 1. 4
Is the primary winding N₁And the connection point of the rectifying and smoothing circuit 1 and the first switch.
It is connected in series to the base of the switching element 3 and supplies the starting current.
The resistance to supply 5 and the base of the first switching element 3
Drive winding N_DSet base current by connecting in series with one end of
Resistance. 6 is a collector for the first switching element
It is connected to the base of the child 3 and the emitter is the drive winding N_DOne end of
Connected to the emitter via the capacitor 7,
The anode of the diode 8 and the phototransistor of the photocoupler 9.
Connected to the collector via the emitter of the transistor
Then, connect the resistor 10 in parallel with the diode 8
Condensed by the phototransistor of the plastic 9 and the resistor 10.
Charge the capacitor 7 and discharge the capacitor 7 by the diode 8.
I do. The photocoupler 9 separates and insulates the primary side and the secondary side.
The light emitting diode on the secondary side of the photocoupler 9 is controlled.
Connected to the control circuit 11. 12 is an output rectifier circuit and is a secondary winding
Line N₂To supply DC. DC output is a control circuit
It is connected to 11 and detects and controls the output voltage.

FIG. 3 shows operation waveforms of a conventional self-excited oscillation type switching power supply device. FIG. 3A shows the first
Collector-emitter voltage V _CE of the switching element 3 of FIG. 3, FIG. 3B is a collector-emitter voltage V _CE of the second switching element 6, and FIG. 3C is a base-emitter of the second switching element 6. Voltage V _BE , Fig. 3 (d)
Indicates the collector current I _C of the second switching element 6.

The operation of the conventional circuit configured as described above will be described below.

First, when an AC input voltage is applied, a DC voltage is generated in the rectifying / smoothing circuit 1, a current flows through the base of the first switching element 3 through the resistor 4, and the first switching element 3 is turned on. A voltage is applied to the next winding N ₁ , an induced voltage proportional to this voltage is generated in the drive winding N _D , and the first switching element 3 is connected via the resistor 5.
The base current of is further increased to enter the ON period.

On the other hand, the voltage induced at the drive winding N _D charges the capacitor 7 by the resistor 10 and the voltage across the capacitor 7 increases, so that the emitter of the second switching element 6
It is a voltage that forward biases the bases. Due to this voltage, the second switching element 6 is turned on, the base current of the first switching element 3 is rapidly drawn, and the base of the first switching element 3 is zero-biased and turned off. In the off period, the induced voltage generated in the drive winding N _D generates a voltage in the direction that reversely biases the base of the first switching element 3, and the excitation energy accumulated in the switching transformer 2 is transferred to the secondary winding N _{D. 2} is discharged to the DC output through the output rectification circuit 12. The excitation energy is lost, the since the first induced voltage a base in the direction of forward bias of the switching element 3 again to drive winding N _D by the ringing voltage generated by the leakage inductance and distributed capacitance of the switching transformer 2 occurs The switching element 3 of No. 1 is turned on.

Hereinafter, the above operation is repeated to supply the output voltage. When the output voltage reaches the voltage set by the control circuit 11, the control circuit 11 operates to activate the photocoupler 9, and the first switching element 3 discharges the electric charge of the capacitor 7 discharged during the off period. The switching element 3 of No. 1 is charged through the resistor 10 and the photocoupler 9 during the ON period. As a result, the charging time of the capacitor 7 is faster than the charging time of the resistor 10 alone, so the ON period is shortened, the duty ratio changes, and the output voltage is kept constant. That is, the output voltage is controlled by the control circuit 11.

[0009]

However, in the above conventional configuration, when the DC output is unloaded, it is necessary to shorten the ON period of the first switching element 3 to reduce the excitation energy, and the charging time of the capacitor 7 is reduced. On the other hand, when the CTR (current transfer rate) of the photocoupler 9 is small, on the other hand, the charging time of the capacitor 7 can be shortened to a certain point, and the ON period of the first switching element 3 is shortened. It is impossible to control the output voltage.

