Re: [TCML] A Question about Solid State Drivers

[Greg Morris <gbmorris@xxxxxxxxx>]

Hi Steve,

Thanks for the advice!

The reason I mentioned such a high "bandwidth" is because I intend to music
modulate the coil. The coil will be nominally operating at around 200kHz,
but I want to be able to modulate it down to a 10% duty cycle, and up to a
90% duty cycle (as well as 0% and 100%, of course, but not 95%, for
example). And so for a 10% duty cycle (or 90%) at 200kHz, the system needs
to be able to operate at a switching frequency of 10*200kHz = 2MHz.

FYI: For the last stage of the digital controller, I plan to use the
digital-out lines on a 40MHz PIC, which I think should work, as long as the
code is extremely streamlined. For the first stage, however, I have some
ideas about using an AC'97 codec, but I'm not clear on the details. Has
anyone tried using anything like that? Any words of wisdom you would like to
pass on?

The intermediate stage of the digital controller is a both more involved,
and will involve interfacing with the user, etc.

Cheers,
Greg

2009/12/6 Steve Ward <steve.ward@xxxxxxxxx>

> Greg,
>
> The transformer is a "gate drive transformer".  Its construction is such
> that it has exceptionally good high frequency response (i wont say high
> bandwidth because it only works down to 10's of kHz generally, this is
> design dependent of course).  Anyway, the combination of a ferrite core and
> inter-twining of the windings gives very low leakage inductance, which is
> what limits response time (self C does too, but its small compared to the
> load C).  Make sure you fully understand the distinction between a
> transformer's magnetizing inductance and leakage inductance.  Despite the
> fact that the magnetizing inductance is large, it does NOT impact the high
> frequency response of the signal being transmitted (it does slow down the
> magnetizing current, which is un-related).
>
> By 2MHZ are you intending to switch at 2MHZ, or do you really only want
> 2MHZ
> for your rising/falling edges?  Careful about the relationship between a
> "square" wave and its frequencies...
>
> If you want a square wave with an Fo of 2MHz and you want it to look
> "pretty
> square" you probably need something thats responsive to 20MHz or better.
>  So
> yes, this should be constructed on a small ferrite ring, something with
> permeability in the 2000 to 5000 range and probably no more than 1" OD.
>  Use
> few turns, maybe only 3 or 4 turns.  This is a trade off between high
> magnetizing current, but gains you low leakage inductance.  The external
> wiring between driver, transformer and switch gates needs to be kept very
> short.  Use twisted pair, or even coaxial cable if the wires *must* be
> long.  If this is your first SSTC, id suggest aiming for a lower frequency
> first because a lot of tricky problems come up with very high frequency
> solid state drives.
>
> Steve
>
> On Sun, Dec 6, 2009 at 7:03 PM, Greg Morris <gbmorris@xxxxxxxxx> wrote:
>
> > I've been looking into interfacing a low-voltage, low-current control
> > circuit with the relatively high-current power circuit of an SSTC. A lot
> of
> > schematics I've found:
> > http://www.angelfire.com/electronic/cwillis/fetcoil2.html
> > http://www.richieburnett.co.uk/sstc02.gif
> > http://www.alansharp.co.uk/page6.htm
> > etc.
> >
> > seem to use signal transformers between the controller and the power
> > circuit, but it seems like you would need an incredibly small inductance
> to
> > be able to get a decent signal at Tesla Coil frequencies, especially when
> > you consider that the secondary of the signal transformer is in series
> with
> > the gate of a power MOSFET or IGBT, which together act as a pretty
> > effective
> > low-pass filter. Am I missing something? What do you folks use? I am
> trying
> > to find a solution with a bandwidth in the area of 2MHz. If you have any
> > suggestions, I would love to hear them.
> >
> > Thanks!
> > Greg
> >
> > --
> > Greg Weyrich Morris
> > VP External - Engineering Undergraduate Society
> > B.Sc. Electrical Engineering (Final Year)
> > University of New Brunswick
> > G.B.Morris@xxxxxx
> > www.unb.ca/robotics
> > _______________________________________________
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> >
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-- 
Greg Weyrich Morris
VP External - Engineering Undergraduate Society
B.Sc. Electrical Engineering (Final Year)
University of New Brunswick
G.B.Morris@xxxxxx
www.unb.ca/robotics
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