
29-6 Power Electronics and Motor Drives
A simplied steady-state thermal equivalent circuit of an LED system is shown in Figure 29.4 [27,28],
assuming that (1) the N LEDs are placed on the same heatsink with a thermal resistance of R
hs
, (2) the
LEDs have their junction-to-case thermal resistance R
jc
, and (3) the thermal resistance of the insulating
material (such as heatsink compound) used to isolate the LEDs from the heatsink is negligible.
Based on the model in Figure 29.4, the luminous ecacy of the LED system can be expressed as
E E k T T k k R NR P= + −
( )
+ +
o e a o e h jc hs d
1 (29.5)
So, the total luminous ux
v
is
φ
v o e a