Problem with 500w AM Linear with mosfet IRFP360

[tzitzikas]

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.

 

[ge]

Assuming you have measured with a ohm meter that nothing is shorted out check your impedance (coils/inductors). A possibilities that come to mind, blown (shorted) transistor. Is this a proven layout?

 

[tzitzikas]

hi. This linear have been successfully constructed by at least 2 radio amateurs. look the following photos:

1) prototype http://www.tzitzikas.webs.com/linear_first.jpg

2) other radio amateur:

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

 

[ge]

That should rule out coupling. Although another possibility is source/load match, my bet would be an inductor with wrong turns, different core/coax on inductors or blown transistor. I've cooked off a few transistors.

 

[vfone]

To get 500W (60dBm) from a 2W (33dBm) input, requires a lot of gain, which is unusual for a single stage power amplifier (even there are a lot of transistors in parallel). Most probably your circuit oscillates.

 

[tzitzikas]

i sort all connections and i think the problem with oscillation stop. I adjust the current at 200mA without driving power, and after with 3w input driving power with 106vdc power supply the output was 200w. The current was 6A. I can't understand what is the problem and my linear gives only 200w. The manufacture claims that with 110v and 2w driving power the linear gives 500w output R.F power. Do you have to propose any change to give the linear more output power? Maybe in R.F transformers? (to change the number of turns?). Thank you very much

new photos

https://tzitzikas.webs.com/fet20.JPG
https://tzitzikas.webs.com/linear20.JPG
https://tzitzikas.webs.com/linear21.JPG
https://tzitzikas.webs.com/linear22.JPG
https://tzitzikas.webs.com/fet21.JPG
https://tzitzikas.webs.com/pompos20.JPG

 

[vfone]

From the pictures I see that you are using thinner supply wires than the working PA. At 6A (or maybe more at the peak) you can get voltage drop. Also the output transformer I think is too small (or improper ferrite core) for 500W output.
Do you use a calibrated power meter and a calibrated 50 ohms load to measure output power? At this high power a home made coupler/diode detector can easily get errors.

 

[tzitzikas]

thank you for your answer. the power supply cable is 1.5mm2. I think is enought for 12-13A current.
The manufacter and other radio amateurs (who have 500w output power with 2w input r.f power) have use 4 cores for output transformer (i use 6 cores). the ferrite core is 43 material that the manufacter suggested.
see the following pictures from their linears:
http://www.tzitzikas.webs.com/linear_first.jpg

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

the swr-power metes is calibrated and tested, and we use dummy load 1kw for the test.

how can i test if a mosfet is not good? all mosfet gives 500ohm resistance between darin-source.

 

[tzitzikas]

Hi. the mosfet that i am using (IRFP360) are no name. Do you believe that the linear gives less power than the prototype because the mosfet are no name instead of the prototype which is using International Rectifier mosfet?

 

[vfone]

Yes, will make a big difference using the original International Rectifier MOSFETs.
Beware that the drive level is very important for this kind of high-power amplifier to get proper efficiency and output power.
You have to drive the MOSFETs just below the gate-rupture point.
I think 2W input power is too low, and you have to increase it up to 5W.

 

[tzitzikas]

https://tzitzikas.webs.com/linear500w.jpg
i sorted all connections.
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??

some pictures
https://tzitzikas.webs.com/linear20a.JPG
https://tzitzikas.webs.com/ferrite_broken1.JPG
https://tzitzikas.webs.com/ferrite_broken2.JPG

the feerite cores that i use are:
https://uk.farnell.com/fair-rite/2643626402/ferritecore-196ohm/dp/1463423
from manufacter:
https://www.fair-rite.com/cgibin/ca...ound+Cable+EMI+Suppression+Cores#select:freq1
Round Cable EMI Suppression Cores
Broadband Frequencies 25-300
MHz (43 material)
some radio amateur told me that maybe this cores are improper for these frequences. But the designer of this linear
claims that we must use 43 material cores for the transformers.
Thank you.

 

[ssankar73]

Hi tzitzikas
Is your problem sorted out?

regards
Sankar

 

[gito nugroho]

HI



Tzitzikas ,is this linear MW transmitter design for Medium Wave transmitter?,maybe not,so the ferrite core You used material 43 is not good for Medium wave,looking at the data sheet that You attached ,It needs a 31 material that's good for 1 t0 100 Mhz.
And maybe You must used more ferrite core to have the right inductance ,for the load of the Transmitter,by the way can You give the exact turns T3.Or can You wrote /attached the article of it.

Once I used a SWR (WELZ) meter that used ferrite rings as the pickup,the power it can handle is 1 Kw,I used the frequency range from 1.8 to 200 mhz,for measuring a 500 watt MW transmitter,and the Ferrite Ring is broken just like Yours.

See also the polarity the feedback winding at T3, the one turn winding ,if the polarity is wrong, You got a positif feed Bck instead of negative feedback.


Gito

 

[tzitzikas]

hi all. i have solve the problem. the problem was the pcb board that hasn't much ground on it's surface. When i change the board the linear hasn't oscillation problems, and i achieve to have 500w r.f power at dummy load with 110vdc - 11A voltage. i post the new pcb board.
Also here you can see the test videos from the transmitter:
YouTube - 1431AM MW TRANSMITTER 500W MOSFETs IRFP360 PLL SWICH ON (auto) (connected with antenna test)
YouTube - 1431am thessaloniki 500w MW transmitter dummy load test.mp4 (dummy load test)
The transmitters is in continuous operation from November 2010 in antenna without problems. We have adjust the FWD r.f output power at 280w with only 0,8w RFL power
the station is 1431 AM Thessaloniki-Greece (frequency 1431Khz) medium waves band
www.1431am. org