AN1895

Power stage design procedure

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2

Power stage design procedure

The step-by-step procedure of an LM-FOT controlled PFC pre-regulator outlined in [2] will

be followed. The design will be done on the basis of a ripple factor (the ratio of the maximum

current ripple amplitude to the inductor peak current at minimum line voltage) Kr=0.3.

1.

.

2.

3.

.

4.

The ripple amplitude on the top of the sinusoid at minimum line voltage, assuming it is

75% of the maximum specified, will be:

5.

The required inductance L of the boost inductor is:

This value will be rounded up to 550 µH; the resulting value of Kr will be slightly smaller

than 0.3, but we will go on using the target value, this will give some additional margin.

6.

7.

(1.6V is the minimum value of the pulse-by-pulse current limiting threshold on the

current sense pin of the L6562). It will be realized with four 0.68

Ω, 1W-rated paralleled

resistors, for a total resistance of 0.17

Ω. This provides some extra power capability. The

inductor peak current that the inductor must be able to carry without saturating will be:

8.

From the formulae in [2], table 4, the MOSFET RMS current is:

;

k

min

2

Vin

(RMS

)min

Vout

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

2

90

400

----------

0.318, k

max

2

Vin

(RMS

)max

Vout

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

2

265

400

----------

0.937

==

=

==

=

t

OFFmin

k

min

f

sw max

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

0.318

100 10

3

⋅

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

3.18

µs

==

=

Pin

0

375

0.9

----------

417W; Ipk

max

2Pin

0

k

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

2417

⋅

0.318 400

⋅

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

6.56A

==

=

=

=

I

Lpk

∆

6Kr

83Kr

–

-------------------Ipk

max

60.3

⋅

83 0.3

⋅

–

------------------------- 6.56

⋅

1.66A

==

=

L1

k

min

–

() Vout

I

Lpk

∆

--------------t

OFFmin

1

0.318

–

() 400

1.66

----------- 3.18 10

6

–

⋅⋅

⋅

523

µH

==

=

I

Lpkmax

8

83Kr

–

-------------------lpk

max

8

83 0.3

⋅

–

------------------------- 6.56

⋅

7.39A

==

=

R

sense max

1.6

I

Lpkmax

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

1.6

7.39

-----------

0.216

Ω

==

=

I

Lpksat

1.8

0.17

-----------

10.6A

==

I

Qrms

()

Pin

0

k

------------------------- 2

16k

min

3

π

⋅

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

–

417

0.318 400

⋅

----------------------------- 2

16 0.318

⋅

3

π

⋅

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

–

3.96A

==

=