hi. i am not use insulators between mosfet and heatsink in order to have the
minimum thermal resistance. I am using only silicon thermal conductive paste to
decrease the thermal resistance. So, the heatsink is connected with the drain
of each mosfet and the heatsink has the drain voltage.
I am using 12 Siliconix's IRFP360 mosfets, but i have order and soon i will
replace them with International Rectifier IRFP360 mosfets. Propably this is the
reason, that the amplifier don't give many watts at outupt.
The other reason, is that the material of ferrite cores (43 material) is not
the proper for 1-2mhz frequences. What do you think??
--- 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, 3:50 AM
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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