Re: Terry`s DRSSTC, gate drive

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Finn,

At 01:37 PM 3/9/2005, you wrote:
> Terry,
>
> First of all, allow me to express my admiration of the way you went about 
> designing and debugging your DRSSTC. I think it is quite remarkable to 
> witness your openness, your skill and quite frankly your restraint to not 
> just "turn up the wick", but wait untill the circuit checks out on the scope.

Thanks!!


> Bravo!
>
> You did once explain how the powering of the gate drivers was acheived, 
> but I missed the point.
> Do you think you could post a schematic of the H-Bridge with one the 
> drivers in place.
> I don`t understand how you derive the 0V reference to the middle of the 
> bridge, for driving the upper 2 IGBT`s, from the rails,

Here is a diagram of 1/2 the bridge with the two drivers and the schematic 
of one gate driver:

http://hot-streamer.com/temp/OpticalGateDriveHook.gif

http://drsstc.com/~terrell/schematics/OpticalGateDrive-1.0.gif

The IGBTs are off most of the time so the output is basically 
floating.  But the two R1s on the gate drivers tend to force the output to 
"around" zero volts.  That allows the voltage across the drivers to be 
about 170VDC so they can charge their power supply storage caps 
(C2s).  Once the C2 caps are charged up, it can drive the gates for about 1 
second which is much more than enough storage time as the voltage across 
the drivers oscillates wildly during the firing cycle.

On the lower side, the collector is tied to the output and the emitter of 
the upper driver is also tied to the output.  When the output is going 
between +170 and -170 during the firing, these circuits "ride" right along 
with the output voltage!!  Not a big deal for the lower one since R1 and R2 
provide a lot of isolation.  But the upper one has the whole "virtual 
ground" connected to the output so everything on the circuit oscillates 
with the output voltage there!!  But it seems to work perfectly fine (heavy 
short wire hook ups!!).

Let me know if this is still not clear.

Cheers,

        Terry



> Cheers, Finn Hammer