Anthony Catalano - EEWeb

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The Important Role of Drive Current on
Lumen Maintenance
The drive current of the LED plays an important role in
determining Lumen maintenance. As discussed at the
outset of this article, recombination in the semiconductor
leads to the multiplication of defects and a reduction
of the radiative efficiency. So it stands to reason that
drive current must play an important role in Lumen
depreciation. Unfortunately, no current standards
recommend testing in this regime, and it is left to the
LED manufacturer to choose the values of current used
in testing. Fortunately, the top-tier manufacturers have
chosen wisely and substantial data is available.
Figure 4 provides a dramatic illustration of the importance
of drive current, particularly at elevated temperatures.
In this chart TM-21 has been used to extrapolate the
Lumen maintenance data for an LED at a semiconductor
junction temperature of approximately 127 o C. It can
clearly be seen that there is a very large increase in the
rate of degradation as the current rises from 0.35 A to
1 Amp. It can also be seen from this chart that the L70
value (where light output decreases to 70% of its initial
value) decreases from over 91,000 hours at 0.35 Amp
to 22,900 hrs at 1 Amp, a huge reduction in effective
useable life. (Strictly speaking TM-21 only permits a
6-fold extrapolation beyond the time-dependent data; I
have clearly gone beyond this to make a point)
Normalised Light Output
1
0.9
0.8
0.7
0.6
T j ≈127ºC
1 Amp Data
0.7 Amp Data
0.35 Amp Data
1 Amp Calc
0.7 Amp Calc
0.35 Amp Calc
0.5
1000 10000
Hours
L 70 =91,400 hrs
L 70 =52,200 hrs
L 70 =22,290 hrs
Figure 4: Chart showing the influence of drive current on LED
degradation.
The Importance of LEDSense® Technology
Given the dramatic reduction in Lumen maintenance
that results from operation at high current levels, it
is important that a means be provided in the drive
electronics to safeguard the operation of LED lighting
products. Thermal fold-back provides that safety net.
Thermal-fold back is important because:
• It allows the maximum brightness and maximum
lifetime regardless of conditions
• It provides a “worry-free” solution to OEMs
• It virtually eliminates the consequences of a bad
installation in the field
TerraLUX’s implimentation of thermal fold-back, called
LEDSense ® , employs a microprocessor-controlled
constant current driver. Figure 5 illustrates a block
diagram of the essential features of the LEDSense circuit.
Various buck or boost topologies are used depending on
the input voltage range and the length of the LED string
which determines the required output voltage. The
temperature of the LED is measured via a thermistor
which is located in the thermal path of the device so that
the operating temperature of the LED can be determined.
The microprocessor, via an A/D converter measures
this value and compares it to the known operating
characteristics of the LED via a built-in algorithm. The
processor then sets the current of the driver through its
D/A converter. With a properly designed and operating
luminaire-lightengine combination, the current (and
temperature) remains at the predetermined safe level.
If temperature exceeds a preset threshold, an algorithm
in the processor gradually reduces the current set point
via the firmware’s algorithm to assure the longevity of
the LED. The microprocessor is also used to analyze
the waveform of the power source in order to tailor the
driver operation to the particular dimmer-transformer
combination in use, proving additional benefit to the
OEM manufacturer.
The resulting performance characteristic is shown in
Figure 6. This figure compares the drive current to the
temperature of the LED for two examples of an LED
lightengine, the first (blue squares) without LEDSense ®
technology and the other (red circles) with LEDSense ®
operating. In this example the temperature that is being
externally controlled is that of the back of the light-
engine heat sink, but we have plotted the LED’s slug
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