RE: DRSSTC -- EMI scope problems

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

Original poster: "Stephen Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>

OK... First of all, I ought to say, this is becoming more of an electonic 
debugging session than a discussion on Tesla coils. Maybe Terry would 
prefer if we carried this on by email off-list?*

*<if it may be a future interest or something to those that follow your 
efforts it is fine.  If it gets into like a solder joint or something 
really basic, then maybe direct e-mails to solve the specific issue would 
be best - T>


  I now get to look at the microprocessor level. It has
>absolutely no 60khz on it, but has high frequency
>"chirps" at every half cycle, adding a choke had
>absolutly no effect on it.

I would suspect either feedthrough of the micro's clock frequency (probably 
OK)... or your gate driver chips have parasitic oscillations... prepare 
another plot in the Silicon Cemetery 8--at-


>The voltage across the IGBT brick may be a problem
>though. It looks like a 60 khz sinewave that goes from
>0 to twice the normal voltage.

Do you mean the gate voltage or the collector voltage? I would expect to 
see 60kHz _square_ waves in both of those places. And why twice the 
voltage? The co-pack diode of the upper IGBT (I assume you're running a 
half or full bridge) should clamp it at the rail.


>it takes a three foot diameter turn to get
>enough inductance. There is no way I have that much
>area

One possibility is that the leakage inductance of your gate drive 
transformer could be resonating with, or forming a low-pass filter with, 
the gate capacitance of the IGBTs. This would tend to turn the square wave 
drive into a sine wave. Hence the collector voltage might end up kind of 
sine wavey too. The gate capacitance is big, and Miller effect makes it 
look even bigger, so the GDT design is a pretty tough challenge... one that 
I went to great lengths to avoid :)

Or of course it could be interference caused by a ground loop picking up 
the primary current. The actual voltages on the coil might be clean. THe 
way to test this is to remove the probe tip from the circuit and touch it 
to the probe's ground lead (which you leave connected to circuit ground) 
Any signal that you can still see is ground loop interference.

Steve C.