Dear Chris,
I use four IRF520 FET's in parallel push pull class B in my
transmitter. The FET's are run at 40 V. The gates are driven via a
transformer. At first I suffered from blown FET's till I found that was
due to parasitic VHF or UHF oscillations. It was cured by inserting
stopper resistors of 120 ohm directly at the gates. The circuit, with the
modification, can be found at page 58 of G3XDV's book LF
Today.
After the modification at irregular intervals one of the
FET's became a short circuit. This was found to be caused by
insufficient cooling of the heatsink. Replacing the small fan by a
larger one cured it. The TX can now be run during the whole night
without any problem.
73, Dick, PA0SE
At 12:40 28-9-05, you wrote:
Thanks for the advice
Jeff,
All points noted.
I must confess that all of the published designs seem to use a logic
driver
IC.
I must be being awkward using an inductor :-(
Maybe I'd better stick to a proven design..
I'll take a photo soon and risk ridicule - HI!
73 Chris G3XIZ
-----Original Message-----
From: F6CWN
[mailto:[email protected]]
Sent: 28 September 2005 10:21:AM
To: [email protected]
Subject: LF: Re: Advice will be gratefully received
Hello Chris,
At first,"RIP poor MOS"!
I prefer use MOS with higher VDS.
The driver must absorb energy, because the Ciss capacitance, i use TSC426
or
TC4429 driver.
It sis better to separate the cooling radiators to minimize the
reaction
between the two parts
of the bridge (capacity Drain / Ground, the ground is
"hot").
A problem with reactive loading, the coils of the LPF are
reversible.
A little photography of your PA can help...
73: Jeff / F6CWN/p
----- Original Message -----
From: "OSBORN, Chris"
<[email protected]>
To: "LF Group (E-mail)"
<[email protected]>
Sent: Wednesday, September 28, 2005 10:43 AM
Subject: LF: Advice will be gratefully received
>
>
>
> 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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