MOSFETs In-Depth

Complete guide to power switching - gate drive, thermal design, and high-efficiency applications.

The Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is a voltage-controlled switch that has revolutionized power electronics. Unlike BJTs that require continuous base current, MOSFETs are controlled by voltage at the gate terminal, drawing essentially zero DC current. This makes them ideal for microcontroller interfacing, high-efficiency power conversion, and high-frequency switching.

Objectives

  • Understand MOSFET structure, operation, and the role of the gate oxide
  • Distinguish enhancement vs depletion mode, N-channel vs P-channel
  • Read and interpret MOSFET datasheets (Vth, RDS(on), gate charge)
  • Design gate drive circuits for logic-level and standard MOSFETs
  • Understand parasitic capacitances and their effect on switching speed
  • Calculate switching losses and thermal requirements
  • Design high-side and low-side switching circuits
  • Implement proper gate drive for P-channel MOSFETs
  • Use gate drivers for high-speed and high-side applications
  • Select appropriate MOSFETs for specific applications
  • Salvage and test MOSFETs safely

Key Takeaways

  • MOSFETs are voltage-controlled: Vgs > Vth turns ON
  • Logic-level MOSFETs can be driven directly by 3.3V/5V microcontrollers
  • Standard MOSFETs need 10V+ gate drive (use gate driver ICs)
  • RDS(on) determines conduction loss: P = I² × RDS(on)
  • Gate charge determines switching speed and gate drive power
  • N-channel for low-side; P-channel for high-side switching
  • High-side N-channel requires bootstrap or charge pump
  • Always use flyback diodes with inductive loads
  • Handle MOSFETs as ESD-sensitive devices
  • Positive temperature coefficient makes paralleling safe

Applications

  • Low-Side LED/Motor Driver: N-channel switches load to ground
  • High-Side P-Channel Switch: P-channel with level shifter controls positive side
  • PWM Motor Speed Control: Fast switching with flyback protection
  • H-Bridge Motor Control: Four MOSFETs for bidirectional control
  • Synchronous Rectification: MOSFETs replace diodes for higher efficiency
  • Buck/Boost Converters: Switch-mode power conversion
  • Electronic Load: MOSFET in linear region for constant current

Practice Problems

Problem 1: A MOSFET with RDS(on)=15mΩ switches 8A at 50% duty cycle. Calculate conduction loss.

Problem 2: A MOSFET has Qg=70nC and switches at 200kHz with Vgs=10V. Calculate gate drive power.

Problem 3: Select heat sink for 5W dissipation. Tj(max)=150°C, Ta=50°C, θjc=0.6°C/W, θcs=0.4°C/W.