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This is the case: 12V bench power supply (current limited to 3A) wired to a 30W LED bulb (H7 bulb for vehicles), controlled by an Arduino via the gate of the 2SK1363.

The signal from the Arduino was sent to the gate by a pushbutton, it was a simple sequence of flashing the LED on and off quickly, 20 times in a 2 seconds or so. Also, the default initial state of the LED was controlled by the code as well, with digitalWrite high or low. This would put the bulb in a "solid state", on or off, and then the pushbutton would initiate fast flashing for 2 seconds.

I used INPUT_PULLUP on the pushbutton pin and everything worked fine but the MOSFET got very hot, and after running the pushbutton flashing on the LED for couple of times in a row, it started to smoke.

I'm a newbie and the only rating I asked for when buying this mosfet was "N-type" and "gate threshold 5V" because of the Arduino.

I later discovered that my particular MOSFET has an Rds rating of 1.4 ohm. When working with my 2.5A bulb it rounds up to 9W, but on the spec sheet I see that "Maximum Power Dissipation" (Pd) is 90 W.

Can someone enlighten me and point me in the correct direction for me to learn proper way of working with MOSFETs? Why did my MOSFET smoke? How can I pick a proper one for my application? What things should I consider?

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  • \$\begingroup\$ Any heatsink? Did you allow for the cold resistance of the lamp being much lower than the hot resistance and the resultant surge current on switch-on? \$\endgroup\$
    – Transistor
    Mar 16, 2021 at 12:14
  • \$\begingroup\$ No heatsink, all just laid out on the bench for testing for short period of time, I think it started to smoke in about 2 minutes or so? About the cold resistance - can't say I know what you mean, do I need some protection from the surge, as a general precaution when fast switching this much current thru mosfet? \$\endgroup\$
    – Varonne
    Mar 16, 2021 at 12:21
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    \$\begingroup\$ Sorry, it's an LED so no cold resistance. I had spotted vehicle and somewhere in my head got a mental picture of a filament lamp. Carry on. \$\endgroup\$
    – Transistor
    Mar 16, 2021 at 12:41
  • \$\begingroup\$ @Varonne if this question is answered you should formally accept one of the answers. That indicates to other readers that you have an answer that is acceptable to you and that there is no urgent need for other answers to come along. It's the normal way things are done on this site. \$\endgroup\$
    – Andy aka
    Jan 14, 2022 at 12:46

2 Answers 2

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The 2SK1363 is a fairly inappropriate choice for this application. Look at \$R_{DS(ON)}\$ for instance: -

enter image description here

You have a 30 watt 12 volt lamp and it will try and take 2.5 amps. That current through a fully-activated 2SK1363 means a power dissipation in the MOSFET of 6.875 watts (this is based on typical \$R_{DS(ON)}\$ = 1.1 Ω).

That would need a heatsink for sure. But it's going to be worse than this because to adequately turn on the MOSFET you need a gate drive of at least 10 volts: -

enter image description here

Pick a MOSFET that has a much lower \$R_{DS(ON)}\$ that can be achieved with logic level signals appropriate to the Arduino you are using. For instance, the IRFZ44 (a long-standing favourite of mine) has this useful graph in its data sheet: -

enter image description here

With a gate-source drive level of 4.5 volts and a load current of 2.5 amps, the IRFZ44 will "drop" about 170 mV. That's a power dissipation of 2.5 x 0.17 = 0.425 watts and more appropriate for something like this however, I'd still try and find something that is a tad better because these are typical values.

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  • \$\begingroup\$ Thank you very much for the rundown! Beginner question - if this mosfet/Arduino combination is controlling this 12V LED and Arduino code tells it to shut down, I know this might sound silly but is there any "work" being done by mosfet to keep the circuit "open"? Is that anything of consideration? \$\endgroup\$
    – Varonne
    Mar 16, 2021 at 12:15
  • \$\begingroup\$ @Varonne it depends on the circuit you have used and you haven't disclosed that. \$\endgroup\$
    – Andy aka
    Mar 16, 2021 at 12:31
  • \$\begingroup\$ This is how the circuit looked - although I didn't use the buck converter. I used a very silly way to show the crocodile clips I used for connecting things, since I have no other way but crocs to quickly test something out, it's not per standards I know :) i.imgur.com/JiUEvce.jpg \$\endgroup\$
    – Varonne
    Mar 16, 2021 at 12:35
  • \$\begingroup\$ @Varonne there will be minimal dissipation of heat in the MOSFET when it is "off" (if that is what you mean by "open"). \$\endgroup\$
    – Andy aka
    Mar 16, 2021 at 12:38
  • \$\begingroup\$ Yeah that's what I meant, since the mosfet will be controlling the light, it won't just be controlling it for short flashes, but for long term on or off states, so I was wondering will mosfet have any "work" doing while keeping the light off. Minimal dissipation, so, nothing of worry I presume. But heatsink is advisable in any way right? \$\endgroup\$
    – Varonne
    Mar 16, 2021 at 12:41
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9W with no heatsinking will cook!

Note that that 90W rating is with Tcase held at 25 degrees C, you are not doing anything like that!

Also, the threshold voltage is below 5V, but that is at 1mA Id, to fully enhance the device you need 10V on the gate, this is not a logic level mosfet in any sense that matters.

What you have there is a fairly old high voltage part made for the primary side of switched mode supplies, there are FAR better choices today.

Try something like a IRLZ34PBF 0.05ohms at 5V on the gate, it will probably not even need heatsinking.

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  • \$\begingroup\$ So when they give 90W rating at 25C that means if I somehow manage to keep the mosfet at a constant 25'C temp, I can run around 8A all the time thru it? I have IRFP4332 at my disposal, gotta see if that one will work! Thanks! \$\endgroup\$
    – Varonne
    Mar 16, 2021 at 12:20
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    \$\begingroup\$ @Varonne that is a more modern part, but again 10V to fully turn on... You need a logic level mosfet for this (Or a gate driver). Rule of thumb with mosfets (and most anything else) is that the real datasheet starts with the first graph, everything up to that point was written by the marketing department not the engineers. Certainly nobody has ever managed to run a mosfet at the front page rating for more then a few tens of ms or so, but that can actually be useful sometimes. \$\endgroup\$
    – Dan Mills
    Mar 16, 2021 at 12:26
  • \$\begingroup\$ I have to rush to my nearby store and pick a better one, but last time I was there I asked exactly that - logic level mosfet for arudino project, and they sold me K1363 and IRFP4332, so I'd say they're not the best salesmen in the city :D but that's all the stores I have, if it's not considered spam - this is the store and mosfets they have for sale...if you can spot the "right" one for me, I'd really appreciate it - bit.ly/3bRBGg4 \$\endgroup\$
    – Varonne
    Mar 16, 2021 at 12:32
  • \$\begingroup\$ They obviously don't understand what 'logic level' means. Try a IRL2203N which they have, should be fine. \$\endgroup\$
    – Dan Mills
    Mar 16, 2021 at 12:43
  • \$\begingroup\$ Awesome, thank you very much! \$\endgroup\$
    – Varonne
    Mar 16, 2021 at 12:44

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