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

[Ken or Doris Herrick]

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 at sonic.net>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 at sonic.net>
>> To:     Tesla Coil Mailing List <tesla at pupman.com>
>> References: <cc2218e80911111413md7c0b57k355ad83baa7e78a4 at mail.gmail.com> <
>> 4AFC3E64.90209 at sonic.net> <
>> cc2218e80911130718k750de578t197b1d7d7677aaa9 at mail.gmail.com> <
>> 4AFDCA69.4090001 at sonic.net>
>>
>>
>>
>>  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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>>     
>
>