Problems when using parallel MOSFETs to switch DC

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Joined Apr 18, 2016
I have got a set of solar panels with Voc arround 200V. I plan to use the generated electricity to heat water in the boiler.
The boiler heating spiral resistance is 24 ohm (cold).

As the boiler is designed to get 240V AC and there is a bimetal Thermal switch - thermostat which would soon die operating on DC, I wanted to substitute it's function using MOSFET. the closest MOSFET I found that would fit the parameters was IRFP264, with Vds max 250V. The idea was to use the boiler's thermostat just with a small current to control MOSFETs.

The problem is that with Rdson 0.075 it dissipates about 3.7W at 7 Amperes flowing into the heating spiral. So I decided to use 2x IRFP264 in parallel, as I prefere to heat up water instead of the MOSFET itself.
As this seemed to be a pretty simple DC circuit I had not considered to add any compensation stuff that would reflect that there are 2 devices in parallel. the circuit seemed to work OK on my test board - in the late winter when there was not much sun during the day. (the thermal switch in the boiler never had to switch off, as the max water temperature had not been reached)
Suddenly when there was more sunshine during the day I found the plastic box with the circuit half melted.fortunately the MOSFETs seemed to be OK.
I supposed that when the boiler thermal switch switched off, the Gate voltage went down very slowly due to the Gate capacitance, the MOSFETS got half open for some time and they shared the power with the boiler. so I added a Gate-Source discharging resistor of 1M to prevent it. (before there was only a zener to limit the Gate voltage:( ) - schematic diagram attached.
Unfortunately I found the box in a bad state again, moreover, one MOSFET of the couple died.
(the resistance between G an S is about 17 ohms, both polarities).
Then I did a more proper PCB and used M1 as a G-S resistance.
Test switch is there to be able to connect/disconnect the circuit without having fireworks. LED glows when the water temperature is lower then pre-set on the thermostat.
the circuit got crazy immediately after connecting it and one MOSFET died again. (26ohm G-S both ways, about 125 ohm D-S both ways - cca 3 ohms difference for the other polarity)

As it would be quite difficult to learn "the hard way" I would really appreciate if someone could help.
I suppose that there must be a kind of situation upon the switch on/off that causes the circuit to oscillate and kill MOSFET by exceeding Vds max or even Vgs max.
would, for example simple, adding dedicated gate resistors to each MOSFET help to keep the circuit stable?
many thanks!!!


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