Re: Series connection of Mosfets/IGBTs

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

Original poster: "gtyler" <gtyler-at-drummond-dot-org.za> 

This is just what I have been contemplating: You get a high voltage,
could be many KV, across the primary without series fets or other high
voltage topology for the electronics. I have an induction heater design
that generates enough voltage on the primary for corona to be a problem
when powered by 220Vac.
     To calculate the primary voltage multiply the current in the fets by
the capacitive reactance of the resonating cap.I have included a circuit
that automatically tunes itself to the resonant frequency, as I menioned
before.
George
----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Thursday, February 19, 2004 5:05 PM
Subject: RE: Series connection of Mosfets/IGBTs


 > Original poster: "Steve Conner" <steve.conner-at-optosci-dot-com>
 >
 > Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz>
 >
 >  >Perhaps I can throw a bit of light on this:
 >
 >  >What is happening is that the primary is no longer an inductance but
 >  >a resistance of very low value at resonance. This allows you to get
 >  >more current in. It also allows a high circulating current to build
 >  >up.
 >
 > That's exactly what is happening. The inverter sees what looks like a
 > resistive load, composed of the AC resistance of the coils, plus a
back EMF
 > that accounts for the energy transfer from primary to secondary.
 >
 > By mutual induction the secondary sees a voltage V2=Mdi1/dt which
causes a
 > current i2 to flow in it. Being at resonance, i2 is in phase with v2.
Again
 > by mutual induction, the primary has a voltage V'1=Mdi2/dt. Two
di/dt's
 > gives a 180 degree phase shift, ie V'1 is a back emf that opposes the
 > original voltage driving the primary.
 >
 > This back emf (or voltage across an "energy transfer resistance" as I
like
 > to think of it) is a transient thing, you don't see it in AC sweep
PSpice
 > simulations because they assume the steady state. As any classical
coiler
 > knows, once all the energy has transferred to the secondary, it turns
round
 > and sloshes back to the primary. In our analysis, the back EMF would
change
 > sign at every "notch", so in the steady state it averages to zero.
 >
 > It is possible to build series tuned SSTCs that work in the steady
state.
 > The sloshing eventually dies down and the output is a pure sine wave
at the
 > driving frequency. In this case streamer loading is all that is
limiting the
 > primary current.
 >
 > But in a pulsed SSTC, we can take advantage of the transient behaviour
to
 > ram large amounts of energy in very quickly. I'm trying to understand
the
 > math at the moment (the above was a gross simplification that would
bring
 > physicists out in a rash) so I can design for a given worst-case peak
 > transient current in the inverter.
 >
 > Steve Conner
 > Power Electronics Misapplications Dept.
 > scopeboy-dot-com
 >
 >
 >