MOSFET started to smoke when switching a 12V bulb - how can I troubleshoot the cause?

Question

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?

Answer 1

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

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: -

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: -

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.

Answer 2

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.