Re: fullbr.SSTC air core pri turns?

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Jan,

On 26 Oct 2001, at 8:01, Tesla list wrote:

> Original poster: "Jan Florian Wagner by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>
> 
> Hi,
> 
> about determining the initial turns on the primary coil for a full-bridge
> with a direct helical TC primary coil drive: is there any constraint that
> gives the minimum number of turns on the primary? 

As always, your main concern is not turns but inductance.
 
> I must say I feel a bit uncomfortable about driving so few uH's at 100kHz
> from 640VppAC - I mean, seems like it'd be destined to fry...?  :o)
> Especially when the secondary is not in place.

How many is "so few"? Suppose Lp is 80uH and has 320V placed across 
it for 5uS. Current ramps up to hit 20A. That is all reactive however 
and under these conditions, the switches don't see a load as well. 
That changes when you add the secondary. 
 
> Btw, is there any reason why one shouldn't place the primary inside the
> secondary coil former? 

Ironically, Tesla developed the classic TC design to move the primary 
outside the secondary as he observed nasties occurring in one of his 
early coils. But it can be done and was done by Greg Leyh in his 
Electrum design. It has been done by others. 

> Just thinking that insulation wouldn't be an issue then, and as a bonus
> it'd give a bit less stray inductance, AFAIK...?

Insulation is always an issue. There is nothing wrong with stray 
inductance as long as it isn't coupled to anything else that can 
dissipate power.

Regards,
malcolm