Motorola M68HC08 Reference Manual page 37

DRM028 — Rev 0
MOTOROLA
Freescale Semiconductor, Inc.
+DC_BUS PHASE A PHASE B PHASE C
560k 560k 560k
560k 560k 560k
560k 560k 560k
1n
2x27k
2x27k 2x27k
1n
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
BLDC Motor Control
For More Information On This Product,
Go to: www.freescale.com
Brushless DC Motor Control Theory
1n
1n
2x27k
Figure 3-13.
BLDC Motor Control
+
–
+
MUX
–
CROSSING
DETECTION
+
–
MUX COMMAND
Designer Reference Manual
ZERO
SIGNAL
37