FOCUS
by
Geoff Archenhold
100
TECHNOLOGY
LED
Figure 7(a) and (b) showing
the decrease in capacitance
and the increase in ESR of
an electrolytic capacitor
with increased operating
temperature.
below the temperature rating. A with time as shown by figures
simple method to test the qual- 7(a) and (b) for a 330µF, 5000
ity of an LED driver would be hour lifetime at 105oC 35V max
to measure the temperature of rated capacitor.
each of its electrolytic capacitors It is important to note that the
and determine how close the LED driver output current will
measured temperature is to the increase as the capacitance
temperature rating displayed on decreases and the ESR increases
the side of each capacitor. If the and therefore the higher the op-
two temperatures are close then erating temperature is the faster
the likelihood is that the LED the output current peak-to-peak
driver will fail very quickly how- value will increase.
ever if the operating tempera- It is being proposed that the end-
ture is small compared to the of-life of an LED driver occurs
capcitors temperature rating then when the output current peak-
the electrolytic capacitor should to-peak value reaches 200% of
last a long time and increase the initial value enabling simple
reliability. LED lifetime predictions to be
Figure 8: A typical LED driver time to failure prediction
Furthermore, the service life of extracted as shown in figure 8
curve verses operating temperature with real data points
an aluminum electrolytic ca- for a single channel 350mA LED
measured.
pacitor for mid- to high-voltage driver.
filters is affected by the applied
• The ambient operating recommend the use of LED
voltage. If the applied voltage Conclusions
temperature where the driver drivers that do not employ such
is between 60% and 100% of An LED drivers’ reliability de-
is used. components.
the rated voltage, the estimated pends upon:
lifetime can be extended by low-
Methods are being developed Next issue
ering the applied voltage below • The number and quality of
to enable reliability testing of In the next issue I will look to
the rated voltage. components used within the
LED drivers by predicting the explain what MTBF means and
Unfortunately, there is no cur- driver design
lifetime of electrolytic capacitors how it is possible to calculate
rent definition for an electrolytic • The rated wattage of the
used within LED drivers. There- a value for complex electronic
capacitors end-of-life scenario LED driver and the maximum
fore, it will soon be possible to products such as LED drivers.
however manufacturers tend to operating temperature of the
determine the good quality LED In addition, I will discuss how to
use: electrolytic capacitors
drivers from the bad. practically measure the efficiency
• The overall efficiency of the
In the meantime, I would suggest of a typical LED driver along
• 10% to 20% decrease in AC-DC and DC-DC stages of
you take your LED driver and with why the PFC of drivers is so
capacitance (measured at the driver.
determine if it contains electro- important for overall LED fixture
120Hz) to define end-of-life • A suitable thermal
lytic capacitors and assess their efficiency.
• 200% increase in Equivalent management system for the
respective maximum rated tem-
Series Resistance (ESR) driver such as an aluminium
perature values against the op- If you have any questions or
(measured at 120Hz) case or forced air cooling fan
erating temperatures measured comments that you would like
if appropriate.
and if they are close look for answering then do not hesitate to
Initial studies of electrolytic ca- • Good driver design where
another LED driver with higher contact the editor of mondo*arc
pacitors undertaken by Han Lei component placement is
rated electrolytic capacitors. or myself.
at the Lighting Research Centre determined by both safety,
High temperature electrolytic
geoff@lowcarbonlighting.com
demonstrates that capacitance EMC and thermal
capacitors are rated from 105oC
decreases while ESR increases considerations.
up to 125oC and I would not
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