JP2007312498A - Regenerative synchronous rectification circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a driving method for a synchronous rectification circuit in a flyback converter and a switching power supply wherein output constant-voltage control can be implemented with a smaller number of parts and high efficiency can be achieved. <P>SOLUTION: An error amplifier used to be connected to an output terminal is disposed between the secondary winding (N2) of a transformer and a synchronous rectifying element (Q1). Thus, both driving of the synchronous rectifying element and constant-voltage control on output can be achieved. Even when a load becomes so heavy that rated power is exceeded and the error amplifier does not work, the synchronous rectifying element can be turned off without fail by a transistor Q2 and a capacitor C1. <P>COPYRIGHT: (C)2008,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 / off time of the MOSFET Q1 is controlled by controlling the on time of the Q2 by a signal from the error amplifier U1. Q1 cannot be turned off when the voltages at the output terminals OUT + and OUT− are low, and Q1 is kept on until the voltage of the N3 winding falls to the threshold voltage of Q1, and the regenerative current is input from the N2 winding to the input. It will flow.
JP-A-8-126317

In the conventional circuit, a complicated circuit is required to accurately control the synchronous rectifier element, and there is a problem of malfunction due to a voltage source connected in parallel to the load.

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 the diode D1. That is, if the voltage of the secondary winding N2 is equal to or higher than the set value, Q2 is turned off and the synchronous rectifier element Q1 is kept on.

When the energy release from the secondary winding N2 is completed at the rated load, Q2 is turned on, the gate charge is extracted, Q1 is turned off, and the main switch element Q3 is turned on by the voltage generated in the NB winding of the transformer. Become. 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 turns off Q2 and keeps turning 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 Q2 is turned on with a time constant of R2 and C1, and Q1 is reliably turned off. Therefore, since synchronous rectification can be maintained regardless of output power, high efficiency can be realized.

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.

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 Resistor C1 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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