Reducing voltage transients across a MOSFET in a synchronous...

Electric power conversion systems – Current conversion – Using semiconductor-type converter

Reexamination Certificate

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Details

C363S021140, C363S089000

Reexamination Certificate

active

06438009

ABSTRACT:

TECHNICAL FIELD
The invention relates generally to DC/DC converters, and more specifically, to a method and an arrangement for controlling synchronous rectifiers in DC/DC converters.
BACKGROUND OF THE INVENTION
To rectify high frequency AC voltages in switched DC/DC converters, synchronous rectifiers in the form of MOSFETs are often used instead of diodes. Since the voltage drop across a MOSFET is lower than across a diode, the efficiency of converters with MOSFETs will be higher. However, since a MOSFET cannot be turned off instantaneously, its turn-off will not occur at exactly the same moment as the turn-on of a primary switch but a little later. This will cause reverse currents through the MOSFET, causing voltage transients to appear across the MOSFET. To eliminate these voltage transients, unique control signals are needed for the MOSFET to function properly. It is known to provide these control signals from the primary side of the converter, e.g. via an additional transformer.
SUMMARY OF THE INVENTION
An object of the invention is to control a MOSFET on the secondary side without any need of transferring control signals from the primary side of the converter. Voltage transients across the MOSFET caused by reverse currents when it is turned off are detected. At least a next on-period of the MOSFET is shortened in response to the detection of such transients. As a result, the MOSFET will be controlled more accurately, and transients are eliminated.


REFERENCES:
patent: 5920475 (1999-07-01), Boylan et al.
patent: 5946207 (1999-08-01), Schoofs
patent: 6243278 (2001-06-01), Jacobs
patent: 6285175 (2001-09-01), Massie
patent: 6304463 (2001-10-01), Krugly
patent: 0 184 963 (1986-06-01), None
patent: 99/52200 (1999-10-01), None

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