From: John Larkin
Subject: Re: Gate drive transformer ferrite?
Date: Fri, 18 Oct 2002 19:26:45 -0700
Organization: Posted via Supernews, http://www.supernews.com
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On Sat, 19 Oct 2002 00:00:58 GMT, John Popelish
>Chris Carlen wrote:
>> I'm planning on accumulating a small stock of Fair-Rite 78 material
>> cores for making SMPS inductors and transformers. Ugh! There are so
>> many choices.
>> I am also interested in making some FET gate drive transformers for
>> various purposes, one possibly being a PWM motor driver (I know there
>> are chips to do this, but I want to perform experiments my way), and
>> another, the more important application, is a gate drive transformer for
>> a solid state tesla coil in the 100kHz-500kHz range.
>> It seems that 78 material is used for power applications, up to about
>> 200kHz. The 61 material is the highest frequency material, but has
>> quite low permeability. It seems that I would want a high permeability
>> for a pulse/broadband transformer, but the high permeability stuff gets
>> very lossy above a few 100kHz. I'm thinking that it needs to be useable
>> into a few MHz in order to preserve fast risetimes into a FET gate, to
>> avoid sluggish turnon/turnoff. I venturing here a bit. Am I in the
>> right universe?
>> Should I just try a 78 core for the gate drivers too? What do other
>> folks use? I'm a bit vague on parameters right now, but in about
>> another few weeks I will be knee deep in magnetic field physics in my
>> Electromagnetics course, so I will re-familiarize myself with all the
>> math that will make sense of this stuff. After that perhaps "magnetics
>> design" won't be so mysterious anymore.
>I hope others will correct my advise if it is not so good.
>I think that for low power pulse forming transformers, permeability is
>king. The 78 material should do fine, but other high permeability,
>lower frequency ferrites are often used as well. This works, because
>low power transformers often use only a fraction of the core
>capability in terms of saturation. Loss per cycle is often
>unimportant if the excitation is low. If you look at the permeability
>versus frequency for several ferrites, you will find that some of the
>low frequency types have a high frequency permeability that equals the
>permeability of a lower frequency lower permeability type over some
>range of frequencies that exceed the flat permeability part of the
sounds good. We're using a Magnetics W-41206-TC torroid (u=10,000) for
gate drive. It's wound with 4 turns of micro-coax, with the braid used
as the primary and the inner as the secondary. You can get sub-ns
risetimes like this. The core mainly supports the big flat parts of
the pulse, and the transmission-line winding handles the fast stuff.
I originally wanted to use exotic metglas cores here (u=100K or so)
but when I told the Magnetics guy what I wanted to do, he just said to
use a high-u ferrite. Turns out they have higher permeability at
medium-high frequencies than the metglas.