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Re: VLF: Class E PA

To: DK7FC <[email protected]>
Subject: Re: VLF: Class E PA
From: Chris Wilson <[email protected]>
Date: Sun, 14 Oct 2018 10:00:26 +0100
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Hello DK7FC,

The biggest voltage and current spikes in my W1VD Class d amp are when
a WSPR or OPERA transmission ceases, from back EMF I presume.

I  am  curious  as tvo whether a "soft voltage start" circuit could be devised
and  added  to  other  power  supplies  similar  to  the one discussed

Sunday, October 14, 2018, 9:30:20 AM, you wrote:

>  Hi Tom, 
>  Good luck with your PA.
>  Tell us the component values when you're finished, as well as the
> actual efficiency. From my LF PA i remember the coil, which was
> quite large already ('air-cored').
>  Regarding current peaks at the first switching: I would simply
> ignore the problem and see if the FET will survive (and how old it
> becomes). If it's lifetime is actually to short, then i would start
> thinking about a solution for the problem. One of them just costs 2
> EUR or so, so it is worth to try...
>  73, Stefan
>  Am 14.10.2018 00:16, schrieb DK1IS: 
> Hello group,
> Just to close this thread:

> Concerning the problem of the first drive pulse with DC power
> already on: Bob, W1XP, made an important contribution in a private
> mail. He pointed out that the current spike I was worried about
> would be limited by the FETs output characteristic anyway. Looking
> to the data sheet of my intended IRFB 4227 this seems to be true
> although there could persist a certain limitation by the save
> operation area. Perhaps I was a little bit too naive estimating the
> peak current as Vdc / Rdson ... Be that as it may, using the newly
> discovered DC soft start feature of my power supply should overcome
> this challenge. Controlled by a logic level input it runs up the
> output voltage with a time constant of about 100 ms. This will
> protect the FET, avoid any key clicks and won´t affect the
> readability of the long lasting vlf signals. To keep the signal
> phase constant (essential on vlf) oscillator and gate drive of the FET won´t 
> be keyed at all.

> I´ve just modified the antenna / loading coil system to resonate at
> 8270 Hz without transformers, - 3dB band width now being 168 Hz.
> Including 25 m RG213U from the arbour to the shack (being later on
> part of the matching network) the feeding impedance is now 480 Ohms
> pure resistive which will be matched to the rest of the class E PA
> by its output network. Thus I`m able to omit the two matching
> transformers being used up to now, see 

> Now I´m waiting for the driver ICs coming from China - in Germany
> up to now without additional tax ;-) !

> Thanks for all the feedback, 73,
> Tom, DK1IS 

Best regards,
 Chris                            mailto:[email protected]

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