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Old 21st June 2004, 04:10 PM   #1
le´flu is offline le´flu  Germany
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Default using Mosfets as diode in PS

Hi all,

while surfing i found this page in the net:

http://peufeu.free.fr/audio/alim_mos_diode/

what do you think of using mosfets in powersupplies to minimise ripple?? Anyone who tried this, any comments or HP`s where one discribes a test with Mosfets like this??

Greetings Ulf
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Old 21st June 2004, 04:52 PM   #2
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It won't work.

There is a "parasitic" diode formed by the substrate that is conductive in the reverse direction. So this device would conduct in *both* directions, with a threshold voltage of ~4 volts in one direction and ~0.7 volts in the other.

MOSFETs are used as "synchronous" rectifiers, where the device is turned "on" when current would flow in the direction of the parasitic diode. But in this case, the IR drop across the channel is lower than the threshold voltage of the parasitic diode.

EDIT: When I wrote the above, I didn't see his schematic at the bottom of the page where he added extra diodes to prevent reverse-biasing the MOSFETs. So perhaps it could work after all.
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Old 21st June 2004, 05:06 PM   #3
le´flu is offline le´flu  Germany
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Hi,

ok shure, but the author of my linked article uses the fet after the rectifierbridge, so reverse current will be inhibited.
But the measuring diagramms show less ripple.
There would be much less noise on the powersupply.....

correct me if i´m wrong

greetings Ulf
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Old 21st June 2004, 05:37 PM   #4
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It seems like it may be worth trying. Let us know how it sounds.
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Old 21st June 2004, 05:48 PM   #5
le´flu is offline le´flu  Germany
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hi,

er..yes, but which MOSfet can i take, the author doesn´t mention one type of fet. So do i have to use a special quality fet or just one for high voltage??

I think i have to collect some data sheets
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Old 22nd June 2004, 03:31 AM   #6
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Hi,

Quote:
But the measuring diagramms show less ripple.
I think the diagrams are only simulations, not actual measurements.
However, the lesser amount of ripple is likely due to the higher internal resitance of the MOSFET.

The author clearly states that the voltage drop is rather high and that the project is not really economical.

Nowadays, SiC Schottky diodes are available at reasonable prices considering the context of a one off DIY project.
They'll probably fall even more within a few years.

There are other alternatives that work very well such as the hybrid tube/silicon diode FW rectifier that has been discussed extensively on this forum before.

Still, I won't stop you from trying the FET rectifier scheme out if you feel so inclined.

Cheers,
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