JP3261646B2 - Self-excited 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 switching power supply.

[0002]

2. Description of the Related Art A conventional self-excited switching power supply is shown in FIG.
As in the circuit example shown in FIG. 2 , the main transformer is provided with a positive feedback winding, thereby obtaining self-excited oscillation.

[0003]

In the conventional circuit, even if both ends of the secondary winding of the main transformer are short-circuited, oscillation does not stop and the secondary winding itself of the main transformer and the switching element may generate heat.

In the conventional circuit, when the output is short-circuited, the diode of the rectifying / smoothing circuit, the secondary winding itself of the main transformer, and the switching accumulator may generate heat.

[0005] Short circuits across the secondary winding and output shorts can be improved by adding fuses or overcurrent limiting circuits, but this increases the number of parts and costs.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a self-excited switching power supply in which an overcurrent does not flow even if both ends of a secondary winding of a main transformer are short-circuited or an output is short-circuited.

Another object of the present invention is to provide a self-excited switching power supply that does not cause an overcurrent even when the both ends of the secondary winding of the main transformer are short-circuited or the output is short-circuited over a wide input voltage range. To provide.

Still another object of the present invention is to stabilize an output voltage by transmitting a feedback signal using a saturable transformer.

[0009]

To achieve the above object, a self-excited switching power supply according to claim 1 of the present invention has a positive feedback from a secondary winding of a main transformer to a control electrode of a switching element via a saturable transformer. Send a signal.

[0010]

According to a first aspect of the present invention, there is provided a self-excited switching power supply device which controls a voltage applied to both ends of a primary winding of a saturable transformer from a secondary winding of a main transformer, thereby controlling a secondary winding of the saturable transformer. This controls the pulse width of the positive feedback signal applied to the control electrode of the switching element.

[0012]

In the self-excited switching power supply according to the first aspect, the saturable transformer has a function of transmitting a pulse signal and a function of limiting the maximum value of the pulse width. Let Φ be the total magnetic flux of the saturable transformer core and N be the number of turns of the primary winding.
Assuming that the amplitude of the forward pulse applied to the primary winding is V1 and the time until saturation is T1, the relationship N1.PHI. = V1.T1 is established between them.

When the saturable transformer saturates, the impedance of the winding decreases to a value close to zero, so that the function as a transformer cannot be maintained, and the amplitude of the forward pulse applied to the control electrode of the switching element becomes zero. , The switching element is turned off.

When the switching element is turned off, the pulse of the secondary winding of the main transformer turns negative, and a pulse in the negative direction is applied to the primary winding of the saturable transformer.
The saturable transformer was saturated by the forward pulse, but the winding has high impedance again due to the application of the negative pulse, and this negative pulse is transmitted from the primary winding to the secondary winding, and Reverse bias the control electrode.

The period until the switching element is turned on again is the time until the excitation energy of the main transformer is completely released. If the core of the saturable transformer is saturated in the negative direction during this period, the switching element is turned on again. The time required for the core of the saturable transformer to saturate after turning on is the longest.

At start-up, since the core of the saturable transformer is not saturated in the negative direction and the output voltage is low, the ON period of the switching element does not reach the maximum value from the beginning but gradually increases. .

In addition, when the output is short-circuited, the voltage of the secondary winding of the main transformer drops, and the core of the saturable transformer cannot be saturated in the negative direction. Overcurrent protection works.

By the above operation of the saturable transformer, a soft start function in which the ON period of the switching element gradually increases at the time of startup and an overcurrent protection function in which the maximum value of the ON period is limited are obtained.

[0019]

[0020]

[0021]

[0022]

[0023] In the self-excited switching power supply according to claim 1, wherein the control transistor controls the amplitude of the negative pulse applied to the primary winding of the saturable transformer.

When the output voltage increases and the current flowing from the feedback control circuit to the base of the control transistor increases, the voltage between the collector and the emitter of the control transistor decreases, and the amplitude of the negative pulse applied to the primary winding of the saturable transformer decreases. Become smaller.

By reducing the amplitude of the negative direction pulse, the amount of change in magnetic flux returning in the negative direction after the core of the saturable transformer is saturated in the positive direction is reduced, and the next ON period is shortened. As the ON period becomes shorter, the output voltage decreases. Thus, the output voltage is stabilized.

