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THOR: performance decay explained



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

Hi everybody.

I eventually found the cause of the decay in performance of the Thor SMPS.

As the SMPS transformers are driven by 560 VDC, I had placed across the
transformers primaries 600 V varistors, from the very beginning.

A few days ago two of those went short (damaging 4 IGBTs too, of course!),
and I
ran some simulation of the voltage across the transformers. I noticed that
actually that voltage rises up to 1300Vpeak, starting from the midpoint of the
charging cycle: this is due to the resonant load topology of the SMPS, that
features a capacitor in series to the transformer primary.

The 600 V VDR have been clipping all the time such a voltage! That's why the
charge profile was bended. It's a miracle they have been working so many hours,
without even getting warm. Very good devices, indeed   :^)

Removed the VDRs, replaced them with a series of two (850 V each, other type)
for a resulting voltage of 1700V, I was able to get:

- improved performance, faster (very near to the theoretical) charge speed: now
I got 3 m long streamers
- stable performance: no more decay within 15 seconds.

And the new faster diodes I am using now are still ice cold.

Regards

http://www.saunalahti.fi/dncmrc/thor.htm