Most probably you have an oscillation problem!
If what is said is true, you have a power gain of 24dB. Way too much for a
single stage!
Given that the amplification factor of the FET's is very much dependent on
the bias, the sudden current jump smells of oscillations. That's intrinsic
in the design, also you probably didn't pay too much attention to grounding,
which is a key issue with high gain amplifiers.
Any wiring running close to the amplifier HAS to be shielded, even bias and
DC lines and VERY well bypassed at the board's edge.
Start from here and let us know!
Alex 4Z5KS
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of sasas asasas
Sent: Sunday, April 11, 2010 7:19 PM
To: amps@contesting.com
Subject: [Amps] Problem with 500w AM Linear with mosfet IRFP360
Hi. I have construct this linear 500w rms (2kw pep) (50ohm) with 12 mosfet
irfp360, http://tzitzikas.webs.com/linear500w.jpg
schematic.
The manufacturer and other radio amateurs from a greek forum have construct
it successfylly.
The manufacturer tells that the linear requires only 2 watts r.f driving
power for full 500w output r.f
power. The supply is 70-110vdc and for maximum power with 110vdc tha current
is 13A (without am modulation).
Without driving power, we must adjust the current at 200mA (when the linear
is cold).
My problem is this: When i tried to adjust the current at 200mA without
driving power, when the current close to
160mA (by tuning the variable resistance 5Kohm) suddenly the current takes
the value 3A and some smoke comes out
from the input transformer.
some photos from my linear and pll here:
http://www.tzitzikas.webs.com/linear10.JPG
http://www.tzitzikas.webs.com/fet.JPG
http://www.tzitzikas.webs.com/linear11.JPG
http://www.tzitzikas.webs.com/linear12.JPG
http://www.tzitzikas.webs.com/pll10.JPG
http://www.tzitzikas.webs.com/pll11.JPG
http://www.tzitzikas.webs.com/pll12.JPG
http://www.tzitzikas.webs.com/pompos.JPG
With 2w input driving power from my pll and supply at 110vdc, and 150mA
current without input r.f power, the output power was
only 60w and the current was only 3,5A without modulation.
Witch do you think is the problem? Thank you very much and sorry for my
English.
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