Message-ID: <59033D72.80705@posteo.de>
Date: Fri, 28 Apr 2017 15:02:42 +0200
From: DK7FC <selberdenken@posteo.de>
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To: rsgb_lf_group@blacksheep.org
References: <1106627522.20170427150315@gmail.com> <5901FD30.5010902@posteo.de> <1907129228.20170428123626@gmail.com>
In-Reply-To: <1907129228.20170428123626@gmail.com>
Subject: Re: LF: Varistor spike management
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Hello Chris,

These FETs are a good choice :-) You still manage to make them dead? And 
your supply voltage is 50V? That's not ideal. You can use 320 V directly 
from the mains, no heavy transformer needed, just a ferrite transformer 
on the RF side, to decouple the RF signal from the mains.
I was running > 1.5 kW on 137 kHz (WSPR-15, no problem, not even in 100% 
TX mode) with just 4x IRFP460A, which are not as good as yours. So i 
don't really understand why you need a combiner at all. The 900V types 
would be fine for a 3-phase PA running at 560 V DC in a H bridge 
arrangement. A nice thought :-)

73, Stefan


Am 28.04.2017 13:36, schrieb Chris Wilson:
>   28th May 2017
>
> Hi Stefan, remarkable work on VLF, congratulations!!
>
> I  see these with all FET's but I acquired very cheaply some Wolfspeed
> FETs  with a lot of overhead. These spikes sometimes kill lesser ones.
> If  you  follow  these  links  you  can  see the normal gate and drain
> waveforms,  and  the  drain waveform without a Varistor as an OPERA TX
> period  ends,  i  took the latter from a series of screenshots showing
> the highest level the close TX spike reaches
>
> http://www.gatesgarth.com/wolfspeed-scope-capture-50v.jpg
>
> http://www.gatesgarth.com/wolfspeed-overshoot-50v.jpg
>
> http://www.gatesgarth.com/wolfspeed.pdf
>
>
>
>
>
> Best regards,
>   Chris  2E0ILY                          mailto:chris@gatesgarth.com
>
>   My part time LF grabber is at http://www.chriswilson.tv/grabber.html
>
>
>    
>> Hi Chris,
>>      
>    
>> Which type of FETs are you using?
>> The IRFP460A for example has a built in Z-diode which seems to handle
>> the spikes. Anyway i put a 150 Ohm / 220 pF (for LF) series RC paralell
>> to the Drain-Source...
>>      
>    
>> 73, Stefan
>>      
>
>