sobrinho:

Hi there,

I'm trying to understand how much a MOSFET will heat up given a load of 5V/3A DC.

My understanding so far is that formula:

P = RDS_{on} * I ^ 2

Yes, although if using PWM you have to consider switching losses too

T = P * R_{thJA} + TC

No, its power divided by thermal resistance that gives temperature difference. Perhaps you meant

thermal conductivity, not resistance?

**RDS**_{on}

My first question is how to interpret RDS_{on}.

Looking at IPP80P03P4L04AKSA1 datasheet it says that it will be around 4mΩ.

At logic levels (5V) the worst-case on-resistance is 7 milliohms. Always look down the table for the worst-case figure

at the gate drive voltage of interest. Any graph is for "typical device", don't use those for design, always

use the worst-case.

P = RDS_{on} * I ^ 2

P = 4 / 1000 * 3 ^ 2

P = 0.004 * 9

P = 0.036

T = P * R_{thJA} + TC

T = 0.036 * 62 + 25

T = 27.232ºC

This means a heat up of ~2ºC over TC assuming -5V of V_{GS} and TC of 25ºC.

Are that formula and calculations correctly?

3A is nothing for a low on-resistance like this - but yes, I-squared-R is the way to figure this out always.

**Alternatives**

What I'm curious about those calculations is that if they are correct, I'm not finding any P-Channel MOSFET for sale in Brazil that is below 0.200Ω.

p-channel FETs are both rarer and intrinsically 3 times worse in performance all else being equal - this is

a physical property of silicon.

One that is widely available here is the IRF9540 and by those formulas, it would heat up to 137,5º even that the datasheet says it supports up to 19A.

Never go anywhere near the maximum current rating of a MOSFET (except for short pulses). Its completely

infeasible as you'd need water-cooling to get there. Max current specification is defined by max power dissipation on

infinite heatsink... You only ever need to check the max current for pulse operation.

Some manufacturers lie about the figure too, quoting a max current rating higher than the package's

maximum current handling!!

It seems odd to the MOSFET heat almost to its maximum operating temperature with only 15% of its maximum load.

Yes, it catches people out - power MOSFETs are very different from BJTs