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Re: Dump the RSG!


From: 	Greg Leyh[SMTP:lod-at-pacbell-dot-net]
Sent: 	Tuesday, November 04, 1997 3:54 PM
To: 	Tesla List
Subject: 	Re: Dump the RSG!

Malcolm Watts wrote:


>              I've been sitting on some Westcode devices (SCRs and
> diodes) for some time now while agonizing over exactly how to use
> them without destroying them. The kind of ratings I am talking about
> (from memory) are 2.5kV holdoff/device (fwd and rev) with current
> ratings around 2kA or so. di/dt should not be a problem with a
> sufficiently low frequency coil. The agonizing bit comes from the
> implications of stacking them. If one turns on fractionally before
> the other (say two stacked), what happens to the sluggard? Bit of a
> worry I think?

At SLAC we have SCR control stacks 15 units high for use in some 
90kV, 3 MW DC regulated power supplies.  Each SCR has its own firing 
circuit which is powered and fired magnetically.  The circuit also
fires its own SCR if the anode volage exceeds a set limit.  This
approach seems to work quite well in protecting the 'sluggard'.

>     The idea is to use DC charging and use the SCR/s to conduct in
> one direction with the diodes to conduct in the other. This should
> allow a per cycle quench capability with a sufficiently narrow gate
> firing pulse (SCRs recover while diodes are conducting).


The back diode does not allow the switch to quench in one direction,
because as the sec energy starts to return to the pri, the diode will
rectify this energy and charge the pri cap once again!  

This sounds undesirable at first, but I believe it's a good thing
since the first beat is doing most of the spark work anyway.  
Why not return the remaining energy present in the second beat 
safely to the primary cap?  Starting with a partially charged cap,
the DC resonant charger will then automatically draw less power 
from the AC mains for the next shot.

My biggest concern on your SCR's would be the di/dt rating. 
Is the rating higher than 2*pi*Fpri*Ipeak?  Ironically, solid-state
switches seem to be better suited to larger, lower frequency coils,
due to the di/dt limit.


-GL