Re: Magnetizing current in SSTCs

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

Original poster: "Malcolm Watts by way of Terry Fritz <teslalist-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

On 12 May 2003, at 17:42, Tesla list wrote:

 > 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?

What happens to the well-planned-for reflected impedance when sparks
start coming off the secondary?

Malcolm