Lesson 9/1753%
MODULE 09

Optocoupler Derating

Designing for the part it will become, not the part you bought: LED current derating against aging curves, CTR margin stacking, temperature derating of every limit, and reliability math connecting operating conditions to expected lifetime.

Learning Objectives

After completing this module, you will be able to:

Apply LED current derating from manufacturer aging data
Stack CTR margins: bin minimum, temperature, and end-of-life
Derate power dissipation and forward current with temperature
Estimate lifetime from stress conditions
Document derating rationale for design reviews
Where this fits. Module 6 built the worst-case CTR equation and Module 7 showed where the curves live. This module designs for the part you will have in year ten: how the LED ages, how margins stack, how temperature erodes every rating, and how to write it all down so a reviewer can check it. Module 12 (Failure Mechanisms) covers the physics of what happens when derating is ignored.
Illustrations and animations are original teaching drawings, not to scale. Values marked representative are typical of common parts; always confirm against the current datasheet or standard you design to.

A new optocoupler and the same part after ten years in a warm cabinet are, electrically, different components. The LED emits less light, the encapsulant may have darkened slightly, and the leakage at the output is higher when hot. Derating means choosing today’s operating point so that tomorrow’s part still meets the specification, with margin.

Derate for survival

Keep stresses (IF, VCEO, IC, P, TJ) well below absolute ratings, so random failure rates stay low.

Derate for function

Keep enough CTR (or LED current above the threshold) at end of life, over temperature, for the circuit to work.

Derate for ageing itself

Lower IF and TJ slow LED degradation, so derating is partly self-rewarding.

Design trap — “it passed the prototype test”. A prototype built with fresh, typical parts at room temperature tests the best case. Field returns come from the worst-case corner: minimum-rank part, hottest enclosure, year eight.

LED light output decreases over time as defects grow in and around the active region, increasing non-radiative recombination (Module 12 covers the mechanisms). Manufacturers publish the result as CTR degradation curves: relative CTR vs time for several IF and temperatures. Three rules emerge from every such dataset:

Current density drives it

Degradation rises faster than linearly with IF. Doubling IF can cut time-to-threshold several-fold.

Temperature accelerates it

Higher junction temperature speeds defect growth (Arrhenius behaviour).

It slows with time

Most loss happens early; the curve flattens on a log time axis.

Two charts of relative CTR versus years: left, curves for 5, 10 and 20 mA at 60 degrees Celsius, with 20 mA degrading fastest; right, curves for 40, 60, 85 and 100 degrees at 10 mA, with the hottest degrading fastest; a dashed 50 percent end-of-life line on both
Figure 1. Ageing trends from the course’s teaching model. The more light you ask for now, the less you keep later.
Animation of relative CTR over 20 years at 70 degrees Celsius for 5, 10 and 20 mA, with markers crossing the 50 percent threshold at very different times
Animation 1. At 70 °C the 20 mA part crosses the 50 % line in about five years; the 10 mA part in decades; the 5 mA part effectively never.
CTR(t)/CTR0 = exp[−K·(IF/10 mA)n·AF(TJ)·(t/105 h)m]    (teaching model: K = 0.25, n = 1.5, m = 0.6, Ea = 0.5 eV)
  • This is a representative model for learning the trends, not vendor data.
  • For real designs, read the vendor’s degradation curves (or request reliability data) and interpolate to your IF and TJ.

Interactive: LED Ageing Simulator

Junction temperature includes the LED’s own heating (IF·VF·RθJA, VF = 1.2 V). The curve shows relative CTR over 30 years with your threshold.

Junction temperature (LED heating only)–
CTR after required life–
Years to reach threshold–
–

Pro tip — drive the LED gently. Many vendors’ ageing data suggest that running phototransistor couplers at roughly 5–10 mA (well under half of IF,max) gives long, flat CTR life. Choosing a higher CTR rank to allow lower IF is usually the cheapest reliability improvement available.

