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BACK EMF DETECTION DURING PWM ON TIME BY ST7MC

3 CONCLUSION

The original direct back EMF sensing scheme has the limitation of duty cycle since it requires

minimum PWM off time to do the detection. The resonant transient caused by motor induct-

ance and power devices' parasitic capacitance will further limit the duty cycle. The improved

direct back EMF sensing scheme, which does the back EMF sensing during PWM on time,

eliminates the duty cycle limitation. It can run at 100% duty cycle, and avoid the parasitic res-

onant transient. During motor start-up and low speed, it is preferable to use the original

scheme since there is no signal attenuation; while at high speed, the system can switched to

the improved back EMF sensing scheme. With the combination of two detection schemes in

one system, the motor can run very well over a wide speed range.

4 REFERENCE

[1] [J.Shao, D.Nolan, and T.Hopkins, “A Novel Direct Back EMF Detection for Sensorless

Brushless DC (BLDC) Motor Drives,” Applied Power Electronic Conference (APEC 2002),

pp33-38.