It’s not hard to notice (or extremely hard not to
notice!) how high brightness LEDs, or HBLEDs, are quickly becoming commonplace
all around us in our daily lives. LEDs are no longer relegated to being an
indicator light on a display panel. HBLEDs have drastically ratcheted up their
output to become sources for illumination.
More and more autos use them for their tail and brake lights. It’s easy
to see the “instant on” they have when the auto in front of you hits its
brakes, not to mention the deep purity of color they have in comparison to the
incandescent predecessors. They are also
turning up in the headlights, the traffic lights, even high power street and
parking lot illumination lights, and in countless other places. A lot of
testing, characterization, and development work has, and continues to take
place, to achieve this level of performance from HBLEDs. This includes making
careful measurements of electrical power being provided and the corresponding
luminous efficacy outputted, in order to assess its performance.
In my title above I am using the term “quickly” for two
reasons in my posting today. First, it is important when trying to capture the
forward characteristics of an HBLED that it is performed in a minimum amount of
time in order to minimize temperature change due to self-heating. The temperature an HBLED is running at has in
impact on its performance. Minimizing the amount of temperature change improves
accuracy of test results in determining the performance of the HBLED, for a
given operating temperature. My second reason for using quickly is providing a means
to make these HBLED measurements with just a little time and effort.
It turned out using the N6784A four-quadrant SMU module
in an N6705B DC power analyzer mainframe worked out really well on both counts
of quickly. This set up is depicted in Figure 1.
Figure 1: HBLED test characterization set up
While the N6784A is an extremely fast voltage source it
is even a faster current source. With current rise and fall times of just a few
microseconds was a simple matter to generate sub-millisecond-long high amplitude
pulses of current with fast settling edges to provide the necessary stimulus
for performing the forward electrical characterization of the HBLED. This
allowed testing to take place in minimum time and avoid significant heating of
the HBLED die.
One of the outcomes of the testing is shown in Figure 2,
displayed graphically by the 14585A software. Here a ramped current pulse was used instead
of a flat top pulse. The HBLED’s voltage and current were simultaneously
digitized as the current was ramped up. This gave a way of characterizing the HBLED’s
forward voltage drop for all levels of drive current, from zero to maximum.
Figure 2: HBLED forward characterization results
The N6705B DC Power Analyzer mainframe and 14585A companion
software made quick work of the setup, testing, and display of results. A ramp waveform from the library of
pre-defined ARBs was selected and used to generate the current ramp. In this instance it was set to ramp up to 1.2 amps in 1 millisecond. The
oscilloscope mode was used to set up the simultaneous capture of voltage and
current, synchronized to the current ramp stimulus. As voltage and current were
captured it is also a simple matter to display the power, being the
point-by-point product of the voltage and current. The electrical power in can then
be correlated with a light output measurement on the HBLED for evaluating its
performance.
