AN1914 Freescale Semiconductor / Motorola, AN1914 Datasheet - Page 11

• Manufacturer Part Number: AN1914
• Description: 3-Phase BLDC Motor Control with Sensorless Back EMF Zero Crossing Detection Using DSP56F80x
• Manufacturer: Freescale Semiconductor / Motorola

3.6 Back-EMF Sensing Circuit
.
An example of the possible implementation of the Back-EMF sensing circuit is shown in
As explained in the theoretical part of this application note, 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 divide sensed voltages down to a 0-15V voltage level. The
comparators sense the zero voltage difference of 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 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 3-8
+DC_Bus Phase A Phase B Phase C
.
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DSP56F80x MC PWM Module in Motor Control Applications
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Figure 3-7. Back-EMF Sensing Circuit Diagram
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Freescale Semiconductor, Inc.
Figure 3-6. Phase Voltage Waveforms
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MUX Command
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MUX
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uVA
uA
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Zero Crossing
Detection signal
Target Motor Theory
Figure
3-7.
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