hi all!
I have got a set of solar panels with Voc arround 200V. I plan to use the generated electricity to heat the water in a 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 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 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 the water instead of the MOSFET itself.
As this seemed to be a pretty simple DC circuit, a slow Speed one with almost purely resistive load, 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 breadboard - 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 incoming power with the Boiler
, so I added a Gate-Source discharging resistor to prevent it. (before there was only a zener to limit the Gate voltage) (picture attached)
unfortunately I found the box in a bad state again, moreover, one MOSFET of the couple died.
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 on switch on/off that causes the device to oscillate and kill it by exceeding Vds max or even Vgs max.
Or maybe another effect I overlooked basing the circuit on the MOSFET ideal model
for example Would plain adding dedicated gate resistors to each MOSFET help to keep the circuit stable?
many thanks!!!
I have got a set of solar panels with Voc arround 200V. I plan to use the generated electricity to heat the water in a 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 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 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 the water instead of the MOSFET itself.
As this seemed to be a pretty simple DC circuit, a slow Speed one with almost purely resistive load, 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 breadboard - 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 incoming power with the Boiler
, so I added a Gate-Source discharging resistor to prevent it. (before there was only a zener to limit the Gate voltage) (picture attached)unfortunately I found the box in a bad state again, moreover, one MOSFET of the couple died.
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 on switch on/off that causes the device to oscillate and kill it by exceeding Vds max or even Vgs max.
Or maybe another effect I overlooked basing the circuit on the MOSFET ideal model
for example Would plain adding dedicated gate resistors to each MOSFET help to keep the circuit stable?
many thanks!!!
