Re: Pushing the IGBT Envelop

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

Original poster: "Greg Leyh" <lod-at-pacbell-dot-net> 


>
>Original poster: "Steve Ward" <steve.ward-at-gmail-dot-com>
>Comments:
>
>
>On Sun, 12 Sep 2004 16:33:26 -0600, Tesla list <tesla-at-pupman-dot-com> wrote:
> > Original poster: "Greg Leyh" <lod-at-pacbell-dot-net>
>[snip]
> > H-bridge drives are great for power converters to 100's of kW, and
> > especially when operation from a unipolar DC source is required.
> > However, an H-Bridge plan wouldn't be cost-effective for the ALF since it
> > requires 4X the silicon for the same primary peak current and voltage swing
>
>Hmm, what would be the alternative?  I sort of had the idea that you
>were going for an OLTC-like scheme, where you charged the tank cap and
>dumped them into the primary like a normal TC, but replaced the spark
>gap with IGBTs or some other thyristor.  Its interesting to make a
>very crude comparison between my DRSSTC-2 and Steve Conners OLTC-2.
>We are both using about the same amount of silicon (i use 4x 300A
>IGBTs, while he uses 2x 600A IGBTs).  We both run just about the same
>input power (around 4kW).  His coil makes a 7' spark, while mine does
>11' .


It's rather impressive just on general principles to produce an 11ft arc 
using 600V mains!
It implies either a lot of voltage multiplication or very precise arc 
creation and maintenence.
I'm not exactly certain what an 'OLTC' entails, but essentially the ALF 
would employ a rather
straightforward capacitor -> series-switch -> primary arrangement, which 
would exhibit
primarily  I2R conduction losses since all switching events would be at 
zero current.
The trickier issues lie in charging the capacitors from the mains, and 
managing the energy properly
as it bounces back and forth between primary and secondary.


>But another big question is, would an OLTC become more efficient at
>higher voltages?  Say if it was possible to get IGBT stacks up to
>15-30kv range?  On the other hand, the DRSSTC seems to work well in
>all cases, even with much lower input voltage. I run 600VDC on my coil
>while Conner uses 1kv if i remember correctly.


True, the efficiency will increase with rising input voltage, due to the 
quickly falling I2R losses.
However at some point the loss improvements become marginal, and the high 
voltage issues start to
rear their ugly head.  The final choice must be dependent on efficiency, 
corona issues, switch
capabilities, total cost, etc.  For the 1:12 ALF prototype I've settled on 
~2400V -at- 2800A peak,
representing a compromise between I2R losses and device ratings.