AN3112

Designing a fixed-off-time PFC

Doc ID 16820 Rev 3

29/36

Equation 65

Equation 66

A commercial value R = 15 k

Ω and a R0 = 1.5 kΩ has been chosen.

Figure 17 and Figure 18 show the trend of the off-time and the switching frequency vs the

input mains voltage. The PFC inner current loop works in the range of 80 kHz-150 kHz.

Due to the tolerance of the capacitor selected (C) and the two discharging resistors, it is

important to take into account a variation on the switching frequency in a real board of about

± 10%.

Finally the Rs limiting resistor should be selected according to the inequalities (

Equation 6):

Equation 67

and the speed-up capacitor Cs using

Equation 7.

Equation 68

This means that after calculations:

1

eq

K

1

R

R

−

=

Ω

=

−

Ω

=

k

79

.

15

903

.

0

1

k

53

.

1

R

1

eq

0

K

R

R

=

Ω

=

Ω

=

k

5

.

1

903

.

0

k

53

.

1

R

0

Figure 17.

Off-time vs. input mains voltage

Figure 18.

Switching frequency vs. input

mains voltage

V

7

.

5

V

6

.

0

V

7

.

5

V

10

k

53

.

1

Rs

k

53

.

1

V

7

.

5

mA

10

V

6

.

0

V

7

.

5

V

15

−

−

⋅

Ω

<

<

Ω

+

−

−

V

6

.

0

V

7

.

5

V

15

V

7

.

5

pF

220

Cs

−

−

⋅

<