Re: MOSFET oscillators

["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 Greg,

On 12 Nov 2002, at 7:22, Tesla list wrote:

 > Original poster: "Gregory Peters by way of Terry Fritz 
<twftesla-at-qwest-dot-net>" <s371034-at-student.uq.edu.au>
 >
 > Jan,
 >
 >  >
 >  > Hmm, interesting idea... :) You probably just have to place an
 >  > additional
 >  > series resistor, 1kOhm? or less, between the mosfet gate
 >  > (ex-NPN-base) and
 >  > feedback winding, and a beefy 15V zener between gate and
 >  > source, pointing
 >  > towards the gate.
 >  >
 >
 > This is exactly what I suspected, but it seemed so simple it wouldn't
 > work. I thought there had to be some reason people hadn't tried it. I
 > would love to try this method to make a "regular" air core SSTC, with
 > many FETs in parallel and 600v input (1.2kV fets). This would
 > essentially have the same topology as a VTTC, using a feedback
 > winding. What do you think?

I tried it in the past but it's so long ago, I've forgotten the
details. That inverter relies on core saturation for its operation.
As the core saturates, coupling between the primary and feedback
winding falls off and starts to reduce base current which in turn
causes the transistor to come out of saturation reinforcing the drop
in base current. As the transistor starts to turn on, it boosts its
base current by transformer action. It's a less civilised circuit
than a pushpull one because the windings are effectively open-circuit
during turn-off and the collector spikes resulting from energy stored
in the leakage inductances can be pretty high. I don't see any reason
why it couldn't be made to work OK with a MOSFET but running it with
air cored coils means a fundamental change in the nature of operation.

Regards,
Malcolm