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BACK EMF DETECTION DURING PWM ON TIME BY ST7MC

will have some transition time during PWM off time. Figure 2 shows the circuit where the PWM

is applied to phases A and B while phase C is floating.

Figure 2. Phase C is floating

We can simplify the circuit by using the neutral voltage Vn=1/2 * ec during PWM off time [1]

and during PWM on time (see next chapter) to get the equivalent circuit in Figure 3.

Figure 3. Simplified equivalent circuit when phase C is floating

When PWM is on in phase A and B, the terminal voltage in steady state will be:

which is the initial condition in the capacitor during PWM off time.

L

r

r

r

L

L

Va

Vb

Vc

Vdc

GND

Vn

r

L

Vc

2

3

v

c

1

2

---v

dc

3

2

---e

c

+

=

(2)