JP2007185059A - Regenerative synchronous rectifier circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a driving method of a synchronous rectifier circuit in a flyback converter, and to provide a circuit capable of regulating a constant output voltage with a small number of parts. <P>SOLUTION: An error amplifier conventionally connected to an output terminal is arranged between the secondary winding of a transformer and a synchronous rectifying device. Therefore, synchronous rectification MOSFET is certainly turned on during an exciting energy releasing time, and an output terminal voltage is automatically given to an error amplifying element at the end of the exciting energy releasing time, thereby turning the synchronous rectifying MOSFET on when a load is light and an output voltage is high after releasing of exciting energy. As a result, extra energy is regenerated to a primary winding side to control the output terminal voltage with high accuracy, thereby enabling combination between driving of the synchronous rectifying device and constant-voltage control of an output. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to a switching power supply device, and more particularly to a synchronous rectifier circuit.

As an example of the conventional system, there is a synchronous rectifier circuit shown in FIG.

In FIG. 2, the on-time of MOSFET Q1 is determined by a voltage generated in auxiliary winding N3 and a differentiation circuit using capacitor C1, resistor R1, and resistor R5. The voltages at the output terminals OUT + and OUT− are fed back to the primary side by the photocoupler PC1 and controlled by changing the on-time of the primary side main switch element Q3.

In the conventional circuit, an element for feeding back the output terminal voltage to the primary side is indispensable, and the switching frequency becomes high at a light load, so that control is difficult.

In the present invention, the error amplifying element is connected between the secondary winding of the transformer and the synchronous rectifying element, so that the excitation energy release time surely turns on the synchronous rectification MOSFET, and the output terminal at the end of the excitation energy release time. Since the voltage is directly applied to the error amplifying element, when the load is light and the output terminal voltage is high, the synchronous rectification MOSFET is kept on even after the excitation energy is released. As a result, excess energy is regenerated to the primary side, and the output terminal voltage is controlled with high accuracy. Further, a photocoupler that feeds back to the primary side is not necessary.

Since synchronous rectification and output voltage control can be performed with a small number of parts, the economic effect is great.

In implementing the present invention, it is important to select a synchronous rectification MOSFET having a sufficiently low on-resistance and a small input capacitance.

Q3 in FIG. 1 is a main switch element. Turns on and off for a certain time by Q4.

A voltage is applied from the secondary winding N2 of the transformer to the error amplifier U1 through R3 and R4.

The output of the error amplifier U1 controls Q2 through a resistor R2. That is, if the voltage of the secondary winding N2 is equal to or higher than the set value, Q2 is turned on, and the synchronous rectifier element Q1 is turned on via R1.

When the energy release from the secondary winding N2 is completed at the rated load, Q1 is turned off, and the main switch element Q3 is turned on by the voltage generated in the NB winding of the transformer. This is the same as a conventional flyback converter.

When the load is light, the output terminal voltage becomes high even when the energy emission from the secondary winding N2 is finished. In this case, the error amplifying element U1 continues to turn on Q1 until the set voltage is reached. As a result, current flows backward from the output to the secondary winding N2 of the transformer, and energy is regenerated to the input side through the primary winding N1.

If the Q1 of the synchronous rectification MOSFET is selected to have a sufficiently low on-resistance, the voltage at the end of energy emission from the N2 winding is equal to the output terminal voltage. Therefore, it is possible to control the output voltage with high accuracy, and feedback to the primary side by a photocoupler or the like is not necessary.

When the load becomes heavier than the rated output, the error amplifier U1 stops outputting so that Q1 does not turn on and is only conducted by the internal parasitic diode of Q1. At this time, since the loss of Q1 becomes large, a problem of heat generation occurs.

As shown in FIG. 3, by adding a capacitor C3 having an appropriate capacity between the collector and emitter of Q2, it is possible to turn on Q1 for a certain period of time even if U1 does not issue an on command. As a result, synchronous rectification works to some extent even under heavy loads, and the heat generation of Q1 can be reduced.

It is a circuit diagram which shows the Example in the flyback converter of this invention. Example 1 It is a circuit diagram which shows an example of a conventional system. It is a circuit diagram which shows the Example in the flyback converter of this invention when heat_generation | fever becomes a problem. (Example 2)

Explanation of symbols

V1 Voltage source OUT +, OUT− Output terminal Q3 Main switch element Q1 Synchronous rectifier element U1 Error amplifying element T1 Transformer N1 Primary winding N2 Secondary winding N3 Auxiliary winding NB Base winding Q2, Q4 Switching elements R1, R2, R3, R4, R5 Resistor PC1 Photocoupler C1, C3 Capacitor

Claims (1)

A main switching element that is turned on and off at a constant cycle is connected in series with the primary winding of the transformer, and a synchronous rectifying element connected in series between the secondary winding of the transformer and the output terminal, and the synchronous A flyback type switching comprising an auxiliary winding for turning on and off the rectifying element and an error amplifying element for controlling the on-time of the synchronous rectifying element between the secondary winding of the transformer and the synchronous rectifying element. In the power supply apparatus, the synchronous rectification switching power supply apparatus is characterized in that when the voltage between the output terminals rises, the on-time of the synchronous rectifier element is lengthened and the current is regenerated to the primary side.

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