which ferrite beads for mos legs?

richwalters · 18th December 2008, 09:22 PM

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 ?

richj

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.

-Antonio

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...

18th December 2008, 09:22 PM	#1
richwalters diyAudio Member Join Date: Sep 2003 Location: Alps:Tube amp designs over 150W, SMPS guru.	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 ? richj

25th June 2011, 08:19 AM	#2
lineup diyAudio Member Join Date: Dec 2005 Location: the north	I don't know. But I like know. __________________ lineup

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25th June 2011, 11:00 PM	#3
ebattleon diyAudio Member Join Date: Jun 2011	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.

26th June 2011, 12:57 AM	#4
magnoman diyAudio Member Join Date: Nov 2010 Location: The City, SanFrancisco	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. -Antonio

28th June 2011, 08:48 PM	#5
hesener diyAudio Member Join Date: Mar 2005 Location: Munich, Germany	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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