Power MOSFET

Power MOSFETs are voltage controlled majority carrier devices designed for high frequency switching. They are the primary choice for SMPS units and low voltage motor drives.

Core Concept

A positive voltage on the gate creates an inversion layer channel connecting the drain and source. Because it is a majority carrier device there is no minority carrier storage time allowing fast switching at 100 kHz.

Devices like the IRF540N have an intrinsic body diode built into the structure. This diode is useful as a freewheeling path in bridge inverter circuits.

The on state resistance Rds on determines the conduction losses. High voltage MOSFETs suffer from higher Rds on due to a thicker drift layer limiting their use in high voltage applications.

Key Formulas

Conduction loss Pcond equals RMS drain current squared multiplied by Rds on. Switching loss increases linearly with switching frequency fsw.

Given: Id rms = 10 A, Rds on = 0.05 ohms
Formula: Pcond = Id^2 * Rds on
Steps: Pcond = 100 * 0.05
Final Answer: 5 W

Exam Tip: Power MOSFETs have a positive temperature coefficient of resistance. This makes them easy to parallel because they inherently share current without thermal runaway.

Performance Parameters

• Switching frequency: 50 kHz to 1 MHz
• Rds on: 5 milliohms to 1 ohm
• Voltage rating: up to 1000 V
• Gate threshold voltage: 2 V to 4 V
• Input capacitance Ciss: 1000 pF typical

Quick Revision

• Voltage controlled device
• Extremely fast switching speeds
• Significant Rds on at high voltages
• Integral body diode included
• Positive temp coefficient allows paralleling
• Exam trap: Confusing MOSFET positive temp coefficient with BJT negative temp coefficient