Sorry to say this, but that is a really lame design. The idea that you can control the output current of a MOSFET by just setting its gate voltage open loop is plain stupid and ignorant. As the MOSFETs heat up, their gate threshold voltage is going to decrease, the MOSFETs will turn on more at the fixed input voltage and hence the current through them is going to increase and you will end up with a failure. All quite expected.
It is equally lame idea to have two MOSFETs be driven by a voltage and think that the current through them would be identical. In real life, any two given parts are never than well matched, except by sheer luck or coincidence.
The way this type of oscillators are supposed to work is by tailoring the inductance of the transformer to the drive frequency so that the current through the inductor does not have the time to increase above the desired limit during the time that the MOSFET is on during the cycle.
It is likely that the circuit is supposed to work only when the input is active and rather than providing interlocks to ensure that, they are just hoping that it will always be true.
I suggest you contact the website where you got the circuit from. Maybe someone there has some additional information for you.
---55p
here you can see some photos from my linear. sometimes i am using 4 mosfet (2+2)
http://tzitzikas.webs.com/linear2xirf640-1.jpg
http://tzitzikas.webs.com/linear2xirf640-2.jpg
http://tzitzikas.webs.com/linear2xirf640-3.jpg
i don't know if the problem is the heatshink, because i am using a fan for cooler and the heatshink is cold.
do you have any idea to modificate the circuit to keep stable the drain's current? Maybe if i will use 2 lm7805 and 2 different gate voltages with 2 variable 5kohm resistances? But i don't know how to modificate the circuit to achive this.
Any other idea, to keep this circuit but to keep stable the drain's current regardless the temperature?? maybe a thermistor or a diode?
see this. i have construct this also:
http://tzitzikas.webs.com/linear500w.jpg (12 mosfets irfp360 6+6) .i have not this problem with this linear. i have test it many times, and i have not destroy any of the mosfets.
(it is using a diode to keep stable the drain's current)
Can i use a 1N4007 diode to the lm7805 like the linear 500w?
(*steve*): The amplifier with 12v voltage with 2 mosfet gives 2watt output r.f power and the whole current is approximately 1,5A, without modulation. I think that the designer have design this linear for linear operation to achive the best quality of the modulation.
The linear's heatsink isn't warm beacause i am also using a fan for extra cooling.
sometimes i adjust the drains current without input signal about 300mA. After some minutes of linear's operation with input signal, and when i stop the input signal this current has been increasedup to 600mA !
i think 400ma is very high current for 2 only mosfets. what value do you think that this current must have?