Impact on audible noise

In motor-drive systems, one of the main sources of audible noise is torque ripple caused by current distortion. For a motor, current distortion depends on multiple factors, including the PWM frequency, dead time and current-sensing accuracy.

The DRV7308 offers significantly reduced switching losses, and enables higher PWM frequencies compared to IGBT- or MOSFET-based solutions. With a higher switching frequency, the reduced winding current ripple enables low torque ripple, beyond the audible range of frequency.

In IGBT- and MOSFET-based systems, the dead time is 1µs to 2μs or more, resulting in considerable motor current distortion. The dead-time distortion occurs at every 60-degree electrical angle and results in sixth harmonics on the current waveform, which typically falls in the audible frequency range. The DRV7308’s adaptive dead-time logic enables less than 200ns of dead time, resulting in minimal current distortion and thus lower audible noise.

Figure 7 compares the motor winding current total harmonic distortion (THD) when testing the DRV7308 with a 0.2μs dead time and an IGBT IPM with 2.5μs of dead time. The DRV7308 distortion is very low compared to the IGBT IPM. This distortion will go exponentially high for the IGBT IPM at a lower power delivery because of the low duty cycle or low inverter modulation index, where the impact of dead time is greater.