A sensorless brushless DC motor senses motor position for the purpose of commutation timing by using the voltage induced in the unused winding at the time. Reliably sensing the motor position for a range of motor constructions and speeds proves a challenge.
Continue reading Some thoughts on zero crossing detection in sensorless brushless DC motors
COMP_PWM is an option on SimonK ESC firmware for sensorless brushless DC motors,
Above is a capture of the waveform during two commutation steps where this FET is low side modulated. You can see that when the low side FET turns off, voltage rapidly rises to just above battery as the energy stored in the winding inductance drives current through the high side FET body diode (which drops 0.7+V) and the other high side FET that is on.