RE: SSTC questions / body diode stuff

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

Original poster: "Jan Wagner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>

Hi,

On Thu, 13 Mar 2003, Tesla list wrote:
 > Original poster: "Justin Hays by way of Terry Fritz 
<teslalist-at-qwest-dot-net>" <pyrotrons2000-at-yahoo-dot-com>
 >
 >  >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!!!
<snip>

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

Actually even dead time won't help, because during the dead time the
freewheeling/internal diode conducts. The current being sinusoidal, the
longer you wait the higher will be the current thru the diode (and
reverse recovery turn-off gets even slower).

http://www.richieburnett.co.uk/sstate2.html
somewhere around the end of the page, "Beware excessive dead-time !"

 > 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.

Yup, it works fine. You just have to be a bit careful to select the proper
ultrafast recovery diode - the instantaneous forward voltage must be
_much_ lower than that of the internal diode. Many ultrafast diodes have
<250mV forward voltage so that selection isn't a big problem, but you
should still check the datasheets first.

cheers,
  - jfw

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