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Old 18th December 2008, 09:22 PM   #1
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Default 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 ?

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Old 25th June 2011, 08:19 AM   #2
lineup is offline lineup  Sweden
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I don't know.
But I like know.
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Old 25th June 2011, 11:00 PM   #3
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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.
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Old 26th June 2011, 12:57 AM   #4
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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.

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Old 28th June 2011, 08:48 PM   #5
hesener is offline hesener  Germany
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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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