Envelope-to: dave@picks.force9.co.uk
Delivery-date: Wed, 28 Sep 2005 11:18:36 +0100
Message-Id: <5.1.0.14.0.20050928121658.02fc4360@u0019445.kuleuven.be>
Date: Wed, 28 Sep 2005 12:21:26 +0200
To: rsgb_lf_group@blacksheep.org
From: Rik Strobbe <rik.strobbe@fys.kuleuven.be>
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 strium.corp>
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Subject: Re: LF: Advice will be gratefully received
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Chris,

if the load differs from 50 Ohm (and especially if ot has a reactive 
component) the drain voltage can easily sing to 5 times the power supply 
voltage.
I would recommend to use FET's with a higher Vds.
I have been using IRFP350's and IRFP450's in my 450 W PA running at 55V 
supply voltage and they survived all kinds of misthreatment.

73, Rik ON7YD

At 10:43 28/09/2005 +0100, you wrote:


>         The PA FET's in my new home brew transmitter keep exploding !
>
>         I'm using push-pull  IRF520's rated at 100 V Vds and my supply is 30
>volts.
>         I am careful not to exceed the V gs.
>
>         The gates are driven with a toroidal transformer and the whole
>circuit is just a scaled up version of my original TX, which works fine.
>         It generates healthy power into a 50 ohm dummy load but on
>connection to the aerial circuit - bang !
>         The aerial system is identical to that used with the old TX !
>
>         Has anyone any ideas on what's causing the problem and how to cure
>it ?
>
>         I'm running out of FET's and my nerves are somewhat frayed (shell
>shock!)
>
>         73 Chris G3XIZ
>
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