AN2030

Rev 2

1/13

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.

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