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BACK EMF DETECTION DURING PWM ON TIME BY ST7MC

Also from the balance three-phase system, considering fundamental frequency only, we have

From (5) and (6),

So, the terminal voltage Vc,

During PWM on time, if the terminal voltage is compared to half of the dc voltage, the zero

crossing of the back EMF is able to be detected.

The following implementation is based on the ST7MC starter kit.

Figure 7. Hardware implementation

At low PWM duty cycle, PE1, PE2 and PE3 are configured as floating input. There is no atten-

uation for the signals since R4, R5 and R6 connect to the floating points. The microcontroller

detects the zero crossing of the back EMF during PWM off time. The reference voltage is in-

ternally set. Then at high duty cycle, PE1, PE2 and PE3 are re-configured as output, set to

logic low. All signals are then attenuated by resistor ratio. Meanwhile, the microcontroller will

use external voltage, attenuated dc bus voltage Vdc, as the reference for zero crossing detec-

tion.

In the starter kit, R43=R44=R49=R50=R53=R54=82k. We should select appropriate resistors

for different dc voltage, as shown in the following table.

e

a

e

b

e

c

++

0

=

(6)

v

n

v

dc

2

-------

e

c

2

----

+

=

(7)

v

c

e

c

v

+

n

3

2

---e

c

v

dc

2

-------

+

==

(8)

MC00

MC01

MC02

MC03

MC04

MC05

MCIC

MCIB

MCIA

ST7MC

motor

Power stage

Back EMF Sensing

PE1

PE2

PE3

MCVREF

Vdc

R43

R44

R8

R4

R5

R6

R7

R9

R49

R50

R53

R54