FOCUS
by
Geoff Archenhold
98 TECHNOLOGY
LED
(environment), as well as electri-
cal factors, applied voltage,
ripple current and charging/dis-
charging conditions. In ca-
pacitors for mid-to-high-voltage
filters, temperature and applied
voltage are the most important
controlling factors. The estimated
service life may be calculated
based on the core temperature
of the capacitor and the applied
voltage.
In general a capacitance change
or tangent change for loss angle
Table 5: The heat dissipation for a 40W LED driver dependant upon
indicates that the product life
varying AC-DC and DC-DC stages
has been affected by tempera-
ture. Generally, as the ambi-
sonable tolerance of components
ent temperature (neighboring
and input voltage. However, if
temperature of the capacitor)
the forward voltage of the LEDs
increases, capacitance decreases
connected to the circuit (due to
and tangent change for loss
thermal heating or increased
angle takes place more rapidly.
forward current settings) or the
This is mainly because electrolytic
number of LEDs connected to
solution generates gas due to
the driver changes significantly
electrode reaction and diffuses it
the efficiency of the driver could
outside via a sealing rubber.
change significantly and become
The Electrolytic capacitors’
much less efficient. Some driver
properties at “hot” temperatures
systems can quickly degrade
are well-explored thanks to the
from 90% efficiency down to
Arrhenius Law:
70% or less with changes on the
output of more than +/- 20%
L = L0 × 2(Tm –T)/10 ×
load variation resulting in excess
(Vm/V)2.5
heating of the driver and possible
Figure 5: LED driver efficiency fall off from a theoretical design limit
early failure.
Where, L0 is the manufacturer-
The reduction in driver efficiency
rated endurance at maximum
higher the efficiency of both has a major impact on operating temperature Tm, hours
stages the better however when temperature of the LED driver T is the operating temperature of
one designs an LED driver the and as such reduces its reliability the capacitor, C
LED characteristics and input especially for current carrying Vm is the maximum manufac-
voltages varying during opera- devices such as MOSFETs or turer-rated capacitor voltage, V
tion thus resulting in a differ- electrolytic capacitors. V is the operating voltage, V
ence between driver efficiency
from design to operation. This Electrolytic Capacitors – Therefore, the lower the core
variation becomes greater if the LED drivers Achilles Heal temperature of the capacitor
number of LEDs (or the forward The majority of high power during actual use, the longer the
voltage of the LED load) being (>15W) LED drivers employ estimated service life is. The core
driven changes significantly from electrolytic capacitors (as shown temperature of a capacitor may
te parameters used in the circuit in figure 6) either on the input be lowered by lowering either the
design. AC stage to enable filtering of ambient temperature or the load
Figure 5 demonstrates what can noise or on the output channel current (operating conditions), or
Figure 6: 85C rated 270uF happen when the output voltage DC stage of the driver. by either boosting capacitance or
electrolytic capacitors.
of the LED driver varies between It is well known that electrolytic lowering internal resistance.
that chosen for the theoretical capacitors are one of the weakest The useful life of an electrolytic
design and what may happen in elements of a driver circuit and capacitor decreases exponentially
practical systems. Here a typical frequently result in failure, espe- as the capacitor body tempera-
LED driver DC to DC stage has cially at elevated temperatures ture increases therefore it is vital
been designed for an output volt- Environmental factors affecting that a high temperature rated
age of 36V and using a standard the service life of an aluminum capacitor is used within the LED
IC chip can achieve a maximum electrolytic capacitor include tem- driver and that the maximum
efficiency of 92% within a rea- perature, humidity and vibration operating temperature is well
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