how can stabilize the drain current help

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ifixit

Joined Nov 20, 2008
652
Hi,

Sorry, I was waiting to see if you would try some of the changes. I have updated some details based on your post on May 15 2010.

Just try the following changes (one at a time?) and see if things improve for a 2 Watt version first…

1. Attached are two transformer re-designs using your available cores. Wind them with the maximum wire size that will fit.
2. Don’t use C4 or C5.
3. Boost Vcc to 15V.
4. Put 1Ω resistor in the sources.
5. Bias the MOSFETs no signal current at ~50mA.
6. Connect gate drive and voltage bias as shown.
7. Use the IRF640 for now. Are you able to get IRF510??
8. Leave the M1 drive circuit as is for now. It may work well enough for your requirements.
9. Let us know how the test goes.

Are the oscillator and modulator located on the same PCB as the linear amp? If so, then we could use the differential drive from the MC1496 later if required.

Good Luck,
Ifixit
 

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tzitzikas

Joined Jun 3, 2007
41
Hi.Thank you for your reply. I had not many time to do the changes. I will try this week, maybe tommorow.

About your suggestions:

5. Bias the MOSFETs no signal current at ~50mA. <-- i think this current is too small, and with this current
the output power will be enought below 2watts.What do you think about this?


6. Connect gate drive and voltage bias as shown. <-- should i remove 4.7ohm series resistors from gates? Also i must remove 10k resistor? (see the picture)

1. Attached are two transformer re-designs using your available cores. Wind them with the maximum wire size that will fit. <-- i can't put 12 turns (6+6) in output transformer M2. The holes are so small. It was difficult
to put 2+4 turns.
About M1: if i put 10+10 turns, this change will help to increase the power which drives the mosfets, so
the circuit will give more output power?

7. Use the IRF640 for now. Are you able to get IRF510?? <---- why i must change the mosfets? i have bought many IRF640 and i would like to
use them. But if it is nessecary i could try to find IRF510.

Are the oscillator and modulator located on the same PCB as the linear amp? If so, then we could use the differential drive from the MC1496 later if required. <---- The oscillator and modulator are in the same
pcb but not in the linear's pcb. you can see 2 photos here:
oscillator-modulator:
http://tzitzikas.webs.com/oscillator-modulator_photo_1.jpg
http://tzitzikas.webs.com/pll-modulator-oscillator-filter-1.jpg

the ferrite for M1 is FT50-43 not FN.
 

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ifixit

Joined Nov 20, 2008
652
Hi,

  1. Make the gate bias adjustable with a pot so that it is easy to try 50mA or 400mA. Bias current should not effect output power directly. You decide what works best.
  2. The 4.7Ω resistor are not required.
  3. Make as many turns as you can for M2. How about 3+3 with the primary center-tapped at 1.5 turns. You could also use another FT50-43 for M2 at 10+10. Do you have another FT50-43 core?
  4. Your bias picture looks okay except add a pot so that it is adjustable. Place the 0.1uF cap close to the transformer. The 100Ω value is not critical.
  5. The IRF640 have 1600pF gate capacitance, which is not good for your application at 500KHz to2MHz. Leave them in and try it with the other changes you can do easily. If you like how it works then don't change them. If most of the time you are at 1600KHz then it may not matter so much.
  6. The IRF510 have only 150pF of gate capacitance so should work much better.
  7. If the differential drive from the modulator is needed, you can run 2 coax cables instead of 1. Try as is for now.
  8. FN versus FT... typo on my part.:(
Why the changes?...
  1. to ensure the amplifier will amplify each frequency in the range by exactly the same gain.
  2. to minimize distortion in the modulated signal, which produces sideband frequencies adjacent to the carrier.
  3. to prevent the MOSFETs from running too hot.
Good Luck,
Ifixit
 

