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Subject: Re: Problems with MOSFET drivers
Date: Mon, 18 Nov 2002 18:05:44 -0000
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"markp" wrote in message
> "Mike Poulton" wrote in message
> > On 18 Nov 2002, Winfield Hill said:
> > > Arghhh! Please let me suggest you measure the phase of the
> > > current out of your driver with respect to the voltage. You'll
> > > find that at resonance it's in phase, below resonance where the
> > > load appears inductive it lags, and above where the load looks
> > > capacitive it leads. Readymade for generating a servo signal,
> > > or at least a meter reading to tell you which way to tune your
> > > oscillator frequency.
> > Yeah, that sounds simpler.
> > --
> Hold on, Why not just use a PLL (software or otherwise) using a squared up
> zero-crossing detector on the feedback winding? When in resonance the
> signals will be the same frequency and zero degrees phase shift w.r.t. the
> feedback signal. A 4046 might just do the job in edge detect mode, but a
> microcontroller would be more flexible.
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.
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..
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