From: John Popelish
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Subject: Re: Gate drive transformer ferrite?
Date: Sat, 19 Oct 2002 00:00:58 GMT
NNTP-Posting-Date: Fri, 18 Oct 2002 20:00:58 EDT
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