Magnetizing current in SSTCs

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

Original poster: "Stephen Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>

Here is something for all the SSTCers to think about...

Recently there has been much talk of FBSSTCs, zero-voltage/current 
switching, and so on. Justin & Aron, Jan Wagner and Richie Burnett have 
good websites explaining this. What I want to look at is magnetizing 
current and how it interacts with these things.

Magnetizing current is the current that would flow in your SSTC primary if 
the secondary wasn't there. It is just due to the inductance of the 
primary, and so lags the drive voltage by 90 degrees. The fewer primary 
turns you use, the bigger the magnetizing current would be. As you can 
imagine, the current being out of phase with the voltage messes up any 
ZVS/ZCS scheme. The current received wisdom is that this puts a lower limit 
on the number of primary turns you can use before your MOSFETs cook.

Now, when you add the secondary, the magnetizing inductance is still there, 
but the reflected impedance of the secondary appears in parallel with it. 
Depending on the drive frequency, this impedance can be inductive, 
resistive, or capacitive. (See http://www.richieburnett.co.uk/ for nice 
graphs) So here's my point: At a carefully chosen drive frequency (it would 
be slightly below the secondary's true resonance) the reflected load would 
surely be capacitive and just the right size to cancel out the magnetizing 
inductance. Therefore you could use as few primary turns as you wanted and 
the current would always be in phase with the voltage.

You can probably make a FBSSTC circuit that runs at this frequency 
automatically. Derive the feedback signal from the primary current instead 
of secondary base current or an antenna. This forces the voltage to switch 
in phase with the primary current, therefore, the circuit can only 
oscillate at the magic frequency (or in practice probably some stupid 
harmonic 8-at-) There is a nice simple half-bridge circuit, used in things 
like CFL lamps and electronic halogen transformers, that works like this.

Hot or not?

Steve C.