Worst-case CTR is a product of independent factors. Each looks modest alone, but together they routinely remove two-thirds of the headline figure:

Animated waterfall bar chart: datasheet CTR minimum 130 percent, then 117 after the I_F factor, 94 after temperature, 84 after V_CE saturation, and 42 percent at end of life
Animation 2. The margin stack for a rank-B PC817-class part in a hot switching application. Only a third of the datasheet minimum remains.
CTREOL = CTRmin × kIF × kT × kVCE × kage × kextra    design rule: CTREOL ≥ 2 × CTRneeded for switching outputs

Why “50 % at end of life”?

Many companies adopt kage = 0.5 as a default when vendor data for the exact conditions is missing. It is conservative for gently driven parts (5–10 mA, moderate temperature), and roughly what the harsher corners of published degradation curves show over 10+ years. The rule is valuable because it is simple and auditable. Replace it with data-backed factors when you have them, and never use a less conservative factor without data.

Calculator: CTR Margin Stack

The bar chart shows each factor’s effect; the verdict compares CTREOL with what the circuit needs (e.g. from Module 4’s saturation checker).

CTREOL–
Margin = CTREOL / CTR needed–
–

Every rating in the absolute-maximum table falls with temperature, but not always along the same line. Read each derating graph separately:

RatingTypical derating (representative)Watch for
IFFlat to 25–55 °C, linear to zero at TmaxThe LED limit often binds first in hot enclosures
PD (LED), PC (output), PtotLinear from 25 °C to zero at TmaxLinear-mode outputs dissipate much more than saturated ones
ICLimited indirectly via PC and TJIC·VCE in feedback loops
IsolationSafety-limiting values (IS, PS, TS)Barrier guarantees only apply within them
Worked example — which limit binds first?
Fictional part: IF 50 mA flat to 25 °C, zero at 110 °C; Ptot 200 mW, same shape. Design: IF = 15 mA (18 mW LED), output saturated at 5 mA and 0.2 V (1 mW). Enclosure 90 °C.
  1. IF limit at 90 °C = 50 × (110 − 90)/85 ≈ 11.8 mA → 15 mA exceeds it.
  2. Ptot limit = 200 × 20/85 ≈ 47 mW → 19 mW is fine.
  3. So the LED current rating binds first. Reduce IF to ≤ ≈ 6 mA (a 50 % derating of 11.8 mA) and compensate with a higher CTR rank.

Two different kinds of “reliability” apply to optocouplers, and mixing them up leads to bad decisions:

Random failures (constant rate)

Counted in FIT: failures per 109 device-hours. MTBF = 109/FIT hours. Handbook models such as SN 29500 (Siemens) and IEC 61709 give reference failure rates and stress factors for temperature and load. Optocouplers typically sit in the tens-of-FIT range at moderate stress (representative).

Wear-out (drift)

CTR degradation is gradual and predictable, not random. A FIT number does not capture it. Handle it with end-of-life margin (Section 3), not with MTBF.

AF = exp[ (Ea/k) · (1/Tuse − 1/Tstress) ]    k = 8.617×10−5 eV/K, temperatures in kelvin
  • Ea — activation energy of the failure mechanism (e.g. 0.3–0.7 eV for many; use what the data supports)
  • FITuse ≈ FITref / AF when converting between temperatures (handbook models add further stress factors)
Log plot of acceleration factor versus stress temperature relative to 55 degrees Celsius for activation energies of 0.3, 0.5, 0.7 and 1.0 electron volts
Figure 2. Arrhenius acceleration relative to 55 °C. The activation energy matters enormously: at 125 °C, AF ranges from about 6 to about 500.
Worked example — what does a 1000 h test at 100 °C prove?
Ea = 0.5 eV, field use at 55 °C.
  1. AF = exp[5802 × (1/328.15 − 1/373.15)] ≈ 8.4.
  2. 1000 h at 100 °C ≈ 8400 h ≈ 1 year at 55 °C, not ten.
  3. Longer claims need longer tests, more parts, or the vendor’s cumulative reliability data.

