JP3595737B2 - Self-excited flyback converter - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a drive circuit for a self-excited flyback converter, among switching power supplies.
[0002]
[Prior art]
As a method for reducing the power consumption of the switching power supply at the time of light load or standby, a converter that can lower the oscillation frequency when a light load is detected is desirable.
In the prior art, a self-excited flyback method for controlling the oscillation frequency corresponding to a light load has not been realized, so a separately-excited flyback converter using a primary-side and secondary-side PWM control method is used. However, at light load or during standby, the oscillation frequency is reduced to suppress the converter output.
[0003]
[Problems to be solved by the invention]
FIG. 4 is a block diagram showing a circuit configuration of the separately excited flyback converter. In FIG. 4, a rectifier circuit 119 is connected to the AC power supply, and a smoothing capacitor 118 is connected in parallel to the DC output side. A series circuit composed of a primary coil of the main transformer 102 and the main switch element 101 is connected in parallel to both ends of a smoothing capacitor 118 to constitute a primary input circuit.
A secondary output circuit including a rectifier diode 114 and a smoothing capacitor 115 is connected to a secondary coil of the main transformer 102.
A starting resistor 103 is connected between one end of the primary coil and the gate terminal of the main switch element 101, and is connected in parallel with a resistor 104 and a resistor 105 connected in parallel between the gate and source of the main switch element 101. A series circuit composed of the connected resistor 106 and diode 107 is connected between the gate terminal of the main switch element 101 and the primary coil of the drive transformer 108 to form a drive circuit.
[0004]
The secondary output circuit voltage is detected by the resistors 116 and 117 and compared with the reference voltage V _ref 113 in the operational amplifier 112.
The detection signal of the operational amplifier 112 and the signal from the oscillator 111 are input to a PWM control circuit 110 to generate a PWM control signal, and supply a gate signal to the main switch element 101 via the transistor 109 and the driving transformer 108.
When a light load is detected by the operational amplifier 112, it is easy to reduce the oscillation frequency of the main switch element 101 by adjusting the PWM control signal from the PWM control circuit 110 and reduce the power consumption of the switching converter. .
However, since the circuit configuration of the separately excited flyback converter described above is complicated, the number of components is large, and it is difficult to reduce cost and space.
[0005]
[Means for Solving the Problems]
SUMMARY OF THE INVENTION The present invention has been made to solve the above-described drawbacks of the related art, and has been described in connection with a gate drive circuit of a self-excited flyback converter which has a simple circuit configuration and a small number of parts, and which detects a light load when a light load is detected. A circuit that lowers the oscillation frequency by clamping the back voltage is added, which not only reduces power consumption during light load and standby, but also enables cost reduction and space saving.
[0006]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram showing a circuit configuration of a self-excited flyback converter according to the present invention, and FIGS. 2 and 3 are waveform diagrams.
[0007]
In FIG. 1, an AC power supply is connected to the input side of the rectifier circuit 17, and a smoothing capacitor 18 is connected in parallel to the DC output side.
The other end of the primary coil 12 of the main transformer 11 having one end connected to the positive pole of the smoothing capacitor 18 is connected to the drain terminal of the main switch element 1, and the source terminal of the main switch element 1 is connected to the negative pole of the smoothing capacitor 18. And constitutes a primary input circuit of the main transformer 11.
[0008]
The polarity of the secondary coil 14 of the main transformer 11 is different from that of the primary coil 12, and a rectifier diode 15 and a smoothing capacitor 16 are connected to form a secondary output circuit.
[0009]
A starting resistor 2 is connected between the gate terminal of the main switch element 1 and one end of the primary coil 12, and a parallel resistor 3 is provided between the gate terminal and the source terminal of the main switch element 1. It is.
In this embodiment, a MOS-FET is used as the main switch element 1, but the present invention is not limited to this.
[0010]
A drive coil 13 is connected in parallel between the gate and the source of the main switch element 1 via a series circuit including a capacitor 4 and a resistor 6.
Between the connection point of the capacitor 4 and the resistor 6 and one end of the drive coil 13, a series circuit including a resistor 5, a diode 7 and a Zener diode 8 having a switch element 9 in a bypass circuit is connected in parallel. Thus, a gate drive circuit of the main switch element 1 is configured.
A control circuit 10 for controlling ON / OFF of the main switch element 1 is connected between the capacitor 4 and the gate terminal in the gate drive circuit.
[0011]
Next, the operation characteristics of the self-excited flyback converter according to the present invention will be described with reference to the block diagram shown in FIG. 1 and the waveform diagrams shown in FIGS.
The switch element 9 of the bypass circuit in the gate drive circuit is controlled to be turned on during rated operation and turned off during light load. Therefore, at the time of rated operation, the Zener diode 8 is short-circuited by the switch element 9 and constitutes a drive circuit of a normal self-excited flyback converter.
