MOSFET Conduction Loss & Heatsink Calculator

Understanding the MOSFET conduction loss calculator — concept and principle

When a MOSFET is fully on, it behaves like a small resistor, R_DS(on), and the load current flowing through it produces heat. This calculator finds that conduction loss and the resulting junction temperature, so you can tell whether the part can run without a heatsink or needs one in motor drivers, load switches, battery protection and DC power circuits.

1.5 W of heat lifts the junction 60 °C above the 25 °C air, to 85 °C.

How it is calculated

Conduction loss: P = I² × R_DS(on), with R_DS(on) converted from mΩ to Ω.
Junction temperature: T_j = T_a + P × R_θJA.

I is the RMS drain current, T_a the ambient temperature and R_θJA the junction-to-ambient thermal resistance in °C/W, which depends on the package and how it is mounted (bare, on copper area, or on a heatsink).

The principle

Power in a resistance is I²R, so conduction loss grows with the square of current: double the current and the heat quadruples. Heat flows from the silicon junction to the surrounding air through a chain of thermal resistances, much like current through resistors in series. Each watt raises the junction temperature by R_θJA degrees above ambient. With a heatsink, R_θJA is replaced by the sum R_θJC + R_θCS + R_θSA (junction to case, case to sink, sink to air). R_DS(on) itself rises with temperature, often by around 50% at 100 °C, so the hot loss is higher than the room-temperature figure.

Worked example

R_DS(on) = 15 mΩ = 0.015 Ω.
P = 10² × 0.015 = 1.5 W.
Temperature rise: 1.5 W × 40 °C/W = 60 °C.
T_j = 25 + 60 = 85 °C.

Each point is the calculator re-run with only “Drain current (A)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Drain current (A)	Conduction loss (W)
2.5	0.09375
5	0.375
7.5	0.84375
10	1.5
12.5	2.34375
15	3.375
20	6

How to read the result

Most silicon MOSFETs are rated for a maximum junction temperature of 150 °C or 175 °C; designers usually keep a margin of 20 to 30 °C or more below that. At 85 °C the part is comfortable, but R_DS(on) at that temperature is higher than 15 mΩ; repeating the calculation with a hotter value, for example 1.5 × 15 mΩ, gives 2.25 W and about 115 °C. If T_j approaches the limit, lower R_DS(on), add copper area or a heatsink, or parallel devices.

Common mistakes

Using the datasheet's typical R_DS(on) at 25 °C instead of the maximum at operating temperature.
Using R_DS(on) measured at 10 V gate drive when the gate is driven at 4.5 V or less.
Ignoring switching losses in PWM circuits, which can exceed conduction loss at high frequency.
Using peak or average current instead of RMS current.

Keep in mind: This estimate covers conduction loss only and assumes steady state. Real thermal design must include switching losses, transient thermal impedance and the manufacturer's ratings, and should be confirmed by measurement.

Frequently asked questions

Do I need a heatsink for 1.5 W?

It depends on the package: a TO-220 in free air is roughly 60 °C/W, so about 1 to 2 W is a practical limit without a heatsink.

Why does R_DS(on) increase with temperature?

Carrier mobility in silicon falls as temperature rises, which increases channel resistance.

Can I parallel MOSFETs to cut losses?

Yes. Two identical devices each carry half the current, so total conduction loss halves, and the positive temperature coefficient helps share current.

Sources: Infineon / International Rectifier application note AN-1084, Power MOSFET Basics · JEDEC JESD51 series, thermal measurement standards for semiconductor packages

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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