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Comparison with resonant switching topology (was Re: My FET T.C.)






Tesla List <tesla-at-pupman-dot-com> on 17.11.99 05:53:10

To:   tesla-at-pupman-dot-com
cc:    (bcc: Marco Denicolai/MARTIS)
Subject:  My FET T.C.



>Original Poster: Alan Sharp <AlanSharp-at-compuserve-dot-com>

>>3.  Driver-- 24, 85A, 500V power MOSFETs electrically in push-pull
>>parallel, together driving the equivalent of 1 primary turn (in a
>>patent-applied-for circuit).  Soon to be increased to 48, then--if my
>>endurance holds out--to 72.  That will double, then treble the spark
>>length (and the input current for a given spark rate).  The primary is
>>untuned.

>Do you mean 85A or 8.5A mosfets - why so many? they will handle
>a peak current many times greater than the continuous current rating.
>I thought that 8 was a lot.

Just a little comparison with the switching topology I am using (resonant load,
full-bridge).
I use 4 1200V 35A IGBT (oversized in terms of current). I switch 560 VDC into a
15 (fifteen!) turns primary transformer, using a primary peak current of only
12A. At the secondary I get 5kV at 1A. All the switcher fits on a 100x160 mm
(eurosized) board. The 4 IGBT use together the same 4 'C/W heatsink and stay
pretty cool all the time.

>>Soon to be increased to 48, then--if my
>>endurance holds out--to 72.

You can convert easily to the resonant topology I am using by just adding a
series capacitor to the transformer primary and setting suitably switching
frequency and duty cycle. Then you'll save tens of MOSFETs and will heat less
your room!

>Also FETs can be killed by over voltage. I tried this and managed
>to fry quite a few FET's.  The problem is that when the FET's switch off
>and
>the magnetic field collapses a very high voltage is induced on the primary
>- it could easily exceed 500V.

No problem with that. Never burned a single IGBT for overvoltage (or
overcurrent). In my topology you switch on/off always at 0 current (so there is
no kickback from the transformer, or primary, and switching losses are
minimal).

Check my design at http://www.saunalahti.fi/dncmrc/ccps.htm

Regards