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Design Guide

(13)

Peak and RMS SW FET currents are calculated along with maximum SW FET power dissipation based on

the SW FET R

DS-ON

value,

(14)

(15)

(16)

Current Limit

The peak current limit I

LIM

should be at least 25% higher than the maximum peak input current,

(17)

The parallel sense resistor combination will need to dissipate the maximum power,

(18)

Re-circulating Diode

The main re-circulating diode (D4) should be sized to block the maximum reverse voltage V

RD4(MAX)

operate at the maximum average current I

D4(MAX)

, and dissipate the maximum power P

D4(MAX)

as determined

by the following equations,

(19)

(20)

(21)

12.2 Transformer

Primary Inductance

The maximum peak input current i

IN-PK(MAX)

occurring at the minimum AC voltage peak V

IN-PK(MIN)

determines

the worst case scenario that the converter must be designed for in order to stay in DCM. Using the

equation for inductor voltage,

(22)

and rearranging with the previously calculated parameters,

(23)

provides an inductance L

CRIT

where the flyback converter will operate at the boundary of CCM for a

switching frequency f

. In order to ensure DCM operation, a general rule of thumb is to pick a primary

inductance L

at 85% of the L

CRIT

value.

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

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