AN1946 datasheet(31/35 Pages) STMICROELECTRONICS

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AN1946

Figure 28. Test result shows the zero-crossing is unsymmetrical

The wrong zero detection will cause wrong commutation which probably will stall the motor.

Also bad zero-crossing will cause bad speed regulation.

(b). At low voltage or low speed, the zero-crossing slope is very flat. The offset and the hyster-

esis loop of the comparator will cause the bad zero-crossing detection. Meanwhile, because of

the low Back EMF, it is very susceptible to the noise.

10.2.1 Solution

To solve the first problem, we need to eliminate the effect of diode voltage drop. Adding an-

other constant voltage before the voltage signal is sent to the comparator can eliminate the ef-

fect of the diode.

To solve the second problem, we need to sharpen the slope of the Back EMF during the zero

crossing period. We can use an amplifier to amply the Back EMF signal only around zero

crossing time because we are not interested in other time periods.

Figure 29 shows the solution.

Figure 29. Solution to improve BACK EMF detection

ero-crossingsignal

Va,b,c

15V

TS274

GND

Vcon

Va,b,c

15V

TS274

GND

Vcon

AN1946 Datasheet(PDF) 31 Page - STMicroelectronics