MOSFET

The power MOSFET is the fast, voltage-driven switch inside most converters below ~600 V. Learn threshold and R_DS(on), conduction and switching losses, gate charge and drivers, the body diode, SOA and thermal design.

A power MOSFET is a transistor switched by the voltage between its gate and source. With enough gate voltage (typically 10 V, or 4.5 V for logic-level parts) the drain–source channel behaves like a small resistor, R_DS(on). With the gate at 0 V it blocks.

Because the gate is insulated, it draws no steady current — only brief charging current when switching. That makes MOSFETs extremely fast (tens of nanoseconds) and ideal for high-frequency converters, motor controllers and load switches.

An analogy

A MOSFET is a water valve operated by pressure on a diaphragm (gate voltage) rather than by turning a handle. Filling the diaphragm takes a little water (gate charge), but holding it open takes none.

A voltage-controlled switch

Below V_GS(th) (≈ 2–4 V) the MOSFET is off. Well above it, the channel is fully enhanced and the device acts as R_DS(on). Using it between those states (the linear region) dissipates lots of power — switches must be driven hard on or fully off.

N-channel vs P-channel, low-side vs high-side

N-channel MOSFETs have lower R_DS(on) for the same size and are used most. As a low-side switch (source to ground), a logic signal can drive them directly. As a high-side switch, the gate must be driven above the supply rail — using a bootstrap or charge-pump driver — or a P-channel device is used.

Losses

Conduction loss I²R_DS(on) dominates at low frequency and high current. Switching loss — the overlap of voltage and current during each transition — grows linearly with frequency. The design balances them: higher f_sw means smaller magnetics but more switching loss.

Gate charge and drivers

The gate is a capacitor network (C_gs, C_gd). Switching in time t requires current I ≈ Q_g/t — often amps, far beyond an MCU pin. Gate drivers supply this current; a gate resistor tunes switching speed against ringing and EMI. The Miller plateau (charging C_gd) is where the drain voltage actually swings.

Body diode, SOA and avalanche

Every MOSFET has an intrinsic body diode from source to drain, which conducts during dead time in bridges; its reverse recovery causes loss and ringing. The safe operating area limits simultaneous V and I in the linear region (important for hot-swap and linear regulators). Avalanche ratings describe how much inductive-kick energy it can absorb.

Wide-bandgap devices

SiC MOSFETs (650–3300 V) and GaN HEMTs (≤ 650 V) switch much faster with lower losses than silicon. GaN enables MHz chargers the size of a matchbox; SiC dominates EV traction inverters and fast chargers. They demand careful layout (low loop inductance) and precise gate drive.

Formulas

• Conduction loss: P_cond = I_rms² · R_DS(on)
• Switching loss (approx.): P_sw ≈ ½ · V · I · (t_r + t_f) · f_sw
• Gate-drive power: P_gate = Q_g · V_GS · f_sw
• Junction temperature: T_j = T_a + P_total · R_θJA

Worked example: Loss budget at 100 kHz

Given: I = 10 A (DC, full duty); R_DS(on) = 10 mΩ; V = 48 V; t_r + t_f = 40 ns; f = 100 kHz

Find: Conduction, switching and total loss.

• P_cond = 10² × 0.01 = 1.0 W
• P_sw = 0.5 × 48 × 10 × 40e-9 × 100e3 = 0.96 W
• Total ≈ 1.96 W

Answer: ≈ 2 W

Worked example: Gate driver sizing

Given: Q_g = 40 nC; Target switching time 20 ns; V_GS = 12 V; f = 100 kHz

Find: Peak gate current and gate-drive power.

• I_g ≈ Q_g/t = 40e-9/20e-9 = 2 A
• P_gate = 40e-9 × 12 × 100e3 = 48 mW

Answer: 2 A peak driver, 48 mW

Common mistakes

• Wrong: Driving a standard MOSFET's gate from a 3.3 V MCU pin. — Right: Use a logic-level MOSFET specified at V_GS = 2.5–4.5 V, or a gate driver; otherwise it sits in the linear region and overheats.
• Wrong: Using R_DS(on) at 25 °C for thermal design. — Right: R_DS(on) rises ~1.5–2× at 100–125 °C; design with the hot value.

Applications

• DC–DC converters: Synchronous buck converters on motherboards use MOSFETs switching at 300 kHz–1 MHz to feed CPUs at 1 V and hundreds of amps.
• Battery systems: Back-to-back MOSFETs form battery-protection and load switches with milliohm resistance.
• Motor controllers: Drones, e-bikes and power tools use MOSFET three-phase bridges driven with PWM.

Key takeaways

• MOSFETs are voltage-driven switches: fully on (R_DS(on)) or fully off.
• P = I²R_DS(on) + ½VI(t_r + t_f)f_sw.
• Gate drivers supply amps of charging current; logic-level parts for 3.3–5 V drive.
• Design thermally with hot R_DS(on); GaN and SiC push speed and efficiency further.

Going further

• Figure of merit: R_DS(on) × Q_g compares devices: lower means less combined conduction and switching loss. Trench and superjunction MOSFETs, GaN and SiC each push this frontier.
• Parallel MOSFETs: MOSFETs share current well when fully on because R_DS(on) rises with temperature (positive tempco), but give each its own gate resistor to prevent oscillation, and avoid paralleling in linear mode.
• Synchronous rectification: Replacing a diode (0.5–1 V drop) with a MOSFET (I·R_DS(on)) cuts rectifier loss dramatically in low-voltage converters, at the cost of timing control to avoid shoot-through.