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BACK EMF DETECTION DURING PWM ON TIME BY ST7MC

2 BACK EMF DETECTION DURING PWM ON TIME

For ST7MC, it is possible to detect the back EMF during PWM on time. At high speed opera-

tion (high duty cycle), the controller can detect the back EMF at the end of PWM on time. In

this way, the controller avoids the resonant transition time. At low speed operation (low duty

cycle), the controller can detect the back EMF at the end of PWM off time. It still has the ben-

efit of no attenuation of back EMF.

First, we can derive the floating phase terminal winding voltage during PWM on time. Assume

phase A and B are conducting current, phase C is floating.

Figure 6. Circuit model during PWM on time

From phase A, we have

From phase B, we have

Voltage drop on power devices is ignored.

From (3) and (4),

L

r

r

r

L

L

Va

Vb

Vc

Vdc

GND

Vn

i

L

r

r

rr

L

L

Va

Vb

Vc

Vdc

GND

Vn

i

v

n

v

dc

ri

–

L

dt

dt

-----

–

e

a

–

=

(3)

v

n

ri

L

di

dt

----- e

–

b

+

=

(4)

v

n

v

dc

2

-------

e

a

e

b

+

2

----------------

–

=

(5)