Re: [TCML] A Question about Solid State Drivers

[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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