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Author Topic: Problem with 500w AM Linear with mosfet IRFP360  (Read 15605 times)
1431
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Posts: 18




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« on: April 11, 2010, 02:32:00 AM »

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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W8JI
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« Reply #1 on: April 11, 2010, 03:36:01 AM »

Your construction is terrible for radio frequencies.

You have long thin leads to the FET's, no groundplane. It is a very poor layout.
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TANAKASAN
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Posts: 933




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« Reply #2 on: April 11, 2010, 03:44:00 AM »

The style of construction is interesting but I am not sure that the design is stable enough for amateur band operation. Your problem could be due to the amplifier oscillating when you turn up the bias. If I was building this amplifier (and I'm not going to) I would have used a groundplane on the PCB with the FETs on two opposite edges using very short connections. For a good example of this construction technique see http://sites.google.com/site/ik0gmm/100w-mosfet-ampli

I would also change the circuit around the bias regulator, using a 6.8K resistor and a 12V 1W zener diode is NOT a safe way to drop from 110V to 12V.

This thread could get interesting in many different ways so before we go on you need to answer a quick question:

Which amateur band(s) is this 500W AM linear supposed to work on? I ask this because I cannot see the size and value of the output filter components?

Tanakasan
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1431
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Posts: 18




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« Reply #3 on: April 11, 2010, 04:09:26 AM »

thank you for your answer. I would like to ask you if the linear can oscillate, without driving power, because tha connections are not sort.
This linear is for 160m but it can also works at lower frquences (medium waves band).
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TANAKASAN
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« Reply #4 on: April 11, 2010, 04:36:09 AM »

"I would like to ask you if the linear can oscillate, without driving power"

Yes, very easily. Most amplifiers make wonderful high power oscillators and causes include:

Incorrect input impedance
Incorrect input reactance (this is why many amplifiers include a resistive attenuator on the input)
Incorrect output impedance
Reactive load on the output including some designs of low-pass filter
Too much bias
Mistakes in construction including phasing of feedback windings
Insufficient power supplies
Inductive components which are supposed to be non-inductive

Building a stable HF amplifier can be a challenge.

Tanakasan
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TANAKASAN
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« Reply #5 on: April 11, 2010, 04:37:51 AM »

Tom, comments please on the schematic, in particular the bias regulator.

Tanakasan
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1431
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Posts: 18




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« Reply #6 on: April 11, 2010, 04:50:48 AM »

been constructed exactly by using the constructer informations (with ferrites material 43). here is the prototype http://www.tzitzikas.webs.com/linear_first.jpg
i would like to ask you, if i put short connections and the problem of oscillation continues what is the next step to solve my problem?
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KF6QEX
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« Reply #7 on: April 11, 2010, 05:15:38 AM »

Quote
The manufacturer and other radio amateurs from a greek forum have construct it successfylly.

I'm curious to see the contruction layout of one of the other working amps.

They layout should look a lot more more like this :
http://frenning.dk/OZ1PIF_HOMEPAGE/50MHz_IRF510.htm

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1431
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Posts: 18




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« Reply #8 on: April 11, 2010, 05:16:25 AM »

been constructed exactly by using the constructer informations (with ferrites material 43). here is the prototype http://www.tzitzikas.webs.com/linear_first.jpg
i would like to ask you, if i put short connections and the problem of oscillation continues what is the next step to solve my problem?
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1431
Member

Posts: 18




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« Reply #9 on: April 11, 2010, 05:24:10 AM »

KF6QEX the follwing photos are from the same linear, which have been constructed by other greek radio amateur who claims that is working perfectly.

http://www.tzitzikas.webs.com/linear_working3.jpg
http://www.tzitzikas.webs.com/linear_working1.jpg
http://www.tzitzikas.webs.com/linear_working2.jpg
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KF6QEX
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Posts: 604




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« Reply #10 on: April 11, 2010, 06:05:39 AM »

Cleanup the layout first.
From the picture of the prototype although it's not completely clear, it looks a lot neater.
Read of the 50MHz amp construction article.
If I were you I would be very tempted to use the board layout of the 50MHz amp. Smiley
There is note there that the parasitic oscillation issue was solved adding 4.7 Ohm resistor at each gate. 



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1431
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« Reply #11 on: April 11, 2010, 06:20:23 AM »

KF6QEX wrote: <<There is note there that the parasitic oscillation issue was solved adding 4.7 Ohm resistor at each gate.>>

Look at the schematic again: http://tzitzikas.webs.com/linear500w.jpg

Each gate has a 12ohm resistor. Do you propose to change it with 4,7ohm resistor?
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KF6QEX
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« Reply #12 on: April 11, 2010, 06:36:20 AM »

Leave the existing resistor alone . Add the 4.7 as close to the Gate as you can like here :
http://frenning.dk/OZ1PIF_HOMEPAGE/images/4r7_resistors-lille.jpg


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1431
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Posts: 18




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« Reply #13 on: April 11, 2010, 08:21:31 AM »

i have 12ohm resistor very close to gate.look the photo:

http://www.tzitzikas.webs.com/fet2.JPG

what do you mean to do with 4,7ohm resistors? i don't understand. thank you
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1431
Member

Posts: 18




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« Reply #14 on: April 11, 2010, 08:27:11 AM »

KF6QEX:

i have 12ohm resistor very close to gate.look at the photo:

http://www.tzitzikas.webs.com/fet2.JPG

what do you mean to do with 4,7ohm resistors? i don't understand. do you mean to add an extra 4,7ohm resistor in series with the existing 12ohm resistor very close to gate?(total 16,7ohm in series with gate).thank you
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