Motorola M68HC08 manual System Description, System Concept

Freescale Semiconductor, Inc.

Freescale Semiconductor, Inc.

Designer Reference Manual — Sensorless BLDC Motor Control

Section 2. System Description

2.1 Contents

2.2System Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.3System Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.2System Concept

The application block diagram is shown in Figure 2-1. The sensorless 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.

DRM028 — Rev 0		Designer Reference Manual
MOTOROLA	System Description	15
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