Paul
Ringing of FETS is an Early Warning sign for a Big Bang
There should be no ringing or singing.
g3kev
-----Original Message-----
From: N1BUG
Sent: Friday, November 30, 2018 8:50 PM
To: [email protected]
Subject: Re: [Lowfer] LF: Re: FET change, hmmm
All I know is this amp had very little drain ringing and worked well
with FQA34N20 FETs but it is not happy with FQA32N20.
I slowly increased drain voltage but at 200W out (of a kilowatt
deck), there were obvious issues related to the ringing.
I've gone back to my old faithful 200W output Class E amp for now. I
am running WSPR-2 33% 137.437.
Paul
On 11/30/18 3:25 PM, Andy Talbot wrote:
Cross conduction spikes can also be dealt with by inserting a bit of
inductance between top and bottom FETs of a totem pole, or in each leg of
a
push-pull pair. As done in the old Decca transmitters, and I cribbed for
my 700 Watt 137 design.
The inductor absorbs the short spike and returns the energy via clamp
diodes to the supply
See http://www.g4jnt.com/137tx.pdf
A lot simpler than worrying about generating a special drive waveform with
gaps
Andy
www.g4jnt.com
On Fri, 30 Nov 2018 at 20:19, Hans-Albrecht Haffa <[email protected]> wrote:
Dear OMs, dear Paul,
To prevend current spikes by cross conduction of both fets it is necesary
to use a gate drive circuit with dead time, as used in power supplies.
But
in this case there is a no current flow time in both transistors, what
may
cause spikes in the drain circuit. Damping of oscillations in the drain
circuit may also reached by quiescent current, not used for class D PAs.
To
load the spikes to the load is not possible in an LW-PA, like in an
flyback-transformer of an switch mode power supply, as the load is an
LP-Filter that filters by reflections of the applied voltage.
|
