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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