JP2008306804A - Regeneration synchronous rectification circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching power supply device which achieves the drive of a synchronous rectification circuit and the constant voltage control of an output in a fly-back converter with fewer parts, and also achieves high efficiency. <P>SOLUTION: An error amplifier, which is connected to an output terminal conventionally, can both drive a synchronous rectification element and perform the constant voltage control of an output by being connected between the secondary winding (N2) of a transformer and the synchronous rectification element (Q1). auxiliary winding and the like is conventionally required to drive the synchronous rectification element, but it is not required by the use of a negative voltage of the secondary winding of the transformer and a voltage of an output. The increase of the windings of the transformer for driving the synchronous rectification element is prevented thereby. <P>COPYRIGHT: (C)2009,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. The constant voltage control becomes difficult.
JP-A-8-126317

In the conventional circuit, a complicated circuit is required to accurately control the synchronous rectifier element, and an auxiliary winding is required for driving the synchronous rectifier element.

According to the first aspect of the present invention, by connecting the error amplifying element between the secondary winding of the transformer and the synchronous rectifying element, the excitation energy release time can be surely turned on the synchronous rectification MOSFET, and at the end of the excitation energy release time. Since the output terminal 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 remains 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.

The invention of claim 2 uses the output terminal voltage and the negative voltage of the secondary winding of the transformer to secure the driving voltage of the synchronous rectifying element, thereby making the auxiliary winding unnecessary.

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 dot mark side of the secondary winding N2 of the transformer is between plus. In other words, the voltage for driving the synchronous rectifier element is charged through D1 to C2 while energy is not released to the secondary side.

The output of error amplifier U1 controls Q5 through R2 and Q2 through D2. That is, if the voltage of the secondary winding N2 is equal to or higher than the set value, Q5 is turned on, 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, Q5 is turned off, Q2 is turned on, the gate charge is extracted, Q1 is turned off, and the voltage generated in the NB winding of the transformer is the main switch element. Q3 turns on. 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 keeps turning on Q5 and Q1 keeps on until the set voltage is reached. As a result, current flows backward from the output to the secondary winding N2 of the transformer, and excess 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 the output, so that the synchronous rectifier does not work and the efficiency is lowered.

Even when the voltage of the secondary winding N2 is low due to C1 placed in parallel with R3, the synchronous rectifying element is only activated while the current of the secondary winding N2 is large by forcibly driving U1 for a certain period of time. It can be driven and efficiency reduction can be prevented to some extent.

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 windings Q2, Q4, Q5 Switch element R1, R2, R3, R4, R5 Resistor C1, C2 Capacitor D1, D2 Diode

Claims (2)

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 In a flyback type switching power supply apparatus in which an error amplifying element for controlling ON / OFF time of a rectifying element is provided between the secondary winding of the transformer and the synchronous rectifying element, when the voltage between the output terminals rises, A synchronous rectification switching power supply device characterized in that the on-time of the synchronous rectification element is lengthened and the current is regenerated to the primary side. 2. The synchronous rectification switching device according to claim 1, wherein a diode and a capacitor are connected in series from the output terminal to the transformer secondary winding to ensure a driving voltage of the synchronous rectification element.

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