SLLSFH8B August 2021 – February 2022 MCT8316A
Figure 8-35 shows the three-phase voltages during 120o trapezoidal operation. It is seen that one of the phases will always be floating within a 60o commuation interval and MCT8316A integrates this floating phase voltage (which denotes the motor back-EMF) in the back-EMF integration method to detect the next commutation instant. The floating phase voltage can either be increasing or decreasing and the algorithm starts the integration after the zero cross detection in order to eliminate integration errors due to variable degauss time. The floating phase voltage is periodically sampled (after zero cross) and added (discrete form of integration). BEMF threshold (BEMF_THRESHOLD1 and BEMF_THRESHOLD2) value is set such that the integral value of the floating phase voltage crosses the BEMF_THRESHOLD1 or BEMF_THRESHOLD2 value at (or very near) to the commutation instant. BEMF_THRESHOLD1 is the threshold for rising floating phase voltage and BEMF_THRESHOLD2 is the threshold for falling floating phase voltage. If BEMF_THRESHOLD2 is set to 0, then BEMF_THRESHOLD1 is used as the threshold for both rising and falling floating phase voltage.
In Figure 8-35 , Vpeak is the peak-peak value of the back-EMF , Vpeak/2 denotes the zero cross of the back-EMF and Tc is the commutation interval or time period of the 60o window. The highlighted triangle in each 60o window is the integral value of back-EMF used by the algorithm to determine the commutation instant. This integral value, which can be approximated as the area of the highlighted triangle, is given by Equation 8.
See for an example application on setting the BEMF threshold.