[0012]
In the continuous switching operation of a normal self-excited flyback converter, after the main switching element 1 is turned off and the energy stored in the main transformer 11 is discharged from the secondary coil 14, the flyback voltage is applied to the main transformer 11. Is generated, and the main switch element 1 is charged and turned on by a circuit including the drive coil 13 → the resistor 6 → the capacitor 4 → the gate terminal of the main switch element 1. The main switch element 1 is turned off by the operation of the control circuit 10. Thereafter, this operation is repeated to continue the oscillation. FIG. 3 is a waveform diagram showing operating characteristics of a normal self-excited flyback converter at a light load. 3A shows a voltage waveform between the drain and the source of the main switch element, FIG. 3C shows a voltage waveform of the drive coil, and FIG. 3D shows a voltage waveform between the gate and the source of the main switch element. FIG. 3B shows a primary coil current waveform and a secondary coil current waveform, and shows that a flyback voltage of the main transformer is generated when the secondary coil current becomes zero.
[0013]
As described above, when the light load is detected, the switch element 9 of the bypass circuit is off, so that the resistor 5 is connected between the connection point between the capacitor 4 and the resistor 6 and one end of the drive coil 13. And a diode 7 and a Zener diode 8 form a series circuit.
When a voltage applied to both ends of Aru resistor 3 connected in parallel between the main gate and the source of the switching element 1 and E _1, E ₁ as compared to the case where Zener diode 8 is not inserted zener diode 8 Since the voltage is substantially reduced, the main switch element 1 remains off. The capacitance between the gate and the source of the main switch element 1 and the capacitor 4 are charged via the starting resistor 2, and the main switch element 1 is turned on when the gate threshold voltage is reached.
That is, the resistor 5 and the diode 7 are inserted in order to improve the switching loss when the main switch element 1 is turned off, and the zener diode 8 and the switch element 9 forming a bypass circuit are further provided. This lowers the oscillation frequency at light load.
FIG. 2 is a waveform chart showing switching operation characteristics under a light load according to the present invention.
2A shows a voltage waveform between the drain and the source of the main switch element, FIG. 2B shows a primary coil current waveform and a secondary coil current waveform, and FIG. 2C shows a voltage waveform of the drive coil. FIG. 2D shows a gate-source voltage waveform of the main switch element.
As is apparent from FIG. 2D, the period during which the flyback voltage is generated is determined by the start-up resistor 2 and the charging time of the gate-source capacitance of the main switch element 1, so that the interval at which the main switch element is turned on is reduced. It can be seen that the oscillation frequency becomes longer and the oscillation frequency decreases.
[0014]
【The invention's effect】
In the self-excited flyback converter according to the present invention, the gate drive circuit of the main switch element is provided with a circuit composed of a zener diode for clamping the flyback voltage and a switch element constituting a bypass circuit, so that a light load or standby state can be achieved. It is possible to reduce the oscillation frequency at the time and reduce the power consumption by the main switch element.
Further, the circuit configuration of the self-excited flyback converter is simple and the number of components is small, so that cost reduction and space saving can be realized.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a circuit configuration of a self-excited flyback converter according to the present invention.
FIG. 2 is a waveform diagram.
FIG. 3 is a waveform diagram.
FIG. 4 is a block diagram showing a circuit configuration of a separately excited flyback converter according to the related art.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Main switch element 2, 3, 5, 6 Resistance 4, 16, 18 Capacitor 7, 15, 17 Diode 8 Zener diode 9 Switch element 11 Main transformer

Claims (1)

A rectifier circuit connected to an AC power source, a smoothing capacitor connected to the DC output side of the rectifier circuit, and a series circuit including a primary coil of a main transformer and a main switch element are connected in parallel to both ends of the smoothing capacitor. A primary input circuit;
A secondary output circuit including a rectifier diode and a smoothing capacitor connected to a secondary coil having a different polarity from the primary coil;
A starting resistor (2) inserted between one end of the primary coil and the gate terminal of the main switch element, a resistor (3) connected in parallel between the gate and source of the main switch element, and a capacitor (4) and a resistor (6). A) a gate drive circuit composed of a primary coil connected in parallel between the gate and the source of the main switch element via a series circuit comprising: and a drive coil having the same polarity;
In the self-excited flyback converter configured by
A Zener including a resistor (5), a diode (7), and a switch element (9) in a bypass circuit between a connection point between a capacitor (4) and a resistor (6) in the gate drive circuit and one end of a drive coil. A self-excited flyback converter, wherein a series circuit including a diode (8) is connected in parallel to form a gate drive circuit of a main switch element.

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