I am looking for cuircuit recommendations for high current MOS FETs with very low R(dson) used as high efficiency diodes with an extremely low forward voltage drop. As far as I remember, a fast single supply comparator was used to drive the MOS FET.
But what about a fullwave rectifier bridge? Is such a circuit possible with such "MOS FET diodes"?

Andy

There are ICs for providing this function.  Often they are listed under hot-swapping.

Keith Cress
kcress - http://www.flaminsystems.com

Check out www.irf.com or www.diodes.com; I vaguely remember seeing some decent application notes there on this topic.  

John D

Look for "synchronous rectification"

TTFN

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If this is a useful approach depends mainly on your output voltage. Currently synchronous rectification may be employed for output voltages up to 48 V.

Main challenge is to provide precise timing for the control of the Mosfets. This may be either achieved by deriving a signal from the voltage to be rectified (difficult due to delays) or by utilizing the signal that is used to swicth the primary side. If timing is not precise -Mosfets turn on to late- the body diodes of the Mosfets involve themselve and this may generate losses due to reverse recovery.

In the other direction, turning on to early, leads to a shoot through condition.

Search for application notes on "synchronous rectification" on http://www.infineon.com
 

18 years ago I designed a power supply to charge or discharge batteries for a medical company.
It acted like a buck converter for charging and acted like a a boost for discharging.

Two FETs were all the switches needed for either conversion, as either FET could act as a switch or as a diode.

I was running 100kHz, 12 Volts, and 100 Watts.

The FET will act like a diode anyway if not turned on as a transistor, but the reverse recovery time is slow, so the FET part needs to get turned on during the on cycle, and sweep the minority carriers out of the parasitic diode before the transistor turns off.

Since then I have worked on a jet engine starter/generator with much higher power, but the principals are the same.

At 100kHz, one does not use a comparator for gate drive, but one could at 60 or 120 Hz.

so Clark, you also used synchronous rect's on a 28V system? Yeah, been there. Succesfully did the same thing for 787 brake control and 373 battery charger. The only challenge was to get non-beleivers to look at the actual oscilloscope traces so they would stop being so negative.
 
How is it working with those negative people at your place nowdays?

Darrell