Further improvement of direct back EMF detection for sensorless brushless DC (BLDC) motor drives

The direct back EMF detection method previously described (J. Shao, et al., 2002) synchronously samples the motor back EMF during PWM "off" time without the need to sense or reconstruct the motor neutral in a sensorless BLDC motor drive system. Since this direct back EMF sensing scheme requires minimum PWM "off" time to sample the back EMF signal, the duty cycle can\´t reach 100%. Also in some applications, e.g. high inductance motors, the long settling time of a parasitic resonant between the motor inductance and the parasitic capacitance of power devices can cause false zero crossing detection of back EMF. In this paper, an improved direct back EMF detection scheme which samples the motor back EMF synchronously during PWM "on" time is proposed to overcome the problems. In the final system, the motor back EMF can be detected during PWM "off\´ time at start-up and low speed, and it is detected during PWM "on" time at high speed. With the combination of the original and the improved back EMF sensing scheme, the duty cycle can be extended to 100% duty cycle, and the impact of the parasitic resonant between motor inductance and the power device capacitance can be minimized. Experimental results verify the analysis