Step 8: Measure the Vf of the DUT at the
heating current (Vf heating).
Step 11: Calculate the power, P, into the DUT by
multiplying the heating/design current by its
corresponding Vf heating as determined in Step 8.
Step 9: Turn off all power to the lamp, and
immediately (≤ 10 ms) reꢀmeasure the Vf of
the DUT at the test current selected in 6).
Step 12: Calculate Rθ using the values of ∆Tj and
ja
P calculated in Steps 10 and 11. Lumileds can
Step 10: Calculate the ∆Tj of the DUT by
dividing the ∆ Vf (∆Vf = Vf (Step 6) ꢀ Vf (Step 9)) by
the appropriate factor in Table 4.3.
provide the Rθ measurements of LED lamp
ja
assemblies as described above as a service to its
LED customers.
Table 4.3
Ratios of the change in forward voltage vs. the change
in junction temperature for high-brightness led materials
∆ Vf /∆ Tj ( mV / °C)
LED Material Type
AS AlInGap
TS AlInGap
-2.0
-2.0
13
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