I see you are using hex socket screws. It is very easy to over-tighten screws
to fix plastic devices to a heatsink, I've seen them crack after a little time
in the field due to expansion and contraction against an over-tight fixing.
The crack can be invisible to the naked eye initially but causes failure of the
device. Mounting torque is 1.13Nm which is quite low.
The mounting tab is the drain and you could look at the heatsink isolation. Is
there an isolation insulator under each device or is the whole heatsink
isolated?
I could be drifting off the subject, but fixing small errors can often chase
away the big ones.
David
G3UNA
----- Original Message -----
From: sasas asasas
To: amps@contesting.com ; David Cutter
Sent: Sunday, April 18, 2010 10:41 AM
Subject: Re: [Amps] 160m mosfet linear amplifier problem with ferrite cores
Some additional photos from my linear:
http://tzitzikas.webs.com/fet20.JPG
http://tzitzikas.webs.com/linear20.JPG
http://tzitzikas.webs.com/linear21.JPG
http://tzitzikas.webs.com/linear22.JPG
http://tzitzikas.webs.com/fet21.JPG
http://tzitzikas.webs.com/pompos20.JPG
--- On Sun, 4/18/10, David Cutter <d.cutter@ntlworld.com> wrote:
From: David Cutter <d.cutter@ntlworld.com>
Subject: Re: [Amps] 160m mosfet linear amplifier problem with ferrite
cores
To: "sasas asasas" <tzitzikas_ee@yahoo.com>, amps@contesting.com
Date: Sunday, April 18, 2010, 2:35 AM
I've looked again at the jpg and I think you have used trifilar
windings, so, that's my first idea more or less gone. Imbalance can be due to
poor coupling between the windings themselves or imbalance in the driving
currents. With high voltage supplies, balance error should be small (compared
to say a 12V amplifier where a small difference in voltages can show a large
imbalance %). If you could measure the current in each half of the primary
that would tell you the whole story: you will need a HF current probe,
preferably 2 probes and dual trace scope which will also tell you about
cross-conduction.
Another thought: you have 6 transistors in parallel in each half and
emitter (sorry - source) degeneration. Did you try matching the FETs for gain
or phase delay? If not, it's conceivable that one side is conducting harder or
for longer than the other side and causing an imbalance. I don't know if the
source resistor is the optimum value for mis-matched FETs, but a higher value
would increase negative feedback and help with this; you have oodles of gain to
sacrifice. Along the same lines, the input drive to each FET may need
optimising for the same reason. Check all your resistor values, one might be a
dud.
This is all armchair speculation, I don't have enough experience for
better detail. Someone with modelling experience could probably do this quite
quickly.
Alex has probably got the answer regarding core material.
David
G3UNA
----- Original
And how can i repair this problem???
You might consider imbalance in each side of the primary
which will cause a net dc to pass, saturation and over-heating. This may be
rubbish but it could get others thinking along a different path.
David
G3UNA
> HI. i have constructed this linear amplifier 500w rms
(2kw pep) (50ohm) with 12 mosfet irfp360,
http://tzitzikas.webs.com/linear500w.jpg for 160m band.
> When i gave 3watts of driving r.f power, it gave to
output only 190w at 106VDC (6A current). Two radio amateurs who have construct
this linear claim tha it gives 500w
> r.f power at 110vdc.
> But when i tried to give 4watts of driving r.f power the
ferrite Cores (43 material)
> of transformer T3 broken! Which do you think is the
problem??
>
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