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Re: Magnetizing current in SSTCs
Original poster: "K. C. Herrick by way of Terry Fritz <teslalist-at-qwest-dot-net>" <kchdlh-at-juno-dot-com>
Notwithstanding what I wrote below, I've played around with a basic
simulation of a 1-turn-primary solid-state Tesla coil; a resonant-primary
one. Its schematic is at http://hot-streamer-dot-com/temp/1-t-pri.pdf.
Might be interesting for anyone with a s.s. driver that will provide just
300 V p-p at Fr and a mere 5 KA of rms current--in bursts, of course,
unless you happen to own a power plant.
One would get the 300 V right off-line via + and - 150 V dc supplies,
then half-bridge transistors to get the drive at Fr.
I found that I could utilize SIMetrix' "ideal transformer" component by
turning it around, i.e., using the primary as the secondary & vice versa.
I can then fix the (new) secondary's inductance while setting the
"primary:secondary" turns ratio to the reciprocal of the desired one. In
the simulation case, to 0.002 for the desired 1:500 step-up. I selected
a coupling coefficient of 0.2 & used a secondary simulation-circuit I
found on the List.
The 7 uF primary-resonator is not too critical: 6-7 makes not much
difference. C3 (output) current is about 2A.
If I find the motivation to do it and have the simulation components,I'll
try to add to the circuit to make it self-tuning using the secondary's
return-current, & then run a multi-step analysis while varying C3 to find
out how large a change in secondary-circuit Fr it will accomodate.
Comments appreciated...
Ken Herrick
On Tue, 13 May 2003 12:16:00 -0600 "Tesla list" <tesla-at-pupman-dot-com>
writes:
> 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.
> >
> >
> >
> >
> >
> >
> >
>
>
>
>