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From: email@example.com (Adam Seychell)
Subject: Re: Problems with MOSFET drivers
Date: 18 Nov 2002 00:05:47 -0800
References: <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
NNTP-Posting-Date: 18 Nov 2002 08:05:47 GMT
Mike Poulton wrote in message news:...
> On 17 Nov 2002, email@example.com (Adam Seychell) said:
> > That method of control cannot stabilize to the resonant frequency
> > since the magnitude of the current falls off either side of Fo. You
> > must have phase information.
> > I'd look at getting a CMOS 4046 PLL to lock in on the primary current
> > using a current transformer. And limit the VCO to narrow frequency
> > range say 500kHz ~ 700kHz.
> Why can't primary current be used to stabilize the frequency? As long
> as the VCO is limited to a narrow frequency range, it seems like it
> would work just fine. I would probably use a microcontroller to read
> current and adjust frequency, but there's probably an analog way to do
> it as well.
Then explain how your going to find the resonant frequency with
knowing only the averaged magnituded of primary current. Especially
for something as fluctuating as a tesla coil.
> > But first get the gate driving problem fixed and don't even start to
> > worry about feed back or loop control. You may need to totally re-plan
> > your entire driver/transformer/bridge setup. If you want the ultimate
> > gate drive transformer then use 4 windings of RG178 coax over a say
> > 30mm E core. Connect all shields together for primary and each inner
> > conductor go to a FET. I think the only leakage inductance here will
> > be outside connections to the FETs and driver. Put gate resistors
> > also, at least while developing. The only disadvantage is reduction in
> > efficiency of the bridge but that's not your priority at the moment
> > and can be optimised at a latter time. Use 3A SMD schottky diodes
> > across the output of driver to supplies in order to stop reverse
> > currents (they must be close to the drivers or they have little
> > effect). Also dramatically increase your supply coupling from the 100n
> > as I thought you mentioned using in one of you other post. Your
> > driving more capacitance than that, so how can that amount of supply
> > capacitance hold charge ! As I said make sure you read up on all the
> > app notes as they go into all this detail.
> Most recently, I've been using .1uF across the supply -- plus whatever's
> inside that multi-$k lab supply. Should be more than enough, but I'll
> add more anyways. The driver outputs are mosfets, so they contain
> reverse diodes already. I'll ad external ones, just in case. I'll make
> a new transformer from coax -- I have tons of RG178. Looks like it'll
> be a few days before I get to work on it, though, since I just got
> several large assignments with plenty of time pressure.
You havn't read the app notes, have you?. Even a multi-$k lab supply
can easily have few hundrend nH of lead inductance. get yourself
familar with the inductor equation V=Ldi/dt.
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