Applications of MOSFETs in Power Circuits | ECExplain

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Concept overview

A MOSFET in a power circuit operates as a voltage-controlled switch or a linear pass element. The gate voltage controls drain-source conduction without drawing steady-state gate current, which allows driving circuits to be simple and efficient. In switching applications, the MOSFET transitions between cut-off and triode regions rapidly, with on-state power dissipation determined by I_D^2 * R_DS(on). In linear regulators, it operates in saturation where V_DS is controlled to drop excess supply voltage. Modern power MOSFETs are optimised for low R_DS(on), fast switching, and high avalanche energy rating.

Real-world applications

Synchronous Buck Converter in Laptop Voltage Regulator Module

Consumer Electronics

Intel Evo laptops use a synchronous buck VRM supplying 1.05 V at up to 45 A from a 19 V adapter. The high-side and low-side switches use Infineon OptiMOS-5 BSZ0901NS MOSFETs with R_DS(on) = 0.9 mohm. Switching at 600 kHz keeps inductor size below 300 nH while maintaining over 95% efficiency at peak load.

Three-Phase Inverter in EV Traction Drive

Electric Vehicles

The Hyundai Ioniq 6 traction inverter uses Infineon CoolSiC 1200 V silicon carbide MOSFETs in a six-switch three-phase bridge. SiC MOSFETs switch at 20 kHz with less switching loss than equivalent silicon IGBTs, allowing a smaller heatsink and higher DC bus voltage of 800 V for faster charging compatibility.

Class-D Audio Amplifier Output Stage

Consumer Audio

The Texas Instruments TAS5760M integrates two N-channel MOSFETs in an H-bridge output stage switching at 384 kHz. The MOSFET R_DS(on) of 120 mohm sets the output resistance and limits efficiency loss at peak audio current. Dead-time control prevents shoot-through between the complementary gate drives.

High-Side Current Sensing with N-Channel MOSFET

Battery Management

In battery protection ICs such as the Seiko S-8261, an N-channel MOSFET is placed in the high-side current path. The IC monitors V_DS in the ohmic region to estimate current using R_DS(on), triggering an over-current cutoff when the estimated current exceeds a threshold. This avoids the insertion loss of a resistive shunt in the main current path.

How it works in practice

In a synchronous buck converter, the high-side MOSFET turns on when the gate driver pulls the gate 10 V above the switching node. Current flows from the input capacitor through the inductor to the load. When the high-side turns off, the inductor current freewheels through the low-side MOSFET turned on by a complementary gate signal, avoiding body diode conduction losses. The duty cycle D = V_out / V_in sets the on-time fraction. Gate driver ICs such as the Texas Instruments UCC27201 provide bootstrap-supplied gate drive for the high-side switch, level-shifting the logic signal from ground-referenced PWM controller output to the floating switch node reference. Dead time between high-side off and low-side on is set in nanoseconds to prevent both devices conducting simultaneously, which would short the input rail.

Examples

Calculating R_DS(on) Conduction Loss

A buck converter low-side MOSFET carries 20 A RMS with R_DS(on) = 2 mohm at 100 degrees C junction temperature. Conduction power loss = I^2 * R = 400 * 0.002 = 0.8 W. Over a 10-hour operation cycle the total energy dissipated is 28.8 kJ. If R_DS(on) were 5 mohm instead, loss rises to 2 W, requiring a heatsink with 10 degrees/W lower thermal resistance. This illustrates why low R_DS(on) selection directly reduces heatsink cost.

Gate Charge and Switching Speed

The Infineon IPD90N04S4L-04 MOSFET has a total gate charge Q_g = 25 nC at V_GS = 10 V. A gate driver sourcing 2 A peak current charges the gate in t = Q_g / I = 25e-9 / 2 = 12.5 ns. The Miller plateau extends this by approximately 10 ns at 400 V bus voltage. Total switching time is roughly 25 ns, allowing reliable 1 MHz switching. Reducing gate drive current to 0.5 A extends switching time to 50 ns, increasing switching losses by a factor of four.

Future scope

Wide bandgap devices using gallium nitride (GaN) are displacing silicon MOSFETs below 650 V in applications requiring high switching frequency and low gate charge. Navitas NV6137 GaN devices switch at 3 MHz in USB-C PD chargers, enabling transformer miniaturisation that would be impractical with silicon. Research into vertical GaN MOSFETs above 1200 V aims to challenge SiC dominance in EV traction applications by the late 2020s. Integrated GaN power stages combining gate driver, bootstrap supply, and protection logic on one die are entering production at Texas Instruments and Infineon.