JPH08191568A - Switching power supply - Google Patents

Abstract

(57) [Abstract] [Purpose] It is an object to provide a switching power supply using an auxiliary winding voltage control method in which the constant voltage characteristic of the secondary side output voltage is improved. [Configuration] A switching element connected in series with the primary winding, which includes a primary winding, a secondary winding, and an auxiliary winding, is turned on / off by an output of a control circuit to induce a voltage in the auxiliary winding. In the switching power supply of the auxiliary winding voltage control method, the phototransistor connected between the output of the auxiliary winding and the power supply terminal of the control circuit and the voltage obtained from the secondary winding A photodiode for changing a current flowing through the phototransistor, wherein the current flowing through the phototransistor is detected by the control circuit, and ON / OFF of the switching element is controlled according to a difference from a reference voltage. The output voltage of the secondary winding is controlled to a constant voltage.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement of a switching power supply using an auxiliary winding voltage control system.

[0002]

2. Description of the Related Art An example of this type of switching power supply is shown in FIG.
Shown in In the figure, 10 is a terminal to which the power supply voltage Vin is input, 20 is a terminal from which the output voltage Eo is taken out, 30
Is a transformer including a primary winding 31, a secondary winding 32, and an auxiliary winding 33, and 40 is a control circuit. A semiconductor switching element 50 such as a FET is connected in series to the primary winding 31 of the transformer 30. This switching element is on / off controlled by the output of the control circuit 40. 11 is a rectifier circuit connected to the input terminal 10;
Is a smoothing capacitor, and the output of the smoothing capacitor 12 is added to the series circuit of the primary winding 31 and the switching element 50.

The output winding 32 of the transformer 30 is connected to the output terminal 20 via a rectifying diode 32a and a smoothing capacitor 32b. The auxiliary winding 33 is connected to the power supply terminal A of the control circuit 40 via a rectifying diode 33a and a smoothing capacitor 33b. Reference numeral 60 denotes a starting resistor, which is connected between the output end of the rectifier circuit 11 and the power supply end A of the control circuit 40.

In such a configuration, the AC voltage Vin applied to the input terminal 10 is full-wave rectified by the rectifier circuit 11, smoothed by the capacitor 12, and applied to the series circuit of the primary winding 31 and the switching element 50. At the same time, it is applied as a power supply voltage to the control circuit 40 via the starting resistor 60, and thereby the control circuit 40 is started. The switching element 50 is on / off controlled by the output of the control circuit 40. As a result, a voltage of N3 / N2 times (N3 is the number of turns of the output winding 32, N2 is the number of turns of the auxiliary winding 33) of the voltage at the power supply terminal A of the control circuit 40 is generated in the output winding 32 of the transformer 30.

Such an auxiliary winding voltage control type switching power supply has been conventionally used, but in the above circuit, the voltage kept constant is the potential at the point A and the output terminal 2
The voltage Eo taken out from 0 has a problem that the constant voltage characteristic cannot be maintained when the forward voltage drop of the rectifying diode 32a caused by the load current or the voltage drop caused by the resistance of the wiring of the output line changes.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems of a conventional circuit, and its purpose is to provide an auxiliary with improved constant voltage characteristic of secondary side output voltage. It is to provide a switching power supply using a winding voltage control method.

[0007]

SUMMARY OF THE INVENTION According to the present invention, a switching element connected in series with a primary winding, a secondary winding and an auxiliary winding is turned on / off by an output of a control circuit. In a switching power supply of an auxiliary winding voltage control system in which a voltage is induced in the auxiliary winding by a phototransistor connected between an output of the auxiliary winding and a power supply terminal of a control circuit, the secondary A photodiode that changes a current flowing through the phototransistor according to a voltage obtained from a winding, and the switching element is turned on depending on a difference between the current flowing through the phototransistor and the reference voltage detected by the control circuit. The output voltage of the secondary winding is controlled to a constant voltage by controlling the off state.

[0008]

In the present invention as described above, negative feedback is performed by detecting the current flowing through the phototransistor in accordance with the output voltage by the control circuit.

[0009]

EXAMPLES The present invention will be described in detail below with reference to FIG. 1 shown as an example. In FIG. 1, the same elements as those in FIG. 5 are denoted by the same reference numerals as those in FIG. In FIG. 1, 71 is a Zener diode, and 72 is a phototransistor. The Zener diode 71 is connected between the power supply terminal A and the control circuit 40, and the phototransistor 72 is connected in parallel to this Zener diode. Assuming that the connection point between the anode pole of the Zener diode 71 and the emitter pole of the phototransistor 72 is B, this point B is connected via the capacitor 73 to the GND on the primary side.
It is connected to the. 74 is a resistance element, one end of which is connected to the switching element 50 and the other end of which is connected to the capacitor 12.

