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From: Mike Poulton
Subject: Re: Problems with MOSFET drivers
Date: 19 Nov 2002 02:51:52 GMT
Organization: MTP Technologies
References: <email@example.com> <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
On 18 Nov 2002, "markp" said:
>> All of these schemes rely on the feedback winding giving an accurate
>> representation of what's happening in the secondary resonant circuit.
>> I'm not sure it does.
> With good inductive coupling from feedback to secondary it *has* to
> follow it, but it relies on good coupling.
Coupling is not good. The feedback winding is better coupled to the
primary than the secondary. This is an air-core transformer with
intentionally loose coupling.
>> The goings-on of the primary also influence it's
>> signal greatly. This makes it much more complicated to deduce what
>> the feedback means.
> Why? If there is good inductive coupling between the the windings the
> only perturbations will be due to things like leakage inductance.
There is extensive leakage inductance, and the coupling is not good.
The primary is in there, too.
> adequate filtering the feedback winding will represent what is
> happening on the output voltage, it *has* to because it is coupled to
> the same flux as the output winding. If this were not true by the way
> your original system of driving the FETs from this is totally invalid.
It was invalid. It didn't work well at all.
> I'm proposing inserting a PLL into all this, which is actually a
> filter to remove such things as leakage inductance noise spikes that
> could end up causing wierd oscillations.
>> Measuring average current over a fraction of a second
>> is easy, and dI/dF near the resonant frequency is quite high. A tiny
>> bump of the frequency knob on the function generator drops the
>> current tremendously.
> OK, assuming a high Q on the output this would be true I guess.
If the Q's not high, it's not a Tesla coil. The bandwidth is very
> Both halves of the primary are equally coupled to the secondary aren't
> they? If not, why not?
They not located in the exact same space, so they are not equally
coupled. Coupling is highly geometry-dependent here. The one closer to
the bottom will see a lower impedance than the one higher up. I suppose
you could use coax, with the shield as one half and the core as the
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