Re: solid-state coils & FETs

["Kennan C Herrick" <kcha1-at-xxxxxxxx> (by way of Terry Fritz <twftesla-at-xxxxxxxxxx>)]

On Mon, 06 Nov 2000 18:38:07 -0700 "Tesla list" <tesla-at-pupman-dot-com>
writes:
> Original poster: "Steve" <Steve-at-g8cyerichmond.freeserve.co.uk> 
> 
> I'm looking towards building my next solid state coil. The current 
> one runs at
> about 100W. What I'm looking for is some higher rated FETs. The 
> circuit I am
> using is basically a class D amplifier. The drive circuit is no 
> problem. Now
> I'm aiming for a couple of kW input. The devices will obviously need 
> low on
> resistance. DC input is 350V at up to 10A. Has anyone got any ideas 
> about
> devices that might be suitable? I've  looked at some data sheets, 
> but so far no
> luck, in each case I've either been outside or very close to the 
> edge of the
> safe envelope.
> 
Hi Steve-

I'm using IRFP460LCs in a "current ring" primary design.  Since you have
350V to work with, you would require higher-voltage MOSFETs.  In a
current pupman posting, Marco Denicolai <Marco.Denicolai-at-tellabs.fi>
writes, "For the Thor SMPS I have been using the IRG4PH40UD IGBT from
International Rectifier. It will work up to 1200V 30A continuous. Farnell
sells it."  I don't know that IGBTs would work too well much above 100
KHz, but maybe they're now improved.  You'd want turn-on & turn-off times
not exceeding 1/2 us or so for 100 KHz operation.

Here are my current spec's:

Solid-state Tesla Coil Design in Progress:

1.  Input, 117 VAC, 60 Hz, 0-1800 W.

2.  Physical size, 2 ft. square x 5 ft. tall.

3.  Primary design:

3.1  One equivalent untuned electrical turn driven by power MOSFETs.

3.2  Present configuration, 24 pairs of 500 V, 85 A MOSFETs in a patented
circuit (U.S. #6,069,413 which you could find at
http://www.patents.ibm-dot-com/) delivering ~450 A through the 1 equivalent
primary turn.  When fully configured, 36 pairs are to deliver ~670 A. 
There are 6 pairs of MOSFETs plus associated energy-storage capacitors
per printed circuit board.  With each such board is associated an
additional circuit board holding the MOSFET drivers and an off-line
switching-current-source to both charge the capacitors and keep line
current limited to a selected maximum.

4.  Secondary design:

4.1  12.5” coil diameter x 52” height.

4.2  Approx. 1100 turns of 20 ga. wire space-wound.  Q = 80-100 -at- ~100
KHz resonance.  See http://www.pupman-dot-com Tesla-postings of 8/19/00 and
8/20/00, HERRICK’S RECIPE FOR A SPACE-WOUND SECONDARY.

4.3  Top toroid, 4” x 14” corrugated aluminum duct, soon to be replaced
with a 6” x 24” commercially-produced toroid, an item of Jim DeLillo's
bulk-buy (jimdel-at-bellatlantic-dot-net).

5.  Features:

5.1  Highest voltage to ground ever present anywhere in primary circuit,
160 V.  Highest MOSFET turn-off transient, n.g.t. ~400 V.

5.2  Self-tuned:  Secondary is the sole resonant element in a feedback
oscillator incorporating the power MOSFETs.  Always dynamically tuned.

5.3  Spark rate continuously variable from 1 per button-press to 10/sec
or more depending on power-line current capability and spark duration.

5.4  Controlled via a small “wand” at the end of a 15 ft. cable.  Wand
incorporates a collapsible ground rod for optionally inducing sparks.

5.5  Spark-length to the ground rod with the previous 24-MOSFET
configuration, ~24”; 48-MOSFET configuration still under test with ~48”
expected.

5.6  Present spark duration, ~6 ms, selectable.  <300 us
secondary-voltage rise time to commencement of spark.

6.  Dissemination of plans:

6.1  I plan to offer my construction plans, for a modest dollar amount
and for a design based on my current one, as soon as I can; I hope within
the next six months.

Regards,

Ken Herrick
Oakland, CA USA 

________________________________________________________________
YOU'RE PAYING TOO MUCH FOR THE INTERNET!
Juno now offers FREE Internet Access!
Try it today - there's no risk!  For your FREE software, visit:
http://dl.www.juno-dot-com/get/tagj.