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Re: [TCML] making a small design bigger
The fundamental limit on this type of SSTC design is impedance matching to
the streamers. Basically, the impedance transformation is set by the
inductance ratio and coupling coefficient of the primary/secondary coil.
Less primary turns gives greater impedance transformation, thus draws more
current from the half-bridge and produces larger sparks. Higher coupling is
good so that the amp-turns in the primary are more linked to the secondary.
You can treat the SSTC much like the standard transformer model that
contains the ideal transformer, a parallel magnetizing inductance on the
primary side, and a series stray inductance on the secondary side (or
As the primary turns are reduced, the magnetizing inductance (Xm) becomes
smaller and thusly magnetizing current goes up (Im). The magnetizing
inductance is kind of mis-named here, its basically the "bare" inductance of
the primary coil, when there is no secondary coil present to use up the
flux. Anyway... as you get fewer turns the current flowing through the
"magnetizing" inductance can be problematic because its not in phase with
the resonant current, which makes it impossible to get true zero current
switching, which is highly desirable from the point of view of keeping the
bridge driver working at high power. Essentially, you have the
super-position of both load current (sinusoidal due to secondary resonance),
and Im, which is triangular because its merely an inductor driven by a
voltage square wave.
This was the reason for the DRSSTC. By adding a primary resonant capacitor,
you could resonate with the equivalent inductance of the primary, keeping
the current sinusoidal. On top of this, the resonance develops higher
primary currents with a lower drive voltage, which in effect is an
additional step of impedance transformation which allows driving more power
into the sparks.
On Tue, Jun 7, 2011 at 7:37 PM, Florin Andrei <florin@xxxxxxxxxxxxxxx>wrote:
> I'm looking at a solid state design for a modulated coil, two examples
> They are pretty much the same thing - a MOSFET half-bridge driven by a
> pulse-width modulated pilot. Sounds simple enough and, except the loose
> couple coil itself, it's mostly technology I'm familiar with (back in the
> day, I designed and built everything from small automation systems to HiFi
> amps to systems with microprocessors, so I know solid state). I want to
> build something like this.
> My question is - what are the general rules to increase the output of this
> type of design? (make it throw bigger sparks) I assume it's not as easy as
> winding more copper into the secondary coil. At some point, the half-bridge
> will go into saturation and won't be able to pump more power into the coil -
> if it's not already there.
> I'm not sure where to begin tweaking the output. I realize a Tesla is not
> the regular low-voltage ferrite-core transformer I used to play with, so I
> figured I should ask before I build something and start popping MOSFETs like
> Florin Andrei
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