AN2030 Datasheet, PDF (1/13 Pages) STMicroelectronics – BACK EMF DETECTION DURING PWM ON TIME BY ST7MC

AN2030
APPLICATION NOTE
BACK EMF DETECTION
DURING PWM ON TIME BY ST7MC
INTRODUCTION
The direct back EMF sensing scheme used by ST72141 synchronously samples the motor
back EMF during PWM “off” time without the need to sense or re-construct the motor neutral
in a sensorless BLDC motor drive system. Since this direct back EMF sensing scheme re-
quires minimum PWM “off” time to sample the back EMF signal, the duty cycle can't reach
100%. Also in some applications, i.e. HVAC using high inductance motors, we see the zero
crossing detection is unsymmetrical in the ST72141 sensorless drive system at high speed. It
is found that the long settling time of a parasitic resonant between the motor inductance and
the parasitic capacitance of power devices causes false zero crossing detection of back EMF.
This application note provides an analysis of the resonant transient during PWM “off” time. As
a result, the back EMF detection during PWM “on” time is used in ST7MC to solve the
problem.
AN2030
Rev 2
1/13
1