AN2870 datasheet(22/41 Pages) STMICROELECTRONICS

22 / 41 page

Designing with L6585DE

AN2870

22/41

The number of turns and the length of the gap can be calculated as follows (L in uH, Bmax

in tesla, Ae in square millimeters and

0=4π*10

-7):

Equation 7

Wire section is selected in order to fit the winding window of the coil former (preferred if

slotted).

In order to evaluate the actual copper losses the DC resistance of the winding must be

multiplied by a factor that depends on skin effect. Using a wire composed by multiple

conductors reduces this factor.

The copper losses can be evaluated as:

Equation 8

Ferrite losses can be checked on ferrite manufacturer catalogs.

ZCD auxiliary winding must be able to develop the triggering pulse for ZCD pin

(Varm = 1.4 V). The voltage across the auxiliary winding will be:

Equation 9

The current flowing in ZCD pin must be limited by means of a resistor connected between

auxiliary winding and ZCD pin. Although the maximum ZCD pin current is around 5 mA, a

smaller value should be chosen in order to limit power dissipation and increase the

application reliability.

Equation 10

gap

max

→

⎪

⎭

⎪⎪

⎬

⎫

wire

RMS

⋅

(

)

out

zcd

aux

⋅

−

ZCD

max

ZCD

⋅

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