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Back-emf sensing circuit diagram – Motorola M68HC08 User Manual

Page 37

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BLDC Motor Control

Brushless DC Motor Control Theory

DRM028 — Rev 0

Designer Reference Manual

MOTOROLA

BLDC Motor Control

37

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

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+

+

MUX COMMAND

ZERO

CROSSING

DETECTION

SIGNAL

560k

1n

2x27k

MUX

560k

560k

560k

560k

560k

560k

560k

560k

1n

1n

1n

2x27k

2x27k

2x27k

+DC_BUS

PHASE A

PHASE B

PHASE C

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Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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