Re: [TCML] Solid state efficiency, was: mini Tesla coil specs]

[Steve Ward <steve.ward@xxxxxxxxx>]

Ken,

This lack of perfect zero current switching is exactly why i use primary
current feedback to determine when to switch the bridge of transistors.

But... im confused.  Your system is an un-tuned primary, just an inductor
which can be modeled as being part of the secondary coil.  How, then, can
your primary current be out of phase with the secondary current?  While
there is some "magnetizing" current in the primary (being 90* out of phase
essentially), it should NOT be so much to cause that much hard switching
(though it is a pain in CW systems).

I suspect there is something not correct about your feedback loop, and you
are getting a 90* phase shift.  Have you verified that your transistors are
switching at the secondary current zero crossing (or within .5uS of it)?

Switching at the primary current *peaks* doesnt seem elegant or like the
correct solution.

Steve


On Sun, Nov 15, 2009 at 3:43 PM, Ken or Doris Herrick <kchdlh@xxxxxxxxx>wrote:

> So almost immediately...a follow-up:  Belatedly I thought to resonate the
> primary & try it again.  This time, a) the current's a sine-wave all the
> way; but b) switching /starts out /near current/ /z.c/./ and after about 4
> cycles it's shifted to current-peaks, where it stays.  The current is, of
> course, a lot greater due to the series-resonance, causing the secondary's
> output to rise faster.
>
> But a) where's the advantage, if switching drifts toward current-peak.  And
> b) how to handle the greater current?  (Bigger hardware, that's how!)
>
> And finally:  Whichever way one goes, it seems as if one will have to put
> up with switching near current-peaks--unless one can produce the spark
> within the 1st 4 cycles or so in a DR design.  I haven't simulated that yet.
>
> Comments?
>
> KCH
>
> -------- Original Message --------
> Subject:        Re: [TCML] Solid state efficiency, was: mini Tesla coil
> specs
> Date:   Sun, 15 Nov 2009 13:19:09 -0800
> From:   Ken or Doris Herrick <kchdlh@xxxxxxxxx>
> To:     Tesla Coil Mailing List <tesla@xxxxxxxxxx>
> References: <cc2218e80911111413md7c0b57k355ad83baa7e78a4@xxxxxxxxxxxxxx> <
> 4AFC3E64.90209@xxxxxxxxx> <
> cc2218e80911130718k750de578t197b1d7d7677aaa9@xxxxxxxxxxxxxx> <
> 4AFDCA69.4090001@xxxxxxxxx>
>
>
>
>  From KCH-
>>
>
> While I have the attention of a few s.s.'ers here, perhaps someone can shed
> light on this:  I'm simulating what amounts to a half-bridge driving an
> untuned-primary t.c.  I record waveforms of a) MOSFET current and b) primary
> current.  I notice the following: At the start, the MOSFETs switch at
> current peaks and the current waveform is triangular.  As the primary
> current--and secondary voltage, of course--builds up, the current waveform
> gradually changes toward a sine shape and the switching events shift toward
> current-zero-crossing.  I have a feeling that that's the case in the real
> world.  Is that to be expected?
>
> KCH
>
>
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