[0026]

FIG. 1 is a circuit diagram showing an embodiment of the present invention. Parts that are the same as or equivalent to the circuit of FIG. 2 showing a conventional example are denoted by the same reference numerals.

Although the embodiment shown in FIG. 1 has a flyback converter configuration, a circuit configuration such as a forward converter or a half bridge can be applied as long as it is a self-excited type.

In the circuit shown in FIG.
The constants of the other components except for may be the same as those of the self-excited flyback converter. As an example of the core of the saturable transformer, a small cylindrical core having an outer diameter of 4 mm, an inner diameter of 2 mm, and a length of 6 mm can be used. The number of turns of the primary winding 14A has an optimum value depending on the output voltage value and the maximum ON period.
If the output voltage is 15 V at W, it may be about 10 times. The number of turns of the secondary winding 14B varies depending on the switching element used, but may be two to four times for a bipolar transistor.

[0029]

[0030]

[0031]

[0032]

[0033]

[0034]

[0035]

In the circuit of FIG . 1 , a change in the output voltage results in a change in the current flowing through the Zener diode 23, thereby changing the collector-emitter voltage of the control transistor 21 and applied to the primary winding 14A of the saturable transformer 14. Change the amplitude of the negative going pulse.

When the amplitude of the negative pulse applied to the primary winding 14A of the saturable transformer 14 changes, the amount of magnetic flux returned from the saturation magnetic flux changes, and the time until the saturation magnetic flux is reached when the forward pulse is applied next time. Changes.

The on-period of the switching transistor 12 varies depending on the time required to reach the saturation magnetic flux. Thus, the ON period is controlled by the output voltage.

In the circuit shown in FIG . 1 , the output voltage is detected by the Zener diode 23 and the Zener breakdown current is used as a feedback control current. However, a circuit comprising a reference voltage source and an operational amplifier may be used. .

By insulating between the primary winding and the secondary winding of the saturable transformer, it is possible to ensure an insulating state and transmit a signal.

[0041]

As described above, according to the present invention, the saturable transformer functions as a foldable overcurrent protection and a soft start, and also has an insulating signal transmission function as a substitute for a photocoupler. As a result, the circuit of the conventional self-excited switching power supply can be greatly simplified. Considering that the self-excited switching power supply device is widely used because of the simplicity of the circuit, the present invention that can further simplify the circuit is highly effective.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of the invention described in claim 1;

FIG. 2 is a circuit diagram showing one example of a conventional system.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 DC power supply 2 Load 11 Main transformer 12 Switching transistor 13 Rectifier smoothing circuit 14 Saturable transformer 15 Starting resistance 20 Resistance 21 Control transistor 22 Backflow prevention diode 23 Zener diode 24 Overcurrent limiting transistor 25 Resistance 26 Resistance 27 Photocoupler 28 Transistor 29 Diode 30 Capacitor 31 Capacitor 32 Resistance 11A Primary winding of main transformer 11B Secondary winding of main transformer 11C Positive feedback winding of main transformer 13A Rectifier diode 13B Smoothing capacitor 14A Primary winding of saturable transformer 14B Saturable transformer Secondary winding

Claims (1)

(57) [Claims] 1. A main transformer having a primary winding and a secondary winding, a switching element having a control electrode connected in series to the primary winding of the main transformer, and a secondary winding of the main transformer Rectifying and smoothing circuit connected to the transformer and the primary winding thereof is connected to the secondary winding of the transformer,
A saturable transformer having a secondary winding connected to a control electrode of the switching element;
A step-down converter inserted in series between the primary windings of a saturable transformer
Voltage resistor and the primary winding of the saturable transformer in parallel.
From the connected backflow prevention diode and control transistor
And a current according to the output voltage detection value.
A feedback control circuit that supplies the base of the
The start-up circuit that applies a start-up current to the control electrode
The primary winding of the main transformer and the switch.
Apply DC voltage to both ends of the series circuit of the switching element,
Self-excited type to obtain DC voltage from the output terminal of the rectifying and smoothing circuit
Switching power supply.