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richwalters 18th December 2008 09:22 PM

which ferrite beads for mos legs?
I've noticed my mosfet psu is quietly oscillating and I've put a ferrite bead in both gate and drain legs. This has "shut it up"and now runs much cooler. Since mos oscillates well into UHF, I chose a material with a fairly low permeability i.e mu=700 philips 4A11 pink . Is this the correct approach and /or what is best ferrite material grade to use. Or broadband ? Anyone got any experience ?


lineup 25th June 2011 08:19 AM

I don't know.
But I like know.

ebattleon 25th June 2011 11:00 PM

This thread is so necro...:)
But if you cool Mosfets tend to "oscillate" when the are hot so the same effect could be obtained with better cooling.

magnoman 26th June 2011 12:57 AM

Its been 25 years and I dont remember what worked best for my application but Fair-Rite makes a nice little evaluation kit. I seem to remember material 43 but could be wrong.
Sorry this isn't much help, the kits were still available just a few years back.


hesener 28th June 2011 08:48 PM

Usually a ferrite bead on the gate should be sufficient. The mu and the losses in the material need to be just high enough so that the oscillation stops with a good margin.
Alternatively, you might beef up the gate driver and/or reduce the gate drive resistor, or move it closer to the gate.
Or, chose a older (slower) MOSFET - the new superjunction devices (think Fairchild SuperFET or Infineon CoolMOS) are getting faster and faster....

just my two cents...

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