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Re: Large ferrite cores for SSTC work

Original poster: "Harri Suomalainen by way of Terry Fritz <twftesla-at-qwest-dot-net>" <harri.suomalainen-at-pp.inet.fi>

> << Absolutely true, but put simply, who runs their coil out of tune? If
>  you wanted an efficient ZVS converter, this would be wonderful. But
>  it's not that...it's an SSTC!
>  My comments may be discredited by the fact that I have never used an
>  output transformer in my SSTC designs. But I know one thing, I had

I've tried one prototype for low losses which had a matching transformer too.

My basic design was simple: fet-based half bridge driving a transformer
primary. The other end of the primary was connected through a couple of
caps to the supply rails (as usual). The secondary was end-feeding my coil.

Transformer primary leakage inductance and the caps were designed to
have a resonant at a suitable point. It was incorporating zero-voltage
switching there. Just to test the design I did drive the prototype close
to 1MHz with loading and had no problems. FETs were vecy cool too :)

Without current-limiting and no transformer resonant protections I had
problems with sparking coils at higher power. They seemed to
probably saturate the core or cause some out-of phase loading or something.
I never really found out the cause because of the lack of a storage scope.

To solve this and to be able to go up in the frequency (switching losses!)
I tried the resonant tank and ZVS solution. Works great and it is
inheritantly short-circuit proof too.

Secondarily this was also a solution for too large leakage inductance causing
problems by limiting the current. I assumed this would be a major problem
with high power circuits with larger transformers.

The ferrite with windings was something like 2x2x2" Amidon E-core.
Feeding something like 500W through that at 250kHz was not a problem
for long period testing. For a smaller periods of time it could take 1kW
nicely. As this was just some prototyping I never investigated more
close about the power range.

The ferrites you'll need will not probably be huge as suggested here
unless you're forced to use small frequency for some reason.