Avago Technologies ACPL-224-500E - Manual (Page 12)

AV02-4387EN 10 Avago Technologies
Calculate the LED Lifetime Performance in Optocouplers to Predict Reliability
Calculate the LED Lifetime Performance
in Optocouplers to Predict Reliability
LED Reliability Stress Tests
CTR is defined as the ratio of the
output collector current (IC) caused
by the light detected by the pho-
todiode to the forward LED input
current (IF) that generates the light,
and is denoted as a percentage. De-
signers can use the change in CTR
over time to gauge the degree of
LED degradation.
2 AlGaAs LEDs are mainly used in
Avago’s optocoupler families that
handle high-speed digital signals
and low-power 10 Mbit/s digital op-
tocouplers. Lastly, the GaAsP LEDs
are used in a broad range of opto-
couplers, from digital optocouplers,
analog optocouplers, gate drivers
for intelligent power management
and many other applications. Tables
1, 2 and 3 show some examples of
Avago optocoupler product families
and their part numbers’ LED type.
Each LED type employs a different
manufacturing process (diffusion
type or epitaxial growth), and differ-
ent doping content concentration.
This allows the LED designer to cus-
tomize the LED light output power
vs current flow to address the differ-
ent speed and power performance
requirements of the optocoupler.
Among the three different LED
types, GaAsP-based LEDs are the
most mature, but have the lowest
light output power, while AlGaAs
type 1 offers the highest light
output power. This allows AlGaAs
type 1 LEDs to be used in the more
stringent isolation applications that
require high creepage/clearance
distances inside the optocoupler
package. AlGaAs type 2 has a per-
formance level that falls between
the other two LED types and can be
used in a wide range of applications
that require speed or power per-
formance. All three LED types have
similar degradation characteristics,
with <10% loss from the original CTR
value after 30 field years of typical
operation conditions.
Optocouplers use a LED to transmit
digital or analog information across
an isolation (or insulation) barrier
(often just an air gap). On the other
side of the barrier is a phototransis-
tor or other light-sensing device that
converts the optical signal back into
an electrical signal. Designers can
set an input current-limiting resis-
tor that defines a recommended
input drive current (IF) to the LED
to produce the desired light output.
However, the optocoupler’s LED
quantum efficiency (total photons
per electron of input current) de-
creases over time due to thermal
and electrical stressing of the LED
PN junction. To determine opto-
coupler reliability, you can perform
stress testing to determine LED deg-
radation for periods of continuous
operation up to 10,000 hours for
the various LED types that are used
in different models of optocouplers.
Unfortunately optocoupler vendors
don’t always provide stress test data
in the device datasheets. Further-
more, each vendor’s stress-test con-
ditions may differ so it can be hard to
directly compare stress-test results.
For example, the IF used by the LED
can be set at any value ranging from
a few milliamps to 20 mA or more,
thus the stress on the LED will differ
and thus reliability results could
span a wide range.
For example, one of the stress tests
performed by Avago – a High Tem-
perature Operating Life (HTOL) test
– is performed with the LED oper-
ating at 125°C and a continuous IF
of 20mA. The Current Transfer Ratio
(CTR) is an electrical parameter
usually specified for an optocoupler.
Due to input current and tempera-
ture, LEDs will degrade due to heat
stress in the LED crystalline struc-
ture. Thus, even though IF stays con-
stant, the light output from the LED
decreases over time. The photodi-
ode’s IC and CTR will thus decrease.
At each pre-determined point of
stress test hours (168hrs, 500hrs,
1000hrs etc), IC is measured and the
CTR is calculated. LED degradation
performance is plotted using this
collection of data points to show
the change in CTR vs the number of
hours that the stress test runs.
The LEDs used in optocouplers are
fabricated from either Aluminum
Gallium Arsenide (AlGaAs) type 1,
2 or Gallium Arsenide Phosphide
(GaAsP). Each vendor will opti-
mize their optocouplers to lever-
age the LED technology that best
suits the application. Additionally,
optocoupler vendors have devel-
oped a wide variety of devices that
have various types of interfaces to
suit many different applications. At
Avago, for example, Type 1 AlGaAs
LEDs are mainly used in optocoupler
product families that include digital
optocouplers, isolation amplifiers,
gate drivers and IPM drivers. Type
×100%
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C
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Current Transfer Ratio, CTR =