Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!zephyr.ens.tek.com!gvgpsa!gold!grege
From: grege@gold.GVG.TEK.COM (Greg Ebert)
Newsgroups: sci.electronics
Subject: Re: high voltage high frequency resonant air core transformer
Message-ID: <1333@gold.GVG.TEK.COM>
Date: 22 Aug 90 18:59:31 GMT
References: <14786@shlump.nac.dec.com>
Organization: Grass Valley Group, Grass Valley, CA
Lines: 23


Using a cascode connection of a MOSFET and a bipolar transistor will give
you the fast switching time from (from the MOSFET), and high-voltage
protection (from the bipolar). I seem to recall a fairly high-voltage
bipolar used for the horizontal-output section on TV's is (used to be ?)
available from Radio Snack.

Since your circuit is resonant, all you really need to do is give it a
'kick-in-the-pants' with an impulse, and it will ring at the resonant
frequency. The pulse rate of the impulse should be an 'integer fraction'
[sorry for such a silly term, but you know what I mean] of the tank
frequency. You can use lower pulse rates for a higher Q.

As far as the 'kickback' is concerned, it's just something you will have
to live with. You can compute it if you know the peak secondary voltage,
which will vary wildly depending upon your load; just use the turns-ratio
of your transformer. Be aware (read: BEWARE!) that under worst-case, the
voltage across the cascode circuit is the kickback voltage PLUS the DC
supply voltage. Egad !

MOSFETs are wonderful devices, and I've made them switch several amps on/off
in less than 100nsec with blatant disrespect for circuit layout and wimpy
gate-drivers.