Re: IR - Half-Bridge Driver, OLTC-II

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "David Sharpe by way of Terry Fritz <twftesla-at-qwest-dot-net>" <sccr4us-at-erols-dot-com>

Dan, Terry, ALL

Tesla list wrote:
>Original poster: "Mccauley, Daniel H by way of Terry Fritz 
><twftesla-at-qwest-dot-net>" <daniel.h.mccauley-at-lmco-dot-com>
>
>Actually, I was looking at your circuit.
>One of the things I have been experimenting with is instead of increasing
>the input voltage to
>the full-bridge circuit (or half-bridge), you can add a step-up transformer
>at the output
>of the full-bridge instead.  I have been experimenting with a possible 5:1
>step-up transformer with
>very successful results.  With a maximum output of a 120VAC half-rectified
>full bridge circuit of 340 V peak-peak
>(at maximum 60Hz peak), the output would be about 1500 V peak-peak.
-------------------------------------------
This work of adding a "driver transformer" (my nomenclature) was
undertaken by Duane Byland in the early 1990's.  It worked fine and
performance tended to improve based on increase in Ns/Np turns ratio
with input power held constant.  This was the closest approximation to
a solid-state magnifier that has been widely demonstrated and repeated.

The problem with this approach is finding suitable ferrites, bobbins, wire,
impregnation materials, etc. etc.  Bottom line is 80-90% of us do not
have the "in house" capabilities to build a transformer capable  of
withstanding >1-2kV pk to pk.  Yes you could oil immerse, but that
adds another level of complexity.

I found an additional circuit developed by MIT called a multilevel single
phase inverter, that was developed for multiple frequency induction
heating applications.  It is basically a cascaded array of half bridge
"modules" which each module can be independently clocked, which
allows voltage multiplication similarly to a Marx Generator.

The URL 
is: 
<http://web.mit.edu/cmse/www/ar02.leeb.pdf>http://web.mit.edu/cmse/www/AR02.leeb.pdf 

Review carefully pages 51 and 52.  The conceptual circuit is shown at
bottom of page 51 ("Four [n] level inverter with voltage self balancing")

The reported advantages to the circuit are (1,2)
1.  Voltage self balance automatically during operation
2.  Number of stages can be cascaded to any arbitrary level, works
      similarly to a bidirectional Marx Generator.
3.  If you take the IR 2153x driver + totem pole IGBT's, add control
      and local rectifier, filter capacitor, and clock optocoupler between
      adjacent half-bridge "stages", you'd have a modular system that
      could be stacked as high as your pocket book is deep. Signal
      clocking between stages could use high speed optocoupler(s) since
      interstage voltages is only Vapplied (<1kV, most opto's rated to
      2-3kV).  This approach will be simple, robust, and low cost.

This may be the follow on circuit for Terry's original OLTC, that now
may allow voltages of up to perhaps 10kV, off of 300-500V input, with
NO transformer required.  Power output is likewise increased based
on number of active modules .

There are A LOT of cutting edge ideas here, but the big one in my
mind is getting away from FET's, and moving toward a REAL power
device (IGBT), and developing a LOW COST, high performance
modular driver system. The reverse recovery of an IGBT copack diode
is 20-50X faster then the body diode of a FET, and that may allow
operation without the typical schottky + HV fast recovery diode.
The only negative is Fo =<100kHz, but a 5 gal PE pail, wound with
#28 will resonate at ~100KHz, and was used by Bylund in his
original design.

Also notice that there is an enormous amount of similarity between
induction heating hardware and SSTC's.  The drivers are IDENTICAL;
the only difference is you're driving a low resistance load for joule heating
with IH, while with a resonator the load is predominately a varying
impedance load based on streamer loading.  The SGH40N60UFD
IH high performance IGBT's I'm using costs a whopping $3.23 ea
at Newark (and they have >300 in stock...  :^)  ).  I'm confident I
can build a driver capable of at least a 1kW output for <$50, the
active silicon cost is <$10 (ignoring future protective circuits).

I've always had good/excellent results with half-bridge circuits, the
only bridge circuit I ever built was a royal pain in the as...  I understand
Marco's earlier comments concerning difficulties with his SMPS.
-------------------------------------------
>At first I was designing my full-bridge circuit to accomodate 480VAC input
>but with very poor reliability.  However,
>using a 120VAC input and a 5:1 output transformer, I am getting similar
>results to the 480VAC.  Of course,
>the tightly coupled primary/secondary must be EXTREMELY WELL isolated.  I
>currently use PVC pipe and another HDMW polyethlyene sheet for added
>isolation.
>
>My goal as of now is too get a very reliable system capable of continuously
>putting out 3 foot + streamers.  I am getting
>very close.  I'd like to work up to maybe a 2 or 3kV p-p output.
>
>For others who are interested, I am using ferrite U-cores to build my
>transformers with.  They are available at
>ferroxcube-dot-com
>
>Dan
><SNIPPO>