PD = internal power dissipation within the LED
LED signal lamps typically use several LED
emitters. Each LED has a slightly different
thermal resistance, based on the proximity of
other heat sources (e.g. adjacent LED emitters,
resistors, power transistors, bulbs, etc) and
printed circuit board layout. Generally, the
thermal resistance value used for thermal
modeling is the highest thermal resistance,
RqJA, of any of the LED emitters within the LED
lamp assembly. Experience has shown that the
LED emitter with the highest thermal resistance
is usually either one of the emitters in the center
of the LED lamp assembly for an x-y
emitter (IF VF),W
Where:
Rq JP = thermal resistance, junction to pin (LED
emitter package), °C/W
Rq PA
= thermal resistance pin to air (printed
circuit and case), °C/W
TP = LED cathode pin temperature on
underside of printed circuit board, °C
These equations are especially useful since the
thermal resistance junction to pin, RqJP, is
specified on the product data sheet and the LED
pin temperature can be measured directly by
attaching a thermocouple on the cathode lead of
the LED emitter on the underside of the printed
circuit board. Thus the junction temperature can
be estimated based on a measurement of the pin
temperature of the LED emitter (please refer to
AB20-4).
arrangement of emitters, the middle emitter in a
single row of emitters, or one of the emitters
adjacent to other heat sources (e.g. resistors,
power transistors, bulbs, etc).
The thermal modeling equation can be further
broken down by separately considering the
thermal resistance of each of the elements of
the LED signal lamp as shown below:
Usually thermal resistance measurements are
done at thermal equilibrium. For an LED signal
lamp, thermal equilibrium usually occurs after
30 minutes of continuous operation. In some
cases, it is important to calculate the junction
temperature under a transient condition
(e.g. 2 minutes at 24 V).
TJ @ TA + (Rq JP + RqPA) PD
TP @ TA + (Rq PA) PD
(3.10)
Table 3.1
Values of k for AlInGaP SuperFlux LED Emitters
Coefficient of
Dominant
Family
LED Material
Wavelength
F V, ( T ), k
HPWA-xHOO
HPWA-xLOO
HPWT-xDOO
HPWT-xHOO
HPWT-xLOO
AS AlInGaP
AS AlInGaP
TS AlInGaP
TS AlInGaP
TS AlInGaP
618 nm
592 nm
630 nm
620 nm
594 nm
-0.0106
-0.0175
-0.0106
-0.0106
-0.0175
20
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