Re: Magnetizing current in SSTCs

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

Original poster: "jimmy hynes by way of Terry Fritz <teslalist-at-qwest-dot-net>" <chunkyboy86-at-yahoo-dot-com>

I dont think that the number of turns affects the ratio of in phase current 
to magnetizing current, as long as k and the pulse duration are unchanged. 
If you cut the primary turns in half, there would be 1/4 the inductance, 
and 4 times the magnetizing current. since the induced voltage on the 
secondary is proportional to the inductance ratio squared (turns ratio) 
there will be twice the voltage induced on the secondary, and twice the 
current. so the power factor doesn't change. If the mode of operation 
changes from cw  to quick pulses, the magnitizing inductance would be 
bigger compared to the in-phase current, because the secondary wouldn't 
have time to fully wind up. Of course reducing the turns would increase the 
current, so you would need bigger FETs. I'm not sure how much magnetizing 
current there is, but from some quick calculations, I'm guessing about 1/3 
of the total current. What is the main advantage to ZCS? Is it just 
switching loss, or do! es switching while current is flowing cause ringing 
type problems? I also dont know how sensitive these things are to tuning. 
Because of the extra capacitive reactance, you will need more volts driving 
the secondary, which means less turns, and low magnetizing current. If it 
is purely an efficiency thing, then you would just have to run the numbers 
and see if it helps or hurts.

Tesla list <tesla-at-pupman-dot-com> wrote:Original poster: "Stephen Conner by way 
of Terry Fritz "

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.






Jimmy