Designing a fixed-off-time PFC

AN3112

14/36

Doc ID 16820 Rev 3

Equation 23

Vout is usually selected in the range of 1.5% of the output voltage. Although ESR does not

usually affect the output ripple, it should be taken into account for power loss calculations.

The total RMS capacitor ripple current, including mains frequency and switching frequency

components, is:

Equation 24

If the PFC stage must guarantee a specified hold-up time, the selection criterion of the

specified maximum dropout voltage (Vout min) that is the minimum output voltage value

(which takes load regulation and output ripple into account). Vout min is the minimum output

operating voltage before the 'power fail' detection and consequent stopping by the

downstream system supplied by the PFC.

Equation 25

A 20% tolerance on the electrolytic capacitors must be taken into account for correct

dimensioning.

Following the previous relationships, after selecting the commercial value of 330 µF the

actual hold-up capability and ripple voltage are recalculated.

In detail:

Equation 26

Equation 27

4.3.4

Boost inductor

In the continuous mode approach, the acceptable current ripple factor, Kr, is typically fixed in

the range between 10% to 35%. For this design, the maximum specified current ripple factor

is 34%.

out

out

l

out

out

l

out

O

V

V

f

2

P

V

f

2

I

C

∆

⋅

⋅

⋅

π

=

∆

⋅

⋅

π

≥

F

338

V

10

V

400

Hz

47

2

W

400

C

O

µ

=

⋅

⋅

⋅

π

≥

2

out

rms

2

Crms

I

ID

I

−

=

() (

)

A

36

.

2

A

0

.

1

A

56

.

2

I

2

2

Crms

=

−

=

min

out

2

out

out

Hold

out

O

V

V

V

t

P

2

C

−

∆

−

⋅

⋅

=

() (

)

F

3

.

242

V

300

V

10

V

400

ms

20

W

400

2

C

2

2

O

µ

=

−

−

⋅

⋅

=

()

out

2

min

out

2

out

out

O

hold

P

2

V

V

V

C

t

⋅

⎥⎦

⎤

⎢⎣

⎡

−

∆

−

⋅

=

() (

)

[

]

ms

22

W

400

2

V

300

V

10

V

400

F

330

t

2

2

hold

=

⋅

−

−

⋅

µ

=

O

l

out

out

C

f

2

I

V

⋅

⋅

π

⋅

=

∆

V

2

.

10

F

330

Hz

47

2

A

0

.

1

=

µ

⋅

⋅

π

⋅

=

∆