AN1627 Freescale Semiconductor / Motorola, AN1627 Datasheet - Page 5

• Manufacturer Part Number: AN1627
• Description: Low Cost High Efficiency Sensorless Drive for Brushless dc Motor Using MC68HC (7)05MC4
• Manufacturer: Freescale Semiconductor / Motorola

It is important to understand how the Back-EMF can be sensed and how the motor behaviour depends
on the alignment of the Back-EMF to commutation events. This is explained in the next sections.
2.2
The Back-EMF sensing technique is based on the fact that only two phases of a DC Brushless motor
are connected at a time (see Figure 1-1.), so the third phase can be used to sense the Back-EMF
voltage.
Let us assume the situation when phases A and B are powered and phase C is free. No current is going
through this phase. This is described by the following conditions:
The branch voltage C can be calculated when considering the above conditions:
It is seen from the Figure 2-1. that the branch voltage of phase C can be sensed between the power
stage output C and the zero voltage level. Thus the Back-EMF voltage is obtained and the zero crossing
can be recognized.
The same expressions can also be found for phase A and B:
There are two necessary conditions which have to be met -
The Figure 2-2. shows branch and motor phase winding voltages during a 0-360°electrical interval.
Shaded rectangles designate the validity of the equation (EQ 2-6.). In other words, the Back-EMF
voltage can be sensed during designated intervals.
AN1627
REV 0.2
•
•
Back-EMF Sensing
Top and bottom switch (in diagonal) have to be driven with the same PWM signal
No current is going through the phase used to sense the Back-EMF
0
30
Freescale Semiconductor, Inc.
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For More Information On This Product,
Figure 2-2.
90
S
u
i
u
A
Ab
VA
iA
u
120
Go to: www.freescale.com

Vx


S

u
Bt
i
u

 
B
150
i B
VC


-- - u


-- - u

i

C
Phase Voltage Waveforms
d

PWM
u
i x

180
i C
u


-- - u

VB


x
d
i C

210

i

C
A B C 




-- -


u
240

d
270
300
330
360
uVA
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390
MOTOROLA
(EQ 2-4.)
(EQ 2-5.)
(EQ 2-6.)
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