[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

Original poster: "Bob (R.A.) Jones" <a1accounting@xxxxxxxxxxxxx>

Hi, Subject: RE: Terry's DRSSTC -"different" H-drive functions...

> Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx> > > >................ > > Here is what I was thinking... > > "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. > > Since "quenching" is not needed in DRSSTCs, the 'having all the IGBTs off' > situation really just adds to device heating for no real good reason. If > the primary loop has say 2 Joules in it when we turn the IGBTs off and all > that energy goes into the reverse diodes at 200BPS, we might burn off 400 > IGBT watts!!!

No most of the energy is returned to the supply. The diodes just dissipate
there normal power with what ever the current is..

> But suppose we instead just turn off the top two IGBTs and turn on the
> bottom two...  No energy is being added now and the H-bridge is still just
> basically a dead short to the primary so the primary loop current can
> continue ringing at a relatively low loss.  By keeping the bottom two
> on, we basically just let the primary loop "freewheel" and not waist the
> stored energy as well as reducing the thermal load on the IGBTs!!  Of
> course, running that loop at say 300 amps for a long time will heat the
> IGBTs too, so that may be a wash...

Yes this is the best freewheeling condition but you still have dissipation
in the IGBT and diodes.
If there was no other losses all the stored energy would be eventually be
dissipated in the lower IGBT and diodes.
Perhaps this  condition  is the one required immediately after a burst or
for cycles or half cycles if an over current condition is detected.
Then switch them all off to get the remaining energy  in to the supply only
after you know its will not be going in to the spark.