AN1914 Freescale Semiconductor / Motorola, AN1914 Datasheet - Page 18

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

Control Technique
5.3 BEMF Zero Crossing Sensing
5.3.1 BEMF Zero Crossing Checking
5.3.2 BEMF Zero Crossing Synchronization with PWM
18
Figure 5-3
arrow lines. This technique is called bipolar (hard) switching. The voltage across the two connected
coils is always ±DC bus voltage whenever there is a current flowing through these coils. Thus the
condition for successful BEMF Zero Crossing sensing is fulfilled as described in
The BEMF Zero Crossing of the 3 phases is checked using hardware comparators as described in
Section 3.
filtration block is used to filter the spike on the Zero Crossing signals.
The software selects the “free” phase at each commutation step and reads the filtered signal to detect
the BEMF Zero Crossing event.
The power stage PWM switching causes the high voltage transient of the phase voltages. This transient
is passed to “free” phase due to mutual capacitor between the motor windings coupling.
shows that free phase “branch” voltage U
voltage U
The non-fed phase “branch” voltage U
BLDC Motor Control application synchronizes the BEMF Zero Crossing detection with PWM.
vb
The outputs of the comparators are led to Quadrature Decoder Inputs. Where the digital
shows that the diagonal power switches are driven by the same PWM signal as shown with
.
Figure 5-4. BEMF Zero Crossing Synchronization with PWM
DSP56F80x MC PWM Module in Motor Control Applications
Freescale Semiconductor, Inc.
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va
va
is disturbed at the PWM edges. Therefore the presented
is disturbed by PWM voltage shown on phase “branch”
u
u
Zero Crossing Sample
s/w flag
va
vb
Section 3.
Figure 5-4