using parallel MOSFET config to replace thermal switch in a water boiler to use with PV panels

Discussion in 'General Electronics Chat' started by duricam, Apr 18, 2016.

  1. duricam

    Thread Starter New Member

    Apr 18, 2016
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    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!!!
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,995
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    Why dont you use a solid state relay..
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    What Vgs are you using?
    Have you measured the actual peak voltage that the panels provide?
     
  4. duricam

    Thread Starter New Member

    Apr 18, 2016
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    Vgs is limited to 12Vmax.
    and yes, if it is very sunny the "open circuit" voltage of the panel set is less than 210 V. definitely it does not exceed Vds max of IRFP264.
    I have attached a bit nicer schematic diagram...
     
  5. duricam

    Thread Starter New Member

    Apr 18, 2016
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    Later I made a more proper PCB and used M1 as a G-S resistance.
    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)
    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.
     
    Last edited: Apr 18, 2016
  6. duricam

    Thread Starter New Member

    Apr 18, 2016
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    well, the idea has not come to my mind... are there any 250V DC / 10A, Ron < 0.04 Ohm ?
    I would be quite surprised... but must look...
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I think the problem arises when the light is dull, so that Vgs is insufficient to turn the FETs on fully. There is likely to be oscillation then, leading to overheating of the FETs. I suggest adding a thresholding circuit so that the gate voltage is clamped low until the voltage across M33 exceeds 10V or so.
    What values are M22 and M33?
     
  8. duricam

    Thread Starter New Member

    Apr 18, 2016
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    M22 means 220k , sorry the notation is used down here....
    your suggestion about low light condition might be correct, will think about an improvement in that area. but I think the problem comes when the sun burns at full power. and mainly when changing the states. (on-off)
    When the light is dull, the MOSFETs will heat a bit, but nothing terrible. (tested, that is why I played with Rgs , first 1Mohm, then 100k to have good discharge at OFF time, now 330k)
    if the panels are connected and they provide ,say, 10V , let Vgs be 3-4V ( Vds similar) and the current into 24ohm spiral is below 0.5 Amps then MOSFETs dissipate 2W, which is nothing for the big beasts with a heat sink. even with oscillation...not enough power available to do the damage
     
    Last edited: Apr 18, 2016
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