AN3009

Designing a transition mode PFC

Doc ID 16032 Rev 4

13/36

Equation 16

A 20% tolerance on the electrolytic capacitors has to be considered to obtain the correct

dimensioning. As per

Equation 14, we have selected for this application a capacitor Co

equal to 47 µF (450 V) so as to maintain a hold-up capability for 12 ms. The actual output

voltage ripple with this capacitor is also calculated. In detail:

Equation 17

As expected the ripple variation on the output is:

Equation 18

3.3.4

Boost inductor

The boost inductor determines the working frequency of the converter, thus it is usually

calculated so that the minimum switching frequency is greater than the maximum frequency

of the L6564’s internal starter (typically 150 µs) to ensure correct transition mode operation.

Assuming a unity power factor, it is possible to write:

Equation 19

Equation 19 shows that the ON time does not depend on the angle of the mains phase, but

is constant over the entire mains cycle.

Equation 20

peak inductor current in a line cycle and

θ the instantaneous line phase in the interval [0,Π]).

Note that the ON time is constant over a line cycle.

related to the input power and input mains voltage. By substituting this relationship in the

expressions of ton and toff, it is possible to find the instantaneous switching frequency along

a given line cycle.

min

out

2

out

out

Hold

out

O

V

V

V

t

P

2

C

−

∆

−

⋅

⋅

=

() (

)

F

7

.

36

V

300

V

20

V

400

ms

10

W

100

2

C

2

2

O

µ

=

−

−

⋅

⋅

=

()

out

2

min

out

2

out

out

O

hold

P

2

V

V

V

C

t

⋅

⎥⎦

⎤

⎢⎣

⎡

−

∆

−

⋅

=

() (

)

[

]

ms

78

.

14

W

100

2

V

300

V

20

V

400

F

47

t

2

2

hold

=

⋅

−

−

⋅

µ

=

O

l

out

out

C

f

2

I

V

⋅

⋅

π

⋅

=

∆

V

02

.

18

F

47

Hz

47

2

A

25

.

0

V

out

=

µ

⋅

⋅

π

⋅

=

∆

VAC

2

IL

L

)

sin(

VAC

2

)

sin(

IL

L

)

,

VAC

(

t

pk

pk

on

⋅

⋅

=

ϑ

⋅

⋅

ϑ

⋅

⋅

=

ϑ

)

sin(

VAC

2

V

)

sin(

IL

L

)

,

VAC

(

t

out

pk

off

ϑ

⋅

⋅

−

ϑ

⋅

⋅

=

ϑ