Low loss synchronous rectifier for application to clamped-mode power converters

Abstract

A synchronous rectifier for use with a clamped-mode power converter uses in one embodiment a hybrid rectifier with a MOSFET rectifying device active in one first cyclic internal of the conduction / nonconduction sequence of the power switch and a second rectifying device embodied in one illustrative embodiment as a low voltage bipolar diode rectifying device active during an alternative interval to the first conduction / nonconduction interval. The gate drive to the MOSFET device is continuous at a constant level for substantially all of the second interval which enhances efficiency of the rectifier. The bipolar rectifier device may also be embodied as a MOSFET device. The subject rectifier may be used in both forward and flyback power converters.

Description

FIELD OF THE INVENTIONThis invention relates to switching type power converters and in particular to forward and flyback converters having a clamp-mode topology.BACKGROUND OF THE INVENTIONSelf synchronized rectifiers refer to rectifiers using MOSFET rectifying devices having control terminals which are driven by voltages of the windings of the power transformer in order to provide the rectification of the output of the transformer. Use of synchronous rectifiers has been limited however by the inefficiency of these rectifiers in buck derived converter topologies. Efficiency is limited due to the nature of switching of buck derived converters (i.e buck, buck-boost, boost converters including forward and flyback topologies) and due to the variability of the transformer reset voltages in the forward type converters. This variability of reset voltage limits the conduction time of one of the MOSFET rectifiers, diminishing the effectiveness and efficiency of the rectifier. This is because ...

AI Technical Summary

Benefits of technology

A synchronous rectifiers is combined with a clamped-mode buck derived power converter. In one illustrative embodiment a hybrid rectifier includes a MOSFET rectifying device active in a first cyclic interval of the conduction / nonconduction sequence of the power switch. A second rectifying device embodied in one illustrative embodiment as a low forward voltage drop bipolar diode rectifying device is active during an alternative interval to the first conduction / nonconduction interval. The gate drive to the MOSFET device is maintained continuous at a constant level for substantially the all of the second interval by the clamping action of the clamping circuitry of the converter. This continuous drive enhances the efficiency of the rectifier.

Problems solved by technology

Use of synchronous rectifiers has been limited however by the inefficiency of these rectifiers in buck derived converter topologies.

Efficiency is limited due to the nature of switching of buck derived converters (i.e buck, buck-boost, boost converters including forward and flyback topologies) and due to the variability of the transformer reset voltages in the forward type converters.

This variability of reset voltage limits the conduction time of one of the MOSFET rectifiers, diminishing the effectiveness and efficiency of the rectifier.

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