RE: SSTC questions / body diode stuff

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Justin Hays by way of Terry Fritz <teslalist-at-qwest-dot-net>" <pyrotrons2000-at-yahoo-dot-com>

Hi All,

 >1) Why do people "disable" the body diodes of the FETs?  With a full
 > bridge, there shouldn't be a problem with too much current through
 > the body diodes, next to none if the switching of the FETs is done
 > correctly and there is at least one rectifying diode on the input.

The body diodes are isolated for ONE reason: because they are too
slow!!!

As an example, let's take the International Rectifier IRFP460 MOSFET.
This is a common MOSFET used in SSTCs. In the datasheet, look at the
specification for the Reverse Recovery Time of the body diode. You
should find it's typical time being 580nS, and up to 1200nS! This is
like hours in an SSTC circuit ; ).

So what's the problem with a slow Trr (rev. rec. time)? Well, say for
instance you have a half-bridge. If the body diode of the top MOSFET
is still conducting when the one below it turns on, you get a massive
pulse of current through both MOSFET's. This pulse of current is
similar to what is called "shoot-through" current, when both MOSFET's
are turned on simultanously.

If dead time is incorporated, you can probably get away with just
using the body diode. Power dissipation in the device package will

Many on this list use series shottky diodes to absolutely, totally,
100% isolate the body diode. I don't.

If you use a fast enough reverse diode (75nS or so) you can leave the
shottkys out. It simplifies the design like you wouldn't believe. And
it's not like I haven't tested this. 16" arcs, 1/2 wave rectified
340VDC input, H-bridge of IRFP460's. No shottkys. Also tested it on
several other, smaller coils, including an IRF640 (only 200V)
half-bridge.

Have a good one,

Justin Hays, www.hvguy-dot-com

=====
Justin Hays
KC5PNP
Email: justin-at-hvguy-dot-com
Website: www.hvguy-dot-com

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