Performance and considerations with LED loads

AN4314

12/31

DocID024821 Rev 2

4

Performance and considerations with LED loads

The board has been designed to source 350 mA nominal value into a string of 23 LEDs of

about 1 W power and a forward drop of 3.2 V typical at ambient temperature. The LEDS

used to test the demonstration board have been furnished by Seoul Semiconductor (part

number X42182) [

6].

In order to reproduce the possible LED string drop variation due to thermal heating of a

single LED, measurements have been done first by loading the SMPS with a string of 23

LEDs having a voltage drop of 71 V

dc

, then loading 22 LEDs (68 V

dc

) and finally 24 (75 V

dc

).

The behavior of the board has been checked also at the limit of the LED tolerance of the

voltage drop, at -10 %, that is, 63 V

dc

and +10 %, that is, 77 V

dc

by using an active load.

Figure 3 represents the different load voltage drops of the LEDs, applied to the converter, at

each point of the wide-range mains. This figure is necessary to understand the following

Figure 4, 5, 6 and 7.

Figure 3. LED string voltage vs. line voltage

Figure 4 shows the LED string current versus the mains input voltage according to the load.

The LED current has been set at 350 mA nominal, using

Equation 2 and considering the

internal Vcsth of SEA05L (see datasheet):

Equation 2

The SEA05L perfectly regulates the current loop over the entire mains voltage and load

range [-10 %, +10 %].

Ω

=

=

14

.

0

350

50

05

_

mA

mV

R

L

SEA

S