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From: <g3kevmal@talktalk.net>
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Date: Sat, 1 Dec 2018 15:47:46 -0000
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Subject: Re: [Lowfer] LF: Re: FET change, hmmm
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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: rsgb_lf_group@blacksheep.org
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 <dk8nd@web.de> 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.