Re: IGBTs are pretty fast

"Terry Given" <my_name@xxxxxxxx> wrote in message
news:1138754474.466290@xxxxxxxxxx
> - can you show a picture of the gate waveform, measured at the igbt?

Sure.
http://webpages.charter.net/dawill/Images/IGBTWaveform4.jpg
5V/div vert, 2us/div horiz, waveform before gate resistors.
http://webpages.charter.net/dawill/Images/IGBTWaveform1.jpg
Ditto, waveform at gate.
Spikes are:
http://webpages.charter.net/dawill/Images/IGBTWaveform2.jpg
http://webpages.charter.net/dawill/Images/IGBTWaveform3.jpg
Same vertical; 200ns/div horiz. (at gate).

Note the spikes coincide with output voltage transitions, but are of the
wrong polarity with respect to voltage.  (In #2, gate voltage is falling,
but the pulse starts negative whereas output is rising.)  Thus I took a pic
of the emitter waveform, about 1mm from the package.
http://webpages.charter.net/dawill/Images/IGBTWaveform5.jpg
(Same phase, time and voltage scales as #1 and 4.)
Period is around 150ns for the pulse, i.e. 6.7MHz.

Incidentially, though the pulse appears on the transistor end of the gate
drive lines, the circuit side is clean.  Alright, so I'll get some PCB and
solder up the drive circuitses...

> - the output voltage waveform looks like it has a 100V spike on the
> rising edge. can you zoom in on that?

That's from the current waveform.  '721 is the output waveform zoomed,
showing about 20V overshoot.

> - can u take a pic of your scope probe setup? that spike might not be
> there....at these low voltages, use a coax scope probe tip adaptor, a
> BNC socket an a short length of tightly twisted wire, soldered directly
> across G-E or C-E.

The current transformer is a black ferrite toroid (probably high mu) with
quadfilar wound 26AWG totalling 280T (4 x 70T in series), and a 2.8 ohm load
resistor.  Twisted pair leads back to the scope, with a few turns around a
ferrite bead for good luck.  (I know, the CT is floating, and hanging on the
ground lead, so it shouldn't pick up much ground loop style hash that a
ferrite bead would be used for, but so what.)

The output waveform is measured with my 10x Tek probe clipped to the output
terminal at the DC side of the coupling capacitor.

> - how far away is your gate drive setup? put it as close as possible
> (creepage/clearance notwithstanding) to the IGBTs.

Not very healthy then... about 12".  I do have twisted pair, which is a
Transmission Line(tm), though..

> - your DC bus inductance is on the order of 500nH or so.

How can that be?  The 0.1 caps are an inch from any transistor, and all
together should be on the order of 100-200nH.

> if you get a
> couple of pieces of double-sided Cu-clad PCB, and some 1mm nomex/lexan
> you can make a DIY multi-layer PCB. just folding your existing assembly
> flat would make a large difference.

So the strips are flat with the heatsink y'mean?

> how fast is your scope? whats its rise-time spec?

SFA.  The output waveform has no difference vs. vertical bandwidth
(switchable 20/100/200MHz).  The oscillographs were taken at 100MHz.  All
the same, it is spec'd at 1.8ns or so.

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


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