rotor position technique detects the zero crossing points of back-EMF
induced in the motor windings. The phase back-EMF zero crossing
points are sensed while one of the three phase windings is not powered.
The information obtained is processed in order to commutate the
energized phase pair and control the phase voltage, using pulse width
modulation.

The back-EMF zero crossing detection enables position recognition. The
resistor network is used to step down sensed voltages to a 0–3.3 V level.
Zero crossing detection is synchronized with the middle of center aligned
PWM signals by the software, in order to filter high voltage spikes
produced by switching the IGBTs (MOSFETs). The software selects by
MUX command the phase comparator output that corresponds to the
current commutation step. The multiplexer (MUX) circuit selects this
signal, which is then transferred to the MCU input.

The voltage drop resistor is used to measure the dc-bus current which is
chopped by the pulse-width modulator (PWM). The signal obtained is
rectified and amplified (0–3.3 V with 1.65 V offset). The internal MCU
analog-to-digital (A/D) converter and zero crossing detection are
synchronized with the PWM signal. This synchronization avoids spikes
when the IGBTs (or MOSFETs) are switched and simplifies the electric
circuit.

Freescale Semiconductor, Inc.

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