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tzitzikas

Joined Jun 3, 2007
41
Hi. I tried to remove the capacitors that you said, but the power was very low after these changes.
After that, i keep the capacitors but i change the secondary coil number of turns (of M1) from 3 to 6 turns. after this, the power was 2w at 12volt dc and 8w at 20vdc with very good modulation and only 300mA current (drain's current of 2 mosfets).
My problem now is the next stage this linear http://tzitzikas.webs.com/linear500w.jpg
The manufacturer claims that with only 2w input r.f power it gives 500w at the output at 110vdc (13A current). But with 5,5w input power at 106vdc my linear gives only 220w output power (7A current). I tried to change C4 capacitor's value but the power was low. Do you have to propose any change at transformers or the mosfet's current without driving power (the manufacturer said to adjust this current at 200ma)
I thing to change the secondary coil of T1 from 6 turns to 9 turns. What do you think?
 

ifixit

Joined Nov 20, 2008
652
Hi,

I don't think adding more turns will work, but with C4 in the circuit I can't be sure. The circuit may work well at certain carrier frequencies, but not at others. If you do try more turns, keep the ratio between primary and secondary the same.

For a 500W output, the T1 core size has to be quite big... in the order of 2 inches by 2 inches. What size is it? If it is not big enough the core will saturate with the high current and limit the output. What are the specs for T1s core?

What's next, 50,000W?:eek:

Regards,
Ifixit
 

Thread Starter

tzitzikas

Joined Jun 3, 2007
41
Hi,

I don't think adding more turns will work, but with C4 in the circuit I can't be sure. The circuit may work well at certain carrier frequencies, but not at others. If you do try more turns, keep the ratio between primary and secondary the same.

For a 500W output, the T1 core size has to be quite big... in the order of 2 inches by 2 inches. What size is it? If it is not big enough the core will saturate with the high current and limit the output. What are the specs for T1s core?

What's next, 50,000W?:eek:

Regards,
Ifixit
the ferrite cores are the FB-43-7351 ( https://www.amidoncorp.com/items/44 )
but i have ordered and i will try the FB-43-7352 and the FB-43-1020 ferrite bead.
Which do you believe is the most proper?
Also see some photos from my linear:
http://tzitzikas.webs.com/linear_photo_1.jpg
http://tzitzikas.webs.com/linear20.JPG
http://tzitzikas.webs.com/linear20a.JPG
http://tzitzikas.webs.com/linear21.JPG
http://tzitzikas.webs.com/fet21.JPG
http://tzitzikas.webs.com/pompos20.JPG
http://tzitzikas.webs.com/T3.JPG

the other problem is that the T3 transformer and the resistors R9,R10,R11,R8 and the r.f choke VK200 are very warm when the linear operates.
Can i use an FB50-43 ferrite for r.f choke, and how many turns i need? I ask this because i believe that, 12A current is too much for
VK200 choke.
But the basic problem is to increase the output power.
The mosfets are from Siliconix but i have ordered and i will try to use International Rectifier's IRFP360 mosfets.
Thank you. (no this is the final stage! :) )
 

ifixit

Joined Nov 20, 2008
652
Hi,

The #43 material should be good for your frequency of operation. The format (shape) of the core can be whatever works for you, use what the designer recommends. The size is dictated by the amount of power you need to handle. In your case 500W, so the cores need to be big.

It is normal for chokes and transformers to get hot under load. The absolute maximum should be set at 100°C for short term reliability reasons, however, if the temperature can be maintained at or below 85°C then reliability will be much improved for the long term. Can you measure the temperature?


Inductors heat up too much because of; losses in the wire where the gage is too small, losses in the core itself due to eddy currents, or saturation. To fix this, use a heavier gage of wire in a larger core size. You still always need lots of ventilation to take the heat away from the core surface.
I don't have any experience with high power RF amps so I don't know what you can get away with. Perhapes someone else in the forum has an opinion. Where are the HAMs when you need one?:)

Regards,
Ifixit
 

Thread Starter

tzitzikas

Joined Jun 3, 2007
41
Could i increase the output transformer T1 number of turns, but keep the ratio 1:2 constant, to increase the output power? It will help? (4 turns primary and 8 turns secondary or 5 turns primary and 10 turns secondary)
Or to increase the secondary number of turns of T3 (with 25ohm coaxial cable)?
If i use bigger ferrite beads?

about choke vk200: i am thinking to change the vk200 with a ferrite bead FB-43-7351 (i have many of them) . how many turns i must use to have the same inductance with the vk200?

If i use 4 vk200 (in parallel ) i will use the same inductance or 1/4 inductance, It will work?
 
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