RE: Series connection of Mosfets/IGBTs

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

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