Junction-to-Ambient Thermal Resistance Measurement Procedure
Step 4: Assemble the modified PCB into the lamp housing
A simple method for measuring the Rθja of a lamp
assembly is possible by assuming the Rθjp of the device
under test (DUT) is of a typical value. By making this
assumption, only the pin-to-ambient thermal resistance,
Rθpa , needs to be measured to calculate the Rθja of the
lamp (Rθja = Rθjp + Rθpa). This simplified procedure for
measuring Rθja is described below:
such that the thermocouple wires are extending
outside the lamp.
Step 5: Energize the entire lamp assembly at the design
voltage for a minimum of 30 minutes. This will allow
the lamp assembly to thermally stabilize.
Step 6: Measure the pin temperature of the DUT along with
the ambient temperature in the room.
Step 1: Assume the Rθjp of the LED emitter is that shown
in the data sheet (typical Rθja for HPWA-xx00 =
155 °C/W, and for HPWT-xx00 = 125 °C/W).
Step 7: Calculate the Rθpa of the lamp assembly using the
following equation:
Step 2: Pick one LED within the assembly to be used as
the DUT. The hottest LED in the assembly should
be chosen, for example an LED in the middle of
the assembly and next to a resistor.
Tp - Ta
Rθpa
=
P
Where the power, P, into the DUT is calculated by
multiplying the heating/design current by its
corresponding forward voltage.
Step 3: Solder a small thermocouple (approximately 0.25
mm in diameter) onto one of the cathode leads of
the DUT near the top surface of the PCB. Large
thermocouples, which can alter the thermal
Step 8: Calculate the Rθja of the lamp assembly by adding
the Rθjp of the emitter from Step 1 to Rθpa from
Step 7.
properties of the DUT, should be avoided.
Junction-to-Ambient Thermal Resistance Measurement
These sections give detailed instructions on
how to perform thermal resistance
not available. An alternate method for measuring
thermal resistance is provided in Appendix 4A.
This method monitors the change in forward
voltage of the LED to determine the change in
junction temperature and thermal resistance.
This method requires an elaborate test setup and
precise measurements. This technique is
commonly used by Lumileds Lighting.
measurements on LED assemblies. The first
method described in the box above, Junctionꢀ
toꢀAmbient Thermal Resistance Measurement
Procedure, allows for simple measurements to
be made on lamp assemblies without an
elaborate test setup. The second method
presented, Estimating JunctionꢀtoꢀAmbient
Thermal Resistance, eliminates the need for
measured thermal resistance. This type of
estimation is ideal for early evaluations, where
an actual prototype and/or test equipment is
Lumileds will evaluate the thermal resistance of
LED assemblies and signal lamps upon request.
Please contact your local applications engineer
for information.
5
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