AN1149-4 Lumileds Lighting, LLC, AN1149-4 Datasheet - Page 5

• Manufacturer Part Number: AN1149-4
• Description: Thermal Management Considerations for SuperFlux LEDs
• Manufacturer: Lumileds Lighting, LLC

Junction-to-Ambient Thermal Resistance Measurement Procedure
A simple method for measuring the R
assembly is possible by assuming the R
under test (DUT) is of a typical value. By making this
assumption, only the pin-to-ambient thermal resistance,
R
lamp (R
measuring R
Step 1: Assume the R
Step 2: Pick one LED within the assembly to be used as
Step 3: Solder a small thermocouple (approximately 0.25
Junction-to-Ambient Thermal Resistance Measurement
These sections give detailed instructions on
how to perform thermal resistance
measurements on LED assemblies. The first
method described in the box above, Junction
to Ambient Thermal Resistance Measurement
Procedure, allows for simple measurements to
be made on lamp assemblies without an
elaborate test setup. The second method
presented, Estimating Junction to Ambient
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
θ
pa , needs to be measured to calculate the Rθja of the
in the data sheet (typical R
155 °C/W, and for HPWT-xx00 = 125 °C/W).
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.
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
properties of the DUT, should be avoided.
θ
ja = R
θ
ja is described below:
θ
jp + R
θ
θ
jp of the LED emitter is that shown
pa). This simplified procedure for
θ
ja for HPWA-xx00 =
θ
ja of a lamp
θ
jp of the device
5
Step 4: Assemble the modified PCB into the lamp housing
Step 5: Energize the entire lamp assembly at the design
Step 6: Measure the pin temperature of the DUT along with
Step 7: Calculate the R
Step 8: Calculate the R
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.
Lumileds will evaluate the thermal resistance of
LED assemblies and signal lamps upon request.
Please contact your local applications engineer
for information.
such that the thermocouple wires are extending
outside the lamp.
voltage for a minimum of 30 minutes. This will allow
the lamp assembly to thermally stabilize.
the ambient temperature in the room.
following equation:
R
Where the power, P, into the DUT is calculated by
multiplying the heating/design current by its
corresponding forward voltage.
the R
Step 7.
θ
pa =
θ
jp of the emitter from Step 1 to R
Tp - Ta
P
θ
θ
pa of the lamp assembly using the
ja of the lamp assembly by adding
θ
pa from