Advanced PWM Strategies: Dead-Time, Filtering, and Power Efficiency

PWM Techniques for Motor Control: Practical Tips and Examples

Control torque/speed: PWM varies average voltage to the motor by changing duty cycle, controlling speed and torque smoothly.
Efficient switching: Switching between full supply and 0V reduces dissipation vs. linear methods.
Compatibility: Works with brushed DC, brushless DC (BLDC via motor driver), stepper drivers, and some AC inverter stages.

Duty cycle (%) — proportion of ON time; directly controls average voltage.
Frequency (Hz) — how fast pulses repeat; affects audible noise, current ripple, and driver switching losses.
Resolution (bits) — timer granularity limits smallest duty-step and control smoothness.
Dead time — brief OFF interval between complementary switches to avoid shoot-through in H-bridges.

Brushed DC motors: 5–20 kHz avoids audible whine while keeping switching losses low. Lower frequencies (a few kHz) increase torque ripple and audible noise.
BLDC with ESCs: Often 8–32 kHz; depends on driver and MOSFET switching characteristics.
High-power/fast control: Higher frequencies (50–200 kHz) reduce current ripple but increase switching losses and require better MOSFETs and gate drivers.
Stepper motors: Match driver recommendations; microstepping drivers may use tens of kHz.

Low-side switching (single MOSFET): Simple for small DC motors; PWM on low side with diode for back-EMF. Example: Arduino PWM pin → MOSFET → motor → supply.
H-bridge (full control & regeneration): Two half-bridges allow forward/reverse and regenerative braking. Use complementary PWM with dead time. Example ICs: L298 (old), DRV8871 (modern low-side), half-bridge MOSFETs + gate drivers.
Synchronous rectification: Use complementary MOSFETs actively switching during OFF periods to reduce conduction losses—important for high efficiency.
Brushless DC (BLDC): PWM applied to inverter/ESC driving three phases; commutation timing is critical (sensor or sensorless). Space vector PWM (SVPWM) improves DC bus utilization vs. simple sinusoidal PWM.

Single-ended PWM: One transistor switching; simplest.
Complementary PWM: Opposite switches used for full-bridge control; include dead time.
Sine PWM / sinusoidal drive: Modulate phase voltages to approximate sine waves for smoother torque in BLDC/AC drives.
Space Vector PWM (SVPWM): Maximizes voltage utilization and reduces harmonic content—preferred in FOC/AC inverters.
PWM with carrier-phase-shifted multi-levels: Reduces EMI by spreading spectral energy.

LC/R filtering: Smooths PWM into near-DC for motors with significant inductance; many motors’ inductance plus winding resistance provide inherent filtering.
Current sensing & control loop: Implement PI/PID or torque-control loop around current measurement for fast, safe response and torque limiting.
Current sampling timing: Sample current during predictable steady portion of PWM (e.g., center of ON or OFF window) to avoid switching noise.

Dead time tuning: Set minimal dead time to prevent shoot-through but avoid excessive delay that distorts effective duty.
Desaturation and current limit: Use MOSFET desat detection or sense resistors