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



Steve (& all)-

I've been trying to stick with an untuned primary + secondary-current feedback so that I could easily switch secondaries without any adjustment. I have two, one with Fr of 100 KHz and the other, about 120. The idea is to just be able to plop one into place, then the other with no fussing.

My (simulated) switching occurs uniformly at ~0.44 us after secondary-current z.c. for a 4.3 us half-period I believe I was remiss in reporting a phase-change: The phase appeared to change because the shape of the primary current was changing due to the magnetizing current. I've now re-checked the phase between the secondary feedback signal's z.c. and the primary's z.c.'s for both the tuned & untuned condition. Tuned, they start out 90 deg. out but within 45 us they are in-phase & remain that way. Untuned, the same 90-deg. out initially with a triangle-wave of current. Then the current shape gradually approaches sine but the z.c.'s never coincide. Always 45 out or so.

Perhaps I might go with tuned.   Gotta study it.  Stay tuned.

KCH

Steve Ward wrote:
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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