(snip)
>
> > I do use this technique in my latest DRSSTC drivers (not published
> > anywhere yet). I use it mostly for current limiting, so that instead
> > of terminating the gate drive upon current trip, i simply let the
> > primary "coast" by leaving off 1 half-bridge (the diodes commutate).
> > This allows the primary to ring with only the damping the secondary,
> > and not regen to the DC bus.
> > I very much like the results of having this added control of the
> > bridge.
>
> I think I understand the diode commutating point, but don't you still need
> to reverse the phase of the driven half-bridge?
Nope. You dont want to "invert" the drive voltage, you just want to "zero"
it. Its funny, when i was thinking about how to originally do it, the
first idea was to make both half-bridges switch together (in phase) during
the coasting time. However, if you just leave off the one half-bridge that
needs to get "inverted" the same behavior will happen naturally via the
free-wheeling diodes. This avoids any difficulties in passing the signal
through the gate drive transformer.
> Further, wouldn't you also
> be better off continuing to drive the other half-bridge, to avoid leaving a
> 'high impedance' gate on the low-side of that half-bridge with possible
> attendant Miller effect issues? (I feel quite out of my depth with that
> last
> question.)
>
Its a great point to raise because that exact problem could exist. My
drives use gate drive transformers, and the driver for the transformer
"clamps" the GDT primary to 0V, which serves as a low impedance "short" on
the GDT secondaries (which connect to the gates). So basically the IGBT
gate looks pretty much shorted. Leaving an IGBT gate "open" is a bad idea.
>
> Turning back to the point of the original post (so this is not a complete
> hijack), do I understand the (at least theoretical) approach you would
> recommend for a DRSSTC then to be: (i) As a goal (but subject to component
> limitations), to time the ending of each burst to correspond with the
> 'first
> notch'?
This is great for achieving the best input power to spark length efficiency.
> (ii) To use your 'coasting' technique to implement current
> limiting, but not end-of-burst?
You may not even need this control, but i like it :-).
> (iii) To implement end-of-burst by not
> driving the IGBTs? (Why would that be better than 'coasting'?)
>
Coasting vs Regenerating at the end of burst? Well, thats something that
needs to be tested for each case. For coils that end the drive at a notch,
i would let it regenerate to the DC bus through the diodes (all IGBTs off
at end of burst) to capture what little energy is left. I dont think
coasting would contribute to the sparks in this case, since the secondary
is presumably already at its peak, and will be ramping down at this point
anyway. For coils that are "detuned" to operate without notching, i would
think the coasting would be better to let that stored primary energy get a
chance to make its way through the secondary to the sparks.
Steve
> MBD
>
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