any help- people - about this illogical problem.?

 

Like I said, it could be due to oscillation. Check the circuit with a 'scope.

 

any help- people - about this illogical problem.?

Could it possibly be because you are ignoring the advise of the people trying to help you? Substituting things that aren't what your told to use?

 

how could it be oscillation just after 2 Ampere ? i told you it is working very well from 0 - 2 A

 

any help- people - about this illogical problem.?

I have revised the circuit for you to reflect some of the changes you have made. I have added decoupling capacitors. Since the LM358 is such a slow opamp, I simulated the step response, and found the circuit to be marginally unstable, so I have added some additional feedback to add phase margin to make the circuit less prone to oscillation. I have added the gate resistors per suggestion.

Try it as shown now. Write back if you change anything...

 

I have revised the circuit for you to reflect some of the changes you have made. I have added decoupling capacitors. Since the LM358 is such a slow opamp, I simulated the step response, and found the circuit to be marginally unstable, so I have added some additional feedback to add phase margin to make the circuit less prone to oscillation. I have added the gate resistors per suggestion.

Try it as shown now. Write back if you change anything...

ok I am working on it , but do you think that this mosfet will do the job because it is written in datasheet that it is good in fast switching . i think that its linear region is narrow.

 

I sent to vishay siliconix and their Product Marketing Manager and he replied with " Unfortunately, the IRF530 is designed to
be operated fully on (Vgs=10V) or fully off (Vgs=0V) as a
switch. Damage in the form of electrical overstress can
occur if the device is operating in the linear region."

 

Actually, the '530 has a very wide range of Gate voltages where the Drain current is more or less proportional to Gate voltage, and is well suited to this circuit. Most more modern FETs are designed to switch from fully off to fully on with less than a two volt change of gate voltage.

You are welcome to try any of hundreds of different NFETs as long as they have a Vds>40V, an Idmax>10A, are in a package capable of dissipating >25W. However, all four NFETS must be of the same type.

 

Since with Mos Fets the gate threshold voltage variation is so large you need an op amp for each one.

If you add 75 or 100 ohms between the output of the op amp and the gate it will restore stability lost when it tries to drive the high capacitance of the FET.

 

Since with Mos Fets the gate threshold voltage variation is so large you need an op amp for each one.

Not really a problem. We are not trying to build a precision current sink.
The goal is to turn a pot and make the current sinked (sunk?) by the combination of four NFETs be proportional to the pot position. Even if the NFETs are mismatched, that will still happen...

If you add 75 or 100 ohms between the output of the op amp and the gate it will restore stability lost when it tries to drive the high capacitance of the FET.

I already incorporated that suggestion into the circuit. In the present version, the OpAmp is operated in unity gain mode where it should be unconditionally stable. The settling time will be much slower, but it shouldn't oscillate...

 

I moved the old circuit to a new PCB and took care of soldering points it gives some good results and there is no oscillations now. but i put a potentiometer instead of the mosfet in the feedback to get flexibility cause it was a narrow area that Vds was changed when turning the control pot. but something is still mysterious , the open circuit voltage (Voc) is near 40 v . i got Voc in the middle of the pot track and it should happen when there is no current flowing when Vgs is small in the beginning of the pot track . i don't remember very well but in the beginning if the track i got Vds=1v then incresed when turning the pot to jump suddenly from 19 to 37 in the middle of the pot track . i need more flexibility here in this area. i will try it again to assure . just i want to inform you about the new .

 

Is this the original pot you bought that you ran many more Watts into than it was rated for? If so change it to a new pot and try again. In the simulation, the control range where the sink current changes occupies more than 60% of the range of a linear pot.

That raises the following question: Is the pot you are using a "Linear Taper" or an "Audio Taper" Pot? You should be using a linear one; not an audio pot...

 

Is this the original pot you bought that you ran many more Watts into than it was rated for? If so change it to a new pot and try again. In the simulation, the control range where the sink current changes occupies more than 60% of the range of a linear pot.

That raises the following question: Is the pot you are using a "Linear Taper" or an "Audio Taper" Pot? You should be using a linear one; not an audio pot...

no the one i used was audio pot like this http://www.ebay.com/itm/2x-Audio-A5...d=100005&prg=1088&rk=3&rkt=5&sd=301031813014&
fortunately the pot that i bought first is a linear one and it is still not corrupted so i will try it i don't think that heat for seconds could burn it up, i measured its resistance and it was good.

 

I wonder why should i use opamp I could use the output of the pot directly to the Mosfet gates .

 

With the opamp, you set a voltage on the pot and that makes a current proportional to that voltage. Current is regulated by the feedback to the opamp. Voltage on the gates is exactly what is required to get the set current. Panel voltage, nor temperature of the NFETs effects the current.

Without the opamp, you set a voltage on pot, it goes to the gates. The FETS work as source followers, their source current is not regulated, so the current will change with drain voltage (the panel voltage), and FET temperature. FETS can go into thermal runaway. No feedback.

 

I made the old circuit without any capacitors and i changed the pot with linear one and i tried it with a pc power supply as a source instead of the pv panel and it works very well. i could decrease voltage consequently the current increased when increasing Vgs i also could measure current and voltage . the weird thing happened when connected the pv panel i increased Vgs but the voltmeter neither notice any change in Vgs nor Vdd , and also the clamp meter didn't read any change in current . I don't know what is the problem here , i replaced the circuit with a power potentiometer 50 watt and i changed the resistance but i disconnect it fast to protect it from burning up and every thing was ok there was a change in current and coltage.

 

I noticed that some Mosfets are burnt up and that is one of the reasons why ? if that is because of protection why won't they all burnt ?

 

I noticed that some Mosfets are burnt up and that is one of the reasons why ?

If a FET isn't turned fully on it will heat up. Using a pot directly to set Vgs, in the absence of a current-limiting circuit, is likely to result in a partly-turned-on state and hence excessive heating.

 

How many times do I have to say this: The NFETs Must be on a HUGE heatsink. If you are burning them up, you don't have a big enough heatsink. Get a larger area heatsink, or blow air across it using a fan, or immerse one end of the heatsink in water or oil.

The NFETs are operated in their linear region. They are dissipating all of the power that an equivalent load resistor would dissipate.

 

How many times do I have to say this: The NFETs Must be on a HUGE heatsink. If you are burning them up, you don't have a big enough heatsink. Get a larger area heatsink, or blow air across it using a fan, or immerse one end of the heatsink in water or oil.

The NFETs are operated in their linear region. They are dissipating all of the power that an equivalent load resistor would dissipate.

I have shown you the heatsink that i used and you didn't object , it's big. also i touched it and the heat wasn't high , it was normal.