Calculator: Arrhenius Acceleration Factor

Convert test time at a stress temperature into equivalent field time. Only valid if the stress does not introduce a new failure mechanism.

Acceleration factor AF–
Equivalent field time–
–

SN 29500 and IEC 61709 are cited here by name only; their tables are not reproduced. Consult the current editions for reference failure rates and stress models.

Products do not live at one temperature. A mission profile lists how many hours are spent at each condition. Converting each slice to an equivalent time at a reference temperature (with AF) gives a realistic ageing budget:

teq = Σ ti · AF(Ti → Tref)    then read CTR loss from the degradation curve at teq
Two bar charts: left, share of a 10-year life spent at 25, 45, 60 and 75 degrees; right, the share of LED ageing caused by each, where 75 degrees causes about 19 percent despite being only 5 percent of the time
Figure 3. A 10-year profile equals about 5.5 years at 60 °C. The rare hot hours matter far more than their share of time.
Worked example — equivalent life
10 years: 20 % at 25 °C, 60 % at 45 °C, 15 % at 60 °C, 5 % at 75 °C; Ea 0.5 eV; reference 60 °C.
  1. AF(25→60) ≈ 0.13, AF(45→60) ≈ 0.44, AF(60→60) = 1, AF(75→60) ≈ 2.1.
  2. teq ≈ 10 × (0.2·0.13 + 0.6·0.44 + 0.15·1 + 0.05·2.1) ≈ 5.5 years at 60 °C.
  3. Read the CTR loss for 5.5 years at your IF and 60 °C, rather than assuming 10 years at the maximum temperature (overly pessimistic) or at room temperature (optimistic).

A derating policy turns engineering judgement into a checkable rule. Typical company policies for optocouplers (examples, not a standard):

StressTypical policy limitRationale
IF≤ 50 % of IF,max derated to TA,maxLED life; thermal margin
VCEO≤ 80 % of rating (including transients)Avalanche and leakage margin
IC, PC, Ptot≤ 50 % of derated ratingJunction temperature and random failure rate
TJ≤ TJ,max − 20…25 °CEncapsulant ageing, dark current
CTRCTREOL ≥ 2 × needed (switching)Function over life
IsolationPeak working ≤ VIORM (no derating credit for VISO)Partial-discharge-free life

Interactive: Derating Report Generator

Fill in your design values. The generator checks them against the policy above and produces a plain-text table you can paste into a design-review file.

Policy used: IF ≤ 50 %, VCE ≤ 80 %, IC ≤ 50 %, P ≤ 50 % of derated ratings; CTR margin ≥ 2.

–
Pro tip — write down the source of every factor. A reviewer should be able to trace each k-factor and derated limit to a datasheet curve, a vendor reliability report, or the policy default. “kage = 0.5 (policy default, no vendor data for 70 °C)” is a good line in a report; a bare 0.5 is not.
Design for year ten

Worst-case rank, hottest enclosure, end-of-life CTR.

Current ages the LED

Degradation grows faster than linearly with IF; gentle drive pays twice.

Temperature accelerates

Arrhenius: AF = exp[(Ea/k)(1/Tuse − 1/Tstress)].

Margins multiply

CTRmin × kIF × kT × kVCE × kage ≈ one-third of the headline.

50 % EOL default

Conservative, auditable; replace only with data.

Every limit derates

Check IF, P and IC separately at TA,max; the LED limit often binds first.

FIT is not drift

Random failure rate and CTR wear-out are different problems.

Document it

Policy, factors and sources in a reviewable table.

Design checklist

Tick items as you review a design. Your ticks are saved in this browser only; use your browser’s print function for a sign-off copy.

Four parts: multiple choice, true/false, a matching exercise and engineering scenarios. Score 80 % on the multiple choice to pass the module.

Part A — Multiple Choice

Question 1 of 10  •  Score: 0/10

Part B — True or False

Score: 0/8

Part C — Match the Term to Its Meaning