Datasheet

IN-AVE

IN-PK(MAX)

Design Guide

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The worst-case average input current is calculated at the minimum peak input voltage and targeted

converter efficiency η,

(7)

where,

(8)

Next the worst-case peak input current i

IN-PK(MAX)

is calculated. From Figure 27, the area of the triangle

(highlighted with the dashed oval) is the average input current. Therefore,

(9)

Figure 27. DCM Flyback Current Waveforms

Switching MOSFET (SW FET)

From its datasheet, the LM3448’s SW FET voltage breakdown rating V

DS(MAX)

is 600V. Due to a

transformer’s inherent leakage inductance, some ringing V

RING

on the drain of the SW FET will be present

and must also be taken into consideration when choosing a turns ratio. V

RING

will depend on the design of

the transformer. A good starting point is to design for 50V of ringing while planning for 100V of ringing if

additional margin is needed.

The maximum reflected voltage V

REFL

based on a turns ratio of “n” at the primary also needs to be

calculated,

(10)

The maximum SW FET drain-to-source voltage is then calculated based on the maximum reflected

voltage V

REFL

, ringing on the SW FET drain and the maximum peak input voltage V

IN-PK(MAX)

(11)

where,

(12)

and the following condition must be met,

AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver SNOA555C–April 2011–Revised May 2013

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