Motorola M68HC08 manual Inc

Freescale Semiconductor, Inc.

BLDC Motor Control

Brushless DC Motor Control Theory

Inc.

+DC_BUS	PHASE A		PHASE B		PHASE C
560k		560k		560k
560k		560k		560k			+
							–
560k		560k		560k
			1n	1n			+
2x27k						1n
							–
	1n	2x27k		2x27k		2x27k
							+
							–

MUX

ZERO

CROSSING

DETECTION

SIGNAL

MUX COMMAND

Freescale Semiconductor,

Figure 3-12. Back-EMF Sensing Circuit Diagram

As explained in the theoretical part of this designer reference manual, the phase zero crossing event can be detected at the moment when the branch voltage (of a free phase) crosses the half dc-bus voltage level. The resistor network is used to step down sensed voltages down to a 0–15 V voltage level. The comparators sense the zero voltage difference in the input signal. The multiple resistors reduce the voltage across each resistor component to an acceptable level. A simple RC filter prevents the comparators from being disturbed by high voltage spikes produced by IGBT switching. The multiplexer (MUX) selects the phase comparator output, which corresponds to the current commutation stage. This zero crossing detection signal is transferred to the timer input pin.

The comparator control and zero crossing signals plus the voltage waveforms are shown in Figure 3-13.

DRM028 — Rev 0		Designer Reference Manual
MOTOROLA	BLDC Motor Control	37
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