Re: Recent s.s.t.c work

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "K. C. Herrick" <kchdlh@xxxxxxx>

Steve (& all)-

Yes, it's a little slow; and at 100 KHz "a little" is too much.  I 
measure about 1 us between the fall at one IGBT gate (crossing 0) and 
the beginning of turn-on at its opposite.  The way I have the drives 
configured now, the gating transistor doesn't start to turn on at all 
until the driving wave goes above the IGBT's source-voltage 
level.  I'm going to change that so that that turn-on starts as soon 
as the driving wave begins to rise from the -25 or so.  That will 
give more drive to the gating transistor by the time the driving wave 
gets above the source-ref. level, & hopefully not cause overlap.

All the drivers are working OK; all the same.  And I have currently 
only 1.1 ohms in series with the gates--& will try getting rid of 
that once I improve the delay.

KCH

Tesla list wrote:
> Original poster: Steve Ward 
> <mailto:steve.ward@xxxxxxxxx><steve.ward@xxxxxxxxx>
>
> Ken, what is the impedance of your gate driver?  I have never seen 
> waveforms that slow and with that peculiar rising edge.  The falling 
> edge looks normal.  Is your gate drive transistor (the one that 
> pulls positive) falling out of saturation perhaps???  The turn on is 
> usually faster than turn off...
>
> The really odd thing is that your gate voltage slows down before you 
> even hit the 0V line!!!  That is *not* right.  Have you checked each 
> driver?  Maybe this one driver is damaged and also causing your 
> imbalance in primary current from hafl-cycle to half-cycle?
>
> Good luck... in any case, i think that those large turn on delays 
> are fix-able.  My gate drivers can supply LOTS of current, as the 
> mosfets i use in it onl have a 55m ohm Rds.  I dont use any 
> resistance on the IGBT gates, so i switch at the maximum speed possible.
>
> Steve