Re: OLTC update - primary IGBT loss

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

Original poster: "Greg Leyh by way of Terry Fritz <twftesla-at-qwest-dot-net>" <lod-at-pacbell-dot-net>

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>Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
>
>Hi Bert, Paul, and All,
>
>I checked it all out and the 3.8nH inductance and 0.4mOhm resistance of the
>caps and it is just is not a factor at all.  The leads are a concern but
>there are ten in parallel and all that.  However, I think I have "found"
>some losses ;-))
>
>I dropped two high voltage high bandwidth scope probes directly between the
>CE leads of a center IGBT (on the leads, right up next to the plastic case)
>and got this:
>
>         http://hot-streamer-dot-com/temp/OLTC09-02-03.gif
>
>The red line is the CE voltage while the IGBT is on.


Hi Terry,

That's an impressive bit of current to push through a PCB
mountable package!  Do you have 3 IGBTs in parallel here,
as per the earlier sketch?  How well do they share the
current?

At 33kHz and 4500A pk that's a max dI/dt of about 940 A/uS.
10nH will drop 9.4V at that speed, so a noticeable percentage
your measured drops might be reactive, instead of purely
resistive.

It appears that the emitter voltage on the first scope trace
starts out the pulse at 115 volts below ground?  Or is this
just a scope offset?

I was also wondering if the forward IGBT voltage drop and the
reverse current drop due to the back diode are about the same?
It's difficult to tell where zero is, on the math trace.

-GL


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