On the other hand, when the CTR of the photocoupler 9 is large, the charging of the capacitor 7 becomes too fast.
Since the second switching element 6 cannot be driven and remains in the ON state for a while, the first switching element 3 is also maintained in the state in which it cannot be turned ON, resulting in an intermittent oscillation state and an audible sound. As described above, the conventional circuit configuration has a drawback in that output control under no load is extremely difficult due to variations in parts.

The present invention solves the above-mentioned conventional problems, and an object of the present invention is to provide a self-excited oscillation type switching power supply device which is highly reliable with a simple circuit.

[0012]

In order to solve the above problems, a self-excited oscillation type switching power supply device according to the present invention comprises a switching transformer having a primary winding, an output winding and a drive winding, and at least a drive winding. First to control the primary winding by wire
A charging / discharging circuit in which a parallel circuit of a switching element of, a phototransistor of a photocoupler, a diode and a resistor, and a capacitor are connected in series is connected between the drive windings, and the capacitor is driven by a voltage induced between the drive windings. Second, the charge and discharge are performed by the diode and the phototransistor of the photocoupler, and the pulse width is changed according to the charging time of the capacitor, that is, at least the charge time constant of the charge / discharge circuit configured by the phototransistor and the capacitor of the photocoupler. The switching element controls the first switching element, and a Zener diode is connected in parallel with the capacitor, and a resistor is connected between the capacitor and the cathode of the Zener diode.

[0013]

With this configuration, since the control of the charging time of the capacitor, that is, the ON period of the first switching element can be stabilized, a self-excited oscillation type switching power supply device capable of controlling the output voltage under no load is provided. It is provided.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows the configuration of a self-oscillation type switching power supply device of the present invention. In FIG. 1, 1 is a rectifying / smoothing circuit, which receives an AC input voltage and converts it into a DC input voltage. A switching transformer 2 has a primary winding N ₁ , a drive winding N _D , and a secondary winding N ₂ . Primary winding N ₁
Has one end connected to the positive electrode of the rectifying and smoothing circuit 1,
The other end is connected to the negative electrode of the rectifying / smoothing circuit 1 via the first switching element 3. Reference numeral 4 is a resistor connected in series to the connection point between the primary winding N ₁ and the rectifying / smoothing circuit 1 and the base of the first switching element 3 to supply a starting current, and 5 is a base of the first switching element 3 and a drive winding. A resistor connected in series with one end of the line N _D to set the base current.

A second switching element 6 has a collector connected to the base of the first switching element 3, an emitter connected to one end of the drive winding N _D , and a base connected to a capacitor 7.
To the emitter, the anode of the diode 8 and the collector of the phototransistor of the photocoupler 9 to the collector respectively, and the resistor 10 is connected in parallel with the diode 8.
The capacitor 7 is charged by the phototransistor of the photocoupler 9 and the resistor 10, and the capacitor 7 is discharged by the diode 8. The photocoupler 9 separates and insulates the primary side and the secondary side, and the light emitting diode on the secondary side of the photocoupler 9 is connected to the control circuit 11.

An output rectifier circuit 12 is connected to the secondary winding N ₂ and supplies DC. The DC output is connected to the control circuit 11 to detect and control the output voltage. Further, a resistor 14 is connected in series to the emitter of the phototransistor of the photocoupler 9 of the charging circuit, and further, the emitter of the phototransistor of the photocoupler 9 and the cathode of the Zener diode 13, the emitter of the second switching element 6 and the Zener diode. By connecting 13 anodes, the charging time of the capacitor 7 can be limited.

The operation of the self-excited oscillation type switching power supply device of the present invention configured as described above will be described below.

First, when the DC output is unloaded, the DC output voltage increases, so that the ON period of the first switching element 3 is shortened to reduce the excitation energy, and the current of the photodiode of the photocoupler 9 is reduced. The current of the phototransistor also increases in accordance with the CTR.
As a result, the charging time of the capacitor 7 is shortened, and FIG.
As shown in (c), the slope of the base-emitter voltage V _BE of the second switching element 6 shifts from (a) to (b).

