Drive Circuitry & Power Stages on Embedded Systems Development

MOSFET Selection & Gate Drive

Mon, 01 Jan 0001 00:00:00 +0000

MOSFET Selection & Gate Drive#

The MOSFET is the workhorse switch in every motor drive, solenoid circuit, and power stage covered in this section. Selecting the right MOSFET means matching its voltage, current, and thermal ratings to the load — and then ensuring the gate drive circuit can switch it on and off fast enough to keep switching losses low. A MOSFET that looks perfect on the datasheet can fail in practice if the gate is driven too slowly, the thermal path is inadequate, or the parasitic inductance in the layout causes voltage spikes.

Flyback & Snubber Protection

Mon, 01 Jan 0001 00:00:00 +0000

Flyback & Snubber Protection#

Every inductive load in this section — motors, solenoids, relays, and actuators — stores energy in a magnetic field. When the current through the inductance is interrupted (FET turns off, relay opens), the stored energy (E = ½LI²) must go somewhere. Without a designed dissipation path, the inductance forces the voltage across the switch to spike upward until it finds a path: avalanche breakdown of the MOSFET, arcing across relay contacts, or conduction through parasitic paths. Protection circuits provide a controlled energy dissipation path that clamps the voltage to a safe level.

Current Sensing Techniques

Mon, 01 Jan 0001 00:00:00 +0000

Current Sensing Techniques#

Measuring the current flowing through a motor, solenoid, or actuator serves three purposes: protection (overcurrent shutdown), regulation (current-loop control), and diagnostics (load monitoring, stall detection). The choice of sensing method depends on the current range, accuracy requirements, bandwidth, and whether the measurement is on the low side (ground-referenced) or high side (supply-referenced) of the load.

Low-Side Shunt Sensing#

A low-value resistor between the load and ground measures current as a ground-referenced voltage:

Thermal Management for Drives

Mon, 01 Jan 0001 00:00:00 +0000

Thermal Management for Drives#

Power MOSFETs, motor driver ICs, and shunt resistors in motor drive circuits dissipate significant heat. A MOSFET switching a 5 A motor at 20 kHz with 20 mΩ RDS(on) dissipates 0.5 W in conduction alone — add switching losses and the total can reach 1–3 W per device. In an H-bridge with four FETs, that is 2–12 W of heat in a small area. Without adequate thermal design, junction temperatures rise until the device derate, malfunction, or fail. Thermal management is not an afterthought — it determines the practical current limit of the drive circuit.

Power Layout & Decoupling

Mon, 01 Jan 0001 00:00:00 +0000

Power Layout & Decoupling#

Motor drive circuits switch amps of current at tens of kHz through inductive loads — the resulting high dI/dt produces voltage transients, ground bounce, and radiated EMI that can reset the MCU, corrupt ADC readings, and interfere with communication buses. The PCB layout of the power stage is as important as the schematic. A motor drive that works on a breadboard may fail on a PCB with poor layout, and vice versa — the difference is parasitic inductance, ground impedance, and capacitor placement.