How well is the mounting tab on the transistors thermally coupled to the heatsink? Are you using the correct mounting kit? Thermal grease? Deburred the hole?

 

Hello,

Have a look at the figures 4A and 4B of the following page:
http://sound.westhost.com/heatsinks.htm

Bertus

 

How well is the mounting tab on the transistors thermally coupled to the heatsink? Are you using the correct mounting kit? Thermal grease? Deburred the hole?

I don't use thermal grease , i already have done as bertus said for holes and all of the back of the transistor is tied very well to the heatsink.i used plastic insulator between the mosfet and heatsink designed for this purpose.
I also redistribute the Fets over the heatsink almost 4 cm between each one and the other . i have never felt excessive heat on the heatsink that mean it can withstand.
notice that i still have random readings on voltmeter when measuring VDs while the panel is connected , but it give good results when the pc power supply is connected so i think it's because of high power.

 

I used smaller solar cell i don't know exactly its power but its open circuit voltage is 18 volt i think it is 50 watt . i used capacitor at its output 100 v 100uf and 1uf for the power supply but the same thing happend when there is no current passing in the circuit the voltmeter reads 12 volt power supply for the op amp and 18 volt for the panel . when the mosfet is fully on the voltmeter reads 1.5 volt for the panel and 12 volt for the supply , but in between ( the linear region ) this is the problem random value on the gates and the drain and also the power supply for the op amp i am afraid that the power supply might be burnt up .

 

thank you for help , the circuit is working now i figured out the problem . there were oscillations in every point in the circuit because my op amp lm 358 couldn't drive large capacitive loads especially in high voltages like mine( that's why it worked with pc power supply and didn't work with the panel which have 40 Voc), i read in its datasheet that it need resistor to make the load resistive . i was first using one resistor after the op amp output but that wasn't enough.
I really appreciate your help