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LF: <Tech>Re:73kHz TX

To: [email protected]
Subject: LF: <Tech>Re:73kHz TX
From: "James Moritz" <[email protected]>
Date: Wed, 13 Jun 2001 12:00:30 +0000
In-reply-to: <[email protected]>
Organization: University of Hertfordshire
Reply-to: [email protected]
Sender: <[email protected]>
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

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