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Re: [TCML] Gate Drive Waveforms Ringing

Hi Brian,

The Vge ringing can often become worse when you actually switch some voltage
with the half-bridge because of the Ccg (aka miller capacitance) will excite
the resonance formed  by the gate driver stray L and the gate capacitance.
The ringing usually isnt a big deal provided it does not ring below say 10V
and above 30V.  If it does overshoot, its good to clamp this with a pair of
back to back zeners or a TVS diode.  The critical part is usually the
transitions of the gate drive.

Anyway, i cant explain why your waveforms "changed".  Were you switching any
voltage with the orignal "good" waveform?

The ringing on your Vce waveform is caused by any stray inductance to your
DC bus capacitors and the IGBT Cce (output capacitance of the junction).
Typically this will ring in the 10-50MHz range.  Its low energy and can be
damped with an RC snubber across each IGBT C-E junction.  Id suggest reading
up on snubbers (a google search should turn up something) but basically you
size the R and C to be an impedance match with the stray L that you are
trying to swamp out.  You can work backwards and figure out the stray L by
measuring the ringing frequency and using the datasheet value for Cce
(sometimes called Coes).  Then, knowing the impedance of this stray L at the
ringing frequency, pick a capacitor that has the same impedance at that
frequency, and pick a resistor that matches that impedance in ohms.  I
believe this yields a critically damped situation, but the cost is power
dissipation in the resistor.  To reduce this, you can decrease the
capacitance and let the system be slightly under-damped, which will reduce
ringing but not completely remove it.

Also, any stray L within the IGBT body itself is basically impossible to
"fix" with a snubber, best to slow down the gate turn ON.

Hope that helps some.


On Tue, Aug 18, 2009 at 7:55 PM, Brian <brianv@xxxxxxxx> wrote:

> Hi Guys,
> I have questions for the SSTC people out there, I have some ringing on my
> gate drives and for the life of me I cannot seem to get rid of it. The
> signal used to be as clean as can be and now it has developed a ringing
> that
> has me totally befuddled.  I rewound the GDT and changed the IGBT so they
> are all good, the GDT used to have a really nice looking waveform. I have
> also changed the lead lengths going back to the control board and nothing
> seems to change. I have uploaded a few images so you can see what it looked
> like and what it looks like now. I have also uploaded the H-Bridge output
> waveform and you can see it has a large amount of ringing at the beginning
> of each transition. Also the other thing I should mention is when only 1 of
> the GDT are connected the signal is very clean again, it makes no
> difference
> which one is connected the signal is good, the moment the second GDT is
> connected the ringing begins.
> Here are the links for the waveforms:
> The Good waveform:
> http://i915.photobucket.com/albums/ac356/Brianvod/GateDrringing001.jpg
> The waveform now:
> http://i915.photobucket.com/albums/ac356/Brianvod/GateDrringing005.jpg
> H-Bridge output waveform:
> http://i915.photobucket.com/albums/ac356/Brianvod/GateDrringing004.jpg
> I did add additional gate resistance and it helped somewhat in fact I added
> until I was way over damped and it of course it began to distort the
> waveform.  But anyway I am not sure why this just all of a sudden showed
> up,
> so I thought I would post it here and see if anyone has a similar issue and
> what they did to correct it.
> Thanks Much
> BrianV
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