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Simpler drsstc half bridge.

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Dylan

Sun Sept 23 2007, 11:16AM

Registered Member #1024 Joined: Sun Sept 23 2007, 10:56AM
Location: Northern NSW, Australia
Posts: 95

Firstly , Hi all , im new here , first post and all.

Anyway Im currently messing with the sstc -> drsstc transition and it seems to me the only difference in basic topology iis the series addition of the primary resonant cap.
Looking at the layout of my average half bridge I add this cap and end up with :
===================================

0v---||-|-||---240v <- Half bridge voltage divider
        |      
        |--||---\  <-Resonant cap
                /
                \ Primary
                /
---Hbridge---\

===================================

My question is : WHY cant the resonant capacitance simply be the capacitative divider of the half bridge? ie:
===================================

0v---||-|-||---240v <- Half bridge voltage divider AND Resonant cap
        |      
        \
        /
        \ Primary
        /
        \
         ----Hbridge---

===================================

I mostly ask as after careful calculation of the required value , adding the resonant cap to my current setup cuts streamer length drastically, and by simple fluke my half bridge divider already has exactly the required capacitance for resonance on both halves of its cap divider!!
- Oh , and BTW I run a half bridge of IGBT's from inverter microwaves (GT60N321) , at full rectified mains in im getting fat strikes to grounded ceiling over 1.5 meters.
The GT60N321's seem to be doing great! Only blown up three so far :) , and FREE :))

Steve Conner

Sun Sept 23 2007, 11:21AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

You're absolutely right, those two capacitors can be combined. The value for your resonance calculations is the value of the two capacitors in parallel, because the ends not connected to the primary are shorted together by the DC bus filter capacitance, so from an RF point of view they are in parallel.

The capacitors in the divider must now have a suitable voltage rating, because you will be generating several kV across them.

Also, they can't be the voltage divider, and the resonant cap, AND the DC bus filter cap. You need a third capacitor across the DC bus, and if you don't have one, it's a miracle that your circuit hasn't splattered itself.

Dylan

Sun Sept 23 2007, 11:35AM

Registered Member #1024 Joined: Sun Sept 23 2007, 10:56AM
Location: Northern NSW, Australia
Posts: 95

So thats why it works !

My setup is as below :

|----||---||----|  <- 2 whopping electros (2200uf @ 200v ea)
|               |
|----||---||----|  <- Half bridge voltage divider AND Resonant cap (2x2 0.1uf 1.6kv)
        |      
        \
        /
        \ Primary
        /
        \
         ----Hbridge---

So , yep ive got the voltage rating in the combined divier/res cap.
Ive got the big electros to cover the filtering.
And well , it produces scary results at full mains in (240) !
So it appears by sheer fluke of "whats in the parts bin" while making my half bridge , ive fluked on at least an almost-ish DRSSTC topology?
If so , thats cool, I like a fortuitous accident.
Anyway Ill post some picks soon.

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