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From: Mike Poulton
Subject: Re: Problems with MOSFET drivers
Date: 18 Nov 2002 21:59:45 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:
> Actually what you need is an edge detector to determine whether the
> edge of the drive signal preceeds the edge of the zero cross signal,
> and decrease the frequency by a small bit if it does. Similarly if the
> drive signal lags the zero cross signal then the frequency is
> increased by a small bit. There's no need to measure current, and I
> don't think you'll be able to do that accurately enough to stay in
> resonance anyway.
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. The goings-on of the primary also influence it's
signal greatly. This makes it much more complicated to deduce what the
feedback means. 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. Consequenty, it won't be hard to measure current
accurately enough. Besides, I want an LCD current meter on this thing
anyways and the same PIC that does the frequency control can do that,
> As an aside, I would change the way the transformer is driven. Use a
> centre tapped primary, and feed this via a constant current source
> (i.e. large inductor) from the main DC suppy. Then use two fets to
> ground, one either end (push-pull). Ensure there is about 5us of
> OVERLAP on the control signals to keep the current flowing. This
> becomes self limiting for output voltage which is now solely dependent
> on turns ratio and supply voltage. This might not be what you want for
> a Tesla coil, but hey..
Yeah, this isn't really a "transformer" in the normal sense of the word.
It's a series LC resonant circuit with an inductively coupled driver.
The turns ratio essentially determines the input impedance, not the
output voltage. The voltage is determined by the Q of the resonator and
the input power. The physical configuration of the primary is critical,
and using a split primary will not provide a balanced drive since the
two halves do not occupy the same space. It is already a challenge to
make a single primary work optimally, and trying to adjust two halves
would be quite unpleasant. The lower half would take significantly more
of the load than the upper half, causing uneven heating of the fets.
Besides, what's the major advantage of that over a full bridge?
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