Reference numeral 80 denotes an output voltage detection circuit for detecting the output voltage Eo, which includes resistors 81 to 84, capacitors 85 and 86, a shunt regulator 87 and the phototransistor 7.
It is composed of a photodiode 88 which is paired with 2 and constitutes a photocoupler. The resistors 81 and 82 are connected in series, and the photodiode 88 and the shunt regulator 87 are connected in series, and each series circuit is connected between the terminal 20 for extracting the output voltage Eo and the common. The connection point between the resistors 81 and 82 is connected to the shunt regulator 87 and the photodiode 8 via the series circuit of the resistor 83 and the capacitor 85.
8 connection points.

Next, the operation of the switching regulator according to the present invention having such a configuration will be described. Input terminal 1
When the AC voltage Vin is applied to 0, the AC voltage is full-wave rectified by the rectifier circuit 11. A current flows through the starting resistor 60 as the input AC Vin increases, and the voltage at the point A, which is the power source end of the control circuit 40, increases. When the voltage at point A reaches (starting voltage + zener voltage) of the control circuit 40, the output of the control circuit 40 drives the switching element 50. Here, since the phototransistor 72 forming the photocoupler is set so as to allow a sufficient current to flow to the control circuit 40, the Zener diode 71 is cut off after startup, and the current of the control circuit 40 is supplied through the phototransistor 72. To be done.

In the steady state, a voltage N3 / N2 times the voltage at the point A (voltage at the point B + VCE of the phototransistor 72) is induced in the secondary winding 32 of the transformer 30. Here, the output voltage detection circuit 80 detects the output voltage Eo, but the output voltage Eo and the reference voltage Vref generated at the connection point of the resistors 81 (R1) and 82 (R2) of the detection circuit 80 are Eo = Vref. ( The relationship of R1 + R2) / R2 (1) is established. If the output voltage Eo rises by ΔEo from such a state, the cathode electrode of the shunt regulator 86 absorbs the current ΔI _F via the photodiode 87 so that the expression (1) is satisfied. Therefore, the collector current of the phototransistor 72 that forms a photocoupler in pair with the photodiode 87 also increases by ΔIc, and this collector current flows into the control circuit 40.

The control circuit 40 comprises a resistor for detecting the collector current of the phototransistor 72 and an error amplifier for comparing the voltage drop across this resistor with a reference voltage. The switching element is designed to reduce the output of the error amplifier. By turning on / off the gate of 50, the voltage at the point B is lowered, whereby the converter operates so that negative feedback is always performed so that the expression (1) is always satisfied. In addition,
In the control circuit 40 of the embodiment, the primary winding 31 of the transformer 30
The ON period of the switching element 50 is controlled so that the peak value of the current flowing through the switching element 50 becomes equal to the output of the built-in error amplifier.

The control circuit 40 is not particularly limited in configuration, but is provided with a resistor for detecting a current flowing into the power supply terminal B and a reference voltage source, and an error amplifier for comparing the detected voltage with the output of the reference voltage source. Just do it. FIG. 2 shows an example of this control circuit. A motorola UC3842 is used as the control IC, and the reference voltage source and the error amplifier are built in this IC.

The circuit shown in FIG. 1 is an embodiment in which the present invention is applied to a flyback type switching power supply. FIG. 3 shows the present invention in which the present invention is applied to a known forward converter. 3 is exactly the same as that of FIG. 1 except that the converter system is different from that of FIG. 1, and the description of its operation is omitted.

FIG. 4 shows a control circuit of the voltage control system. This control circuit uses an FA7610P manufactured by Fuji Electric Co., Ltd.

[0017]

According to the present invention, the secondary side output voltage is detected and the negative feedback is performed through the photocoupler. Therefore, the forward voltage drop of the rectifying diode caused by the load current or the output line There is an effect that it is possible to obtain a switching power supply using an auxiliary winding voltage control method having a constant voltage characteristic that is not affected by a voltage drop or the like caused by wiring resistance.

[Brief description of drawings]

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

FIG. 2 is a configuration diagram of an example of a control circuit used in the present invention.

FIG. 3 is a circuit configuration diagram showing another embodiment of the switching power supply according to the present invention.

FIG. 4 is a configuration diagram of an example of a control circuit used in the present invention.

FIG. 5 is a circuit configuration diagram of a conventional switching power supply.

[Explanation of symbols]

 30 transformer 31 primary winding 32 secondary winding 40 control circuit 50 switching element 60 detection resistor 72 phototransistor 88 photodiode

Claims (1)

[Claims] 1. A switching element connected in series with the primary winding, which comprises a primary winding, a secondary winding, and an auxiliary winding, is turned on / off by an output of a control circuit, and a voltage is applied to the auxiliary winding. In a switching power supply of an auxiliary winding voltage control system for inducing a voltage, a phototransistor connected between an output of the auxiliary winding and a power supply terminal of a control circuit and a voltage obtained from the secondary winding are connected. A photodiode for changing the current flowing through the phototransistor in accordance with the control circuit, and detecting the current flowing through the phototransistor by the control circuit and controlling the on / off of the switching element based on the difference from the reference voltage. A switching power supply configured to control the output voltage of the secondary winding to a constant voltage.