Re: Magnetizing current in SSTCs

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

Original poster: "K. C. Herrick by way of Terry Fritz <teslalist-at-qwest-dot-net>" <kchdlh-at-juno-dot-com>

Stephen (& all)-

It's a good point about the magnetizing current.  To educate myself a bit
more, if possible at my age, I simulated a sine wave source, a
transformer and a resistive load in SIMetrix.

1.  100 V pk applied at 100 Hz, starting with 0 V at t = 0 and connected
via 1 ohm to a "perfect" transformer having 150 mH primary inductance, a
1:1 ratio, the primary: secondary coupling of 1 and a 1 meg load
resistor:  1st half-cycle primary current, ~2.0 A pk with the wave
becoming symmetrical about 0 A, at ~1.0 A pk, after about 600 ms.

2.  Load resistance reduced to 100 ohms:  1st half-cycle primary current,
~2.5 A pk with the wave becoming symmetrical about 0 A, at ~1.4 A pk,
after about 600 ms.  Load current is ~1 A pk.

3.  Same as #1 except coupling = 0.2:  Trivial; essentially the same as
#1.

4.  Load resistance reduced to 100 ohms and coupling at 0.2:  1st
half-cycle primary current, ~2.0 A pk with the wave becoming symmetrical
about 0 A, at ~1.0 A pk, after about 600 ms.  Load current is ~150 mA pk.

What that tells me is that, a) with a decent transformer, the load
current mostly diminishes the magnetizing current; but b) in Tesla-coil
work where the coupling is low (like something between #3 and #4),
essentially all the magnetizing current is "going to waste" since it is
not significantly diminished by the load current...is that right?  So
those of us who are looking into very low impedance, e.g. 1-turn,
primaries are perhaps barking up the wrong tree...right?

Further, at 0.2 coupling or so, the reflected secondary impedance is not
going to do a whole lot...right?

Sadly,

Ken Herrick

On Mon, 12 May 2003 17:42:10 -0600 "Tesla list" <tesla-at-pupman-dot-com>
writes:
 > 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.
 >
 >
 >
 >
 >
 >
 >