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Re: [TCML] OLTC midi problem

Dave,
It sounds like you were getting very narrow spikes in the drive to gate of your IGBT which were triggering it, probably between charging cycles.
The addition of a small cap or scope has created a small low pass filter. It is likely that the duration of these spikes is too small to have been picked up by the scope.
They may be created by the controller and pass unnoticed by the scope, through the opto (assuming the opto is a fast one)
Having some resistance in series with the gate will also add a low pass filter element using the gate capcitance and resistor as an RC filter.
It may also be true that you are getting a small parasitic spark somewhere that is creating these short pulses.
I find adding caps across 0v and/or VCC to earth and/or case often removes this sort of issue.
Another possibility is an earth loop, ensure where possible all your earth paths are not paralleled and terminate to a single 0V point. This reduces induction and the pickup from any stray spark.
I have spent hours of time trying to pin down the causes of this sort of issue. Good luck. 

    Derek

On 07/09/2013 13:40, dave pierson wrote:

[embedded thoughts:]


A small problem that I would like to present to more experienced
builders...
I have been building a small OLTC that is still in testing phase.
The coil produces about 2.5 feet long arcs and runs great with the
basic PWM controller I have made to give the 0-500bps.
IGBT is a 1200v 600A, and is driven by a simple 30A driver IC and
circuit, with isolated PSU, and opto coupler isolates between the
driver and the PWM control box.
PWM control box is metal, negative to case, powered by battery and
connected via a coax cable with the shield connected to mains supply
earth at the driver end.

So with this set up, the coil runs great...

I then bought a basic midi interface from EVR, and assembled, tested
etc the board and set pulse output to suit the needs of my setup.
(also running off battery...)
Connected in place of my manual controller and runs great
up to about 75% power input to the coil.
Even a fraction above the 75% mark and suddenly, nothing but the
occasional splutter...
Moved controller further away... still faults at the same 75% mark.
Scope across output of midi controller is clean and runs continuously
even when spluttering, so I ruled out the controller.
So I put scope across the Gate of the IGBT... and what do you know,
the problem went away!
Next I removed the scope and connected the Emitter side of gate to
ground via a 1000pf capacitor (i can't ground it directly due to
circulating currents) and it also solved the problem, running
up to full power no problem...
So my question is, what was the real cause of the problem?
Any ideas?

     It sounds like a classic case of RFI getting into
     the gate.  The RFI is bypassed by the stray capacitance
     of the scope probe, or by the explicit capacitor.
     (I have vague memories of typical scope probe
      capacitance being 15 pF?  check probe specs.)

     Why does this show up at '75.111%'?  Speculation:
     somewhere a stray spark starts?  General rise
     in RFI as output rises exceeds some threshhold
     in the controller?

     I recall, tho it may not be applicable that having
     'some' resistance in series with gate to reduce
     sensitivity to stray RFI.

     Suggest seeing what other advice appears.

  best
   dwp
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