This document is an application note for the ST72141 chip. It introduces the back EMF detection algorithm of the ST72141 chip. This algorithm uses the on-time of the PWM to detect the back EMF. It solves the problem of the long settling time of the parasitic resonant between the motor inductance and the parasitic capacitance of power devices in the ST72141 sensorless drive system at high speed.
This document analyzes the resonant transient that occurs during the PWM off time in sensorless BLDC motor drives. It explains that parasitic capacitance and inductance can cause false zero-crossing detection, limiting the maximum PWM duty cycle. The note details why the standard direct back EMF sensing scheme fails at high speeds due to this settling time.
The long settling time of a parasitic resonant between the motor inductance and the parasitic capacitance of power devices causes false zero crossing detection.
The ST7MC detects the back EMF during the PWM on time instead of the off time to allow the resonant transient to settle down.
The signal is attenuated by the resistor ratio, and the microcontroller uses an external attenuated DC bus voltage as the reference.