Re: Terry's DRSSTC -"different" H-drive functions...

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Antonio,

At 06:09 PM 1/12/2005, you wrote:
> Tesla list wrote:
> >
> > Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>
>
> > "Normally", when the gate drives are off, the IGBTs are pretty much
> > slamming on the breaks for the primary loop current.  If all the IGBTs are
> > not conducting, then the primary Q is very low and the energy stored in the
> > primary is very quickly being dissipated in the free-wheeling diodes in the
> > IGBTs.
>
> Maybe I am thinking in a different circuit, but if the IGBTs are off,
> in a full bridge, the reverse diodes don't conduct,
> unless for some reason the voltage across the primary circuit (L+C)
> becomes higher than the DC power supply that feeds the bridge.
>
> Antonio Carlos M. de Queiroz

After the primary voltage rings up to say 10kV, we turn off the IGBTS.  At 
that point, the primary capacitor voltage appears across the now open 
H-bridge.  But the reverse diodes will catch that voltage and limit it to 
+- the buss voltage:

http://hot-streamer.com/temp/DRSSTC-H-Bridge_sch.gif

http://hot-streamer.com/temp/DRSSTC-H-Bridge_Vdiff.gif

In the second picture, you can see to voltage across the bridge when the 
gate drives are turned off (yellow circle).  We get about +- 300V at say 50 
amps average for roughly 100uS.  That is ~~1.5 watts into the reverse diode 
heating.  At say 200BPS that is 300W!!  That is what I am trying to avoid 
by keeping the lower IGBTS conducting during the turnoff and keeping that 
power in the primary LC system.

Cheers,

        Terry