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power MOSFET discussion notes

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I’m planning to control 3 LEDs and a heating element (5 Ohms) using a Raspberry Pi.

The main power supply outputs 12V & 5A. Using an L78S05CV Voltage regulator, I’m planning to power the RPi. So far I didn’t decide how I’m gonna power the heating element. I mean, It could possibly use either the 5V or the 12V.

Anyway, in order to switch on and off the heater, I searched for some logic-level MOSFETs and calculated the power dissipation (P) that the heater will cause to the MOSFET as well as the maximum MOSFET dissipation without using a heatsink (Pd).

Here is a gathering table of what I’ve found and calculated so far:

+-------------+-------------+----------+---------------------+--------+
|   MOSFET    | VGS(th) (V) | P@5V (W) |      P@12V (W)      | Pd (W) |
+-------------+-------------+----------+---------------------+--------+
| IRLB8743PbF | 1.35 - 2.35 |   0.0032 | 0.018               |   2.41 |
| IRL540      | 1.00 - 2.00 |    0.077 | 0.443               |   2.41 |
| FDN337N     | 0.40 - 1.00 |    0.065 | 0.374               |    0.5 |
| IRLD014     | 1.00 - 2.00 |    0.200 | 1.152(needs a sink) |   1.25 |
| IRLZ44N     | 1.00 - 2.00 |    0.025 | 0.126               |   2.41 |
+-------------+-------------+----------+---------------------+--------+

So, since I’m new to this I would like to ask for your opinion on which one or maybe more than one is better for that case.

Maybe my approach is totally wrong and these calculations make no sense in such a project. So any idea hint or other suggestion is welcome.

  • Hi @J.Doe, Welcome and nice to meet you. Your research notes on power MOSFET is good. I happened to have also done some tests on the same subject. My interest area is a bit different from you so is my approach. Now let me make some quick comments: (1) As you did, first thing first is Vgs(th). The reason is that most old generations eg IRF504N have high Vgs around 7V. So we need to find low Vgs, (2) What you do not emphasize is switching current. I need to switch current from 500mA to 50A, but luckily power MOSFET can entertain big current happily. / to continue, … – tlfong01 33 mins ago
  • Their On Resistance is very small, comparing with power NPN Darington such as TIP120. So there is no worry of using bulky heat sink. (3) I have been using the old IRF540N, so my new replacement is IRL540N. (4) I should have forgotten perhaps other important points. So you might like to skim my old posts on IRL540N and feel free ask me any newbie questions! 🙂 And here is my IRL540N Engineering Experimentation Notes: penzu.com/p/d9fef62f – tlfong01 27 mins ago
  • One thing that might puzzle you is that my goal is to turn on/off home heater of 200VAC/DC (or 10A 2000W), but I am using only low voltage 12VDC (and very low resistor) to switch high current. As I said, what is important is the current, never mind the voltage 200VDC or 12VDC. So 12VDC is good for experimentation. – tlfong01 17 mins ago
.END

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