How to select a MOSFET for an automotive water pump

1. Voltage margin comes first

A nominal 12 V system is not a 12 V environment. Jump start can push the rail to roughly 26 V, and load dump plus inductive switching from other loads on the same harness produces short transients well above that.

2. Peak current, not nominal current

A pump draws several times its running current at start-up, and blocked-rotor current is higher again. The continuous rating in the datasheet headline is a thermal statement at a defined case temperature — it says little about the start transient.

• Measure start-up current at the lowest ambient temperature you must operate at — winding resistance falls and current rises.
• Check the pulsed drain current rating and the safe operating area (SOA) curve, not just the continuous figure.
• Design the protection trip below the point where the MOSFET leaves its SOA.

3. R_DS(on) at temperature — and a worked example

R_DS(on) roughly doubles between 25 °C and 150 °C. Sizing conduction loss with the 25 °C value understates the loss and the resulting housing temperature.

Example: 10 A continuous, RDS(on)(25 °C) = 8 mΩ, ratio at 150 °C ≈ 1.9
R(150 °C) ≈ 15.2 mΩ  →  P = I² × R = 100 × 0.0152 ≈ 1.52 W per device
Two devices conducting at a time: ≈ 3.0 W to remove from the housing

4. Avalanche and switching behaviour

Motor windings store energy that appears across the switch at turn-off. Check the single-pulse avalanche energy E_AS, and whether the datasheet allows repetitive avalanche — many parts specify it only as a single event.

• Gate charge Q_g sets driver loss: P_drive ≈ Q_g × V_GS × f_PWM.
• Faster switching lowers switching loss but raises EMI and drain ringing.
• Set the gate resistor from measured ringing, not from a rule of thumb.

5. Qualification and common mistakes

Discrete MOSFETs are qualified to AEC-Q101; AEC-Q100 applies to integrated circuits. A part listed as “automotive” in a distributor search is not automatically qualified — check the manufacturer's datasheet ordering information.

• Mistake: sizing from 25 °C R_DS(on) and nominal current.
• Mistake: assuming a similar industrial part is interchangeable.
• Mistake: ignoring package thermal resistance — the same die in a larger package often runs cooler for less money than a premium die.

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