However, when the CTR is small due to the variation in the CTR of the photocoupler 9, the current from the phototransistor is small and the charging time of the capacitor 7 cannot be shortened, and the ON period of the first switching element 3 is reduced. It can only be shortened to a certain time. Therefore, it becomes impossible to control the DC output voltage rise under no load. The constants of the Zener diode 13, the resistor 14, and other parts are determined in order to set the potential of the emitter of the phototransistor of the photocoupler 9 and the charging current of the capacitor 7 so that the DC output voltage can be controlled even at this time.

On the contrary, when the CTR of the photocoupler 9 is large, the current charged in the capacitor 7 increases, and FIG.
As shown in (c), the slope of the base-emitter voltage V _BE of the second switching element 6 becomes large, and the charging time of the capacitor 7 is shortened to a region where the second switching element 6 cannot be driven. Therefore, the ON state of the second switching element 6 continues, and the state in which the first switching element 3 cannot be turned ON is maintained.

As described above, when the CTR of the photocoupler 9 is large, the current flowing to the emitter of the phototransistor of the photocoupler 9 increases, but when the CTR of the photocoupler 9 is small, the Zener diode 13 and the resistance determined From the constant of 14, the upper limit of the current flowing from the emitter of the phototransistor of the photocoupler 9 is limited by the Zener diode 13 and the resistor 14 to limit the charging current of the capacitor 7 and the second switching element 6 cannot drive. The charging time of the capacitor 7 is not shortened to the area.

Further, by preventing the charging time of the capacitor 7 from becoming too fast, the slope of the base-emitter voltage V _BE of the second switching element 6 becomes small, and the driving of the first switching element 3 becomes possible, and the output The voltage can be controlled. As described above, the influence of the variation in the CTR of the photocoupler 9 is reduced by limiting the charging current of the capacitor 7 by the Zener diode 13 and the resistor 14, and the output voltage can be controlled under no load.

[0024]

As described above, the present invention is characterized in that a resistor is connected in series to the emitter of the phototransistor of the photocoupler of the charge / discharge circuit of the capacitor, and a Zener diode is connected in parallel to the resistor and the capacitor. With such a configuration, it is possible to realize an excellent self-excited oscillation type switching power supply device that stably controls the output voltage even under no load.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram showing an embodiment of a self-oscillation type switching power supply device of the present invention.

FIG. 2 is a conventional circuit configuration diagram.

FIG. 3 is an operation waveform diagram in the present invention and the conventional circuit configuration.

[Explanation of symbols]

1 rectifying / smoothing circuit 2 switching transformer 3 first switching element 4 resistance 5 resistance 6 second switching element 7 capacitor 8 diode 9 photocoupler 10 resistance 11 control circuit 12 output rectifier circuit 13 zener diode 14 resistance (a) first Collector-emitter voltage V _CE of the switching element (b) Collector-emitter voltage V _{CE of} the second switching element (c) Base-emitter voltage V _BE (d) of the second switching element Second switching element Collector current I _C

Claims (1)

[Claims] 1. A switching transformer having a primary winding, an output winding, and a drive winding, and at least one drive winding.
A charging / discharging circuit in which a first switching element for controlling the next winding, a phototransistor of a photocoupler, a parallel circuit of a diode and a resistor, and a capacitor are connected in series is connected between the drive windings, and The voltage is induced by the capacitor to charge and discharge the diode and the phototransistor of the photocoupler, and the pulse width depends on the charging time of the capacitor, that is, at least the charging time constant of the charging and discharging circuit composed of the phototransistor and the capacitor of the photocoupler. The first switching element is controlled by the second switching element that makes the
A self-excited oscillation type switching power supply device in which a Zener diode is connected in parallel with the capacitor and a resistor is connected between the capacitor and the cathode of the Zener diode.