ambient temperature of 50 °C at the maximum
input voltage. The forward current at Ta < 50 °C
is 55 mA, and due to the increase in resistance
the forward current at Ta > 50 °C is 30 mA. In
such a case, the maximum junction
temperature will be achieved at 50 °C,
therefore, 50 °C can be used as Ta(max) in the
current derating calculations.
Figure 4.11 Current control circuit using
temperature compensation.
An example of a current control circuit using
temperature compensation is shown in
Figure 4.11.
Appendix 4A
Alternate Junction-to-Ambient Thermal Resistance
Measurement Procedure
Step 1: 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.
wires to one cathode lead and to one anode lead
of an LED, which is of the same type as the DUT.
Next solder the other end of these wires directly
to the PCB in such a way as to have this dummy
LED take the place of the DUT in the circuit.
Step 2: Electrically isolate the DUT from the rest
of the circuit by cutting the appropriate Copper
traces on the printed circuit board (PCB).
Step 5: Assemble the modified PCB into the lamp
housing such that the dummy LED and the DUT
wires are extending outside the lamp.
Step 3: Solder long thin wires onto one cathode
lead and one anode lead of the DUT. These
wires should be long enough to extend outside
the lamp housing once it is reassembled
Step 6: Measure the initial Vf of the DUT at a very
low test current. This test current should be low
enough such that it causes a minimum amount
of heating (1 mA is recommended).
because they will be used to apply the heating
current and to measure the ∆Vf of the DUT.
Step 7: Energize the entire lamp assembly at the
design voltage, and DUT at the design current for
the individual LEDs for a minimum of 30 minutes.
This will allow the lamp assembly to thermally
stabilize.
Step 4: Complete the original circuit of the PCB
assembly by attaching a dummy LED onto the
PCB to take the place of the isolated DUT. This
can be accomplished by soldering long, thin
12
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