Loss and heat: calculating as far as the heat sink

Add conduction and switching losses and calculate the temperature rise from thermal resistance. The steps for checking that a MOSFET can take it.

Loss calculation

Conduction loss: P_cond = I_rms² × Rds(on) (hot)

Rds(on) rises to 1.5 to 2 times as the temperature goes up. Use the value near 100 °C, not the 25 °C figure in the datasheet.

Switching loss: P_sw ≈ ½ × V × I × (t_rise + t_fall) × fsw

A larger gate-drive current shortens the transitions and reduces the loss, but increases ringing and EMI.

Temperature calculation

Heat leaves from the junction to the air through thermal resistances. The temperature rises by loss times thermal resistance.

Tj = Ta + P × Rθ

Add up the resistances along the path the heat takes: junction → case (RθJC) → thermal pad (RθCS) → heat sink → air (RθSA).

Example: P = 3 W, Ta = 40 °C, total thermal resistance 25 °C/W → Tj = 115 °C. There is margin against a maximum rating of 150 to 175 °C.

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Cooling through the PCB

To remove heat through the PCB without a heat sink, lay wide copper under the MOSFET pad and pack in thermal vias to carry the heat to the opposite layer. 2 oz copper and a 4-layer board help a great deal.

Verification

After the calculation, always run a long test at maximum load with a thermal camera or thermocouple. Also allow for the ambient temperature rising a lot inside a sealed enclosure.