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I am trying to toggle the power on a 24V LED strip using a IRFZ44N MOSFET. I am able to power the LED strip and vary the current using different PWM duty cycles, but the MOSFET gets really hot, especially when the lights are "dimmed", i.e. 0 < duty-cycle < 100. With duty-cycle = 0 there are no heating issues though.

The LED strip is rather power consuming, 24W/m, and I'm utilizing 4m so i figured a 4A power supply would suffice. The idea is to toggle the voltage, ~10V, on the T3 MOSFET gate by regulating the current at the base of T1 and T2. To my help is a Raspberry Pi 4B, using 3.3V GPIO, the 12V input is 1A

circuit

I am not an EE and apologize if there is anything unclear about the above schematic, would be happy to elaborate.

I found a similar post here on SE, which made me suspect the PWM frequency and T3 input voltage: I've tried many different PWM frequencies, from 1Hz to 10KHz, and also measured the voltage on the T3 gate, which according to my multi-meter is around 12v, 10V+ should be enough from what i understand. I didn't quite get the points they made about capacitance, but i have tried to lower the resistance on R4, which seems to delay the heating issue, but not fix it.

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The MOSFET is acting as a source follower and therefore, the voltage it develops at the source is somewhat less than the voltage applied at the gate. So, if T2 collector can produce (say) 12 volts then T3's gate receives 12 volts and T3's source will be a volt or so lower at 10 or 11 volts (maybe even lower with some FETs).

This means that the power dissipation in the MOSFET is quite high because it is dropping 24 volts to around 10 volts at maybe around half an amp. The current is a guesstimate based on what current the LEDs are taking with 10 volts applied. If 0.5 amps flow then the MOSFET power dissipation is around 14 volts x 0.5 amps = 7 watts.

Solution: move the LEDs to the drain and connect the source to ground and now you will fully activate the FET and get 24 volts across the LEDs.

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  • \$\begingroup\$ Thank you for the quick reply Andy. Moving the LED did indeed cause the MOSFET to dissipate less heat, but after a while it still gets very hot, in minutes rather than seconds tho - not like before. Would you be able to give me some pointers on how to debug the heating issues further? I am a bit out of ideas \$\endgroup\$
    – ohmybuddha
    Jul 21 '20 at 8:00
  • \$\begingroup\$ @ohmybuddha what current is flowing through the MOSFET from drain to load? What PWM frequency are you operating at? Does it get warm on low brightness or does it appear to be proportional to LED brightness? Why are you using a TIP2955 for driving the MOSFET gate? Is the schematic in your question precisely what you are using? \$\endgroup\$
    – Andy aka
    Jul 21 '20 at 8:26
  • \$\begingroup\$ According to my MM, the current across the load is 4A and from the drain to ground is 1.8A, and 0A between source and ground. It seems to get just as hot at lower brightness/duty-cycles, not at 0 though. I had the TIP2955 lying around, never given it a second thought tbh. The schematic is as above, except for the LED being moved to between +24 and the MOSFET. \$\endgroup\$
    – ohmybuddha
    Jul 21 '20 at 8:59
  • \$\begingroup\$ Those numbers don't make sense. If there is 4 amps flowing through the load (current is through, voltage is across) and the load is in series with the drain then there will also be 4 amps flowing into the drain. Are you in fact measuring by connecting your meter across the load and therefore shorting the load out and causing excessive current to flow into the transistor? Maybe you need to check how to use the meter when measuring amps. There should be some on-line information to help you. \$\endgroup\$
    – Andy aka
    Jul 21 '20 at 9:05
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    \$\begingroup\$ My apologies Andy, i had it all backwards. Followed your advice and learned how to use the MM properly. Apparently there was very little current running through the LED, turns out that using a breadboard was a poor choice, it could hardly conduct enough. New wires and some soldering, problem fixed, no more MOSFET heating. Many lessons learned. Thanks again! \$\endgroup\$
    – ohmybuddha
    Jul 23 '20 at 14:07

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