Dear Andy, David, Petr, Rik, LF Group,
The static gate capacitance of each STW34NB20 is over 2000pF,
and to switch 4 devices in a small fraction of a microsecond
requires several amps peak, which is quite a tall order for most
gate driver chips. There are some very powerful driver ICs around,
but I did not have any to hand at the time. The discrete booster
uses a TC4427 with the addition of 2 small MOSFETs, 2 diodes
and a resistor. It is probably not optimised yet , but is simple and
does the job. The present limiting factor is the driver transformer
leakage inductance, which limits the rise time at the output
MOSFET gates, and causes some ringing also.
I expect it would be possible to apply G3YXM's keying method, but
this would increase the stress on the output devices, and I have
not tried to evaluate the effect this might have - since I already had
a modulator, it was not required.
The TX is very much (almost literally!) in the breadboard phase at
the moment, and I will be trying some different ideas with it in the
near future, hopefully.
Cheers, Jim Moritz
73 de M0BMU