Re: SSgap Diagrams

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

Original poster: "Marco Denicolai by way of Terry Fritz <twftesla-at-uswest-dot-net>" <marco.denicolai-at-tellabs-dot-com>

Hi Terry (again),

Tesla list wrote:
> 
> Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>
> 
> Hi Marco,
> 
> Thanks for the great tip about the gate resistance.  I now think high speed
> switching noise is what killed IGBT #2.  I setup to look for noise and it
> was not hard to find!  Very high frequency (~4MHz) bad stuff was going on
> in the turn off and on times.  

You didn't mention *where* you measured that noise.

>I now have ended up with 680 ohms in the
> gate.  

This is a very high value (too high). Again, something must be wrong.

>This will probably be tweaked further but it eliminates all the
> noise and does not seem to hurt operation.  The IGBT remains cool.  A scope
> picture of the noises are at:
> 
> http://hot-streamer-dot-com/temp/GateR.jpg
> 
> The R1 (white) trance is with no gate resistance. R2 is with 100 ohms and
> the top yellow trace is with 680.  The noise was not bad at low voltage but
> it got super bad at higher voltages and was causing some false switching
> (really bad for H bridge SMPSs ;-)) and general loss of control.  With the
> 680 ohm in there things are much more stable and I don't see little noise
> burst in things I shouldn't.  I am sure I will have to work on this more
> but you probably save me a bunch of IGBTs :-))
>

A sad fact you have to accept is that with a breadboard assembly you
probably can't get any further. Noise, glitches and nasty transients
will make your life an hell. It makes an enormous difference to use a
PCB, even a prototyping PCB with your own (short) wirings.

I suggest that you:

- abandon the breadboard
- use a small prototyping PCB (say 3 x 3 cm) where you SOLDER the
IR2118, bootstrap cap and diode, and some good 0.1uF + 10 uF filter
capacitors
- on one side of the PCB solder DIRECTLY the IGBT, which is properly
fixed to its heatsink
- mount the *heasink* somewhere, with screws: the PCB is so light that
can simply hang from the IGBT.
- drive this thing with several cm long wires (as you need) feeding
power and drive (to the IR2118, this time).
- use series termination at the drive side for the IR2118 inputs: use
star topology (NOT bus), use resistor for each star "leg"

I hope you got the idea. It shouldn't take you more than 1/2 hour to get
that little PCB done.

Cheers

-- 
_____________________________________________________________

 Marco Denicolai           Senior Design Engineer 
 Tellabs Oy                tel: +358 9 4131 2769
 DSL Products              mobile: +358 50 353 9468
 Sinikalliontie 7          fax: +358 9 4131 2410
 02630 Espoo  FINLAND      email: marco.denicolai-at-tellabs-dot-com
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