Hi Scott I think Jim M0BMU is the guy you need to discuss this with but from
my experiments (mainly with class E single ended) 200V working FETs might be
ok at about 36 V volts but would certainly be marginal at 48. the drain
volts can rise to 4 times the dc supply voltage on peaks. Add in a little
"off-tune" and it can be higher. I have 500 volt devices pencilled in for
the 50 volt supply. I do find a "click" from my feed choke when the key goes
down.... there is a physical movement of the wires round the core. (that is
36 V and about 5 amps) even into a resistive dummy load. I suppose the only
way to soft keying would be to "PWM" the drive during keying. I believe that
if you use the keying / drive circuit used by Dave G0MRF and also G3YXM with
the TI fet driver chip (TC4426).....this does soften the edges a bit in this
way.
Cheers de Alan G3NYK
----- Original Message -----
From: "Scott Tilley" <[email protected]>
To: <[email protected]>
Cc: <[email protected]>
Sent: 17 July 2004 21:31
Subject: LF: clicking matching transformer...
Any thoughts on if a matching transformer should 'click' when the key is
released... Obviously it is likely an inductive reaction to the rapid change
in current supplied... When I connect the 48V supply to the system the
finals don't last long... OK at 12 and 24V so I feel it is a inductive
kickback that's blasting the 200V rated FETs... The system works great into
a 6 ohm load driver pushpull without the match xfmr... It must be the
inductive kickback...
Perhaps the inductance of the windings it too great?
Damn physics... To quote a power engineer friend of mine "it's a damn shame
that FETs behave the way they do, otherwise they would be the perfect
device..."
Class E time?
Check the status of radio beacon 'TIL' at:
www.scottboxx.com/radio
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