Re: A s.s.t.c. problem (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Gerry Reynolds <greynolds@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Wed, 20 Dec 2006 23:19:56 -0600
From: Steve Ward <steve.ward@xxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: A s.s.t.c. problem (fwd)

Hi Ken,

My instinct tells me that you certainly want to use the ferrite cored
CT, since you want the transformer to be very *ideal*, meaning very
high coupling between the windings.  With an air core, there is
certainly far less coupling than with a toroid that provides a much
better magnetic path to follow.  Im not quite sure why you are seeing
any discrepency with the operating frequency.  Its possible that the
switching delay is working to reduce the operating frequency to some
degree, but this is just a guess, and may be completely wrong.

I notice you use an interesting loading scheme for your CT.  Im not
sure, but it might lend itself to unwanted phase shifts when the
primary current is low.  In my CT termination scheme, i simply have
clamping diodes, so that until the output voltage reaches -0.7V or
5.7V, the output is high impedance, meaning that even small primary
currents will cause the output to swing up and down between 0 and 5V.
I find this simple termination method to work very well for my needs,
though im not sure what constraints your system puts on this part of
the design (i realize you must feed in a constant oscillator to keep
your gate drivers powered inbetween drive periods).  Anyway, i think
the important thing is to avoid loading the CT too much, and to avoid
any reactive components that may add a phase shift to the feedback
signal.

Steve Ward

On 12/20/06, Tesla list <tesla@xxxxxxxxxx> wrote:
> Original poster: Gerry Reynolds <greynolds@xxxxxxxxxx>
>
>
>
> ---------- Forwarded message ----------
> Date: Wed, 20 Dec 2006 16:24:05 -0800
> From: K. C. Herrick <kchdlh@xxxxxxx>
> To: Tesla List <tesla@xxxxxxxxxx>
> Subject: Re: A  s.s.t.c. problem
>
> Well, I was a bit hasty in the concern about my "problem".  I only had
> about 3 coil turns active in the primary.  And eyeballing the setup, the
> IGBT/mains-capacitor/primary-connection loop does appear to add the
> equivalent of perhaps 1 coil-turn to the overall circuit inductance.
> And also, when I drive the system open-loop with a signal generator, I
> find that I need ~150 KHz to resonate the (smaller of 2) secondaries
> that I'm presently using.  I'd thought it was at about 125 KHz -- but
> that was with the big toroid; I'm temporarily using a smaller one for
> testing.
>
> So...I need to reduce the primary capacitance to allow for a few more
> turns in the coil.  And perhaps try to diminish the extraneous
> inductance.  And maybe I'll start using the longer secondary for its
> lower Fr.  But I'm getting to be almost too feeble to pick it up
> anymore, in my old age.
>
> KCH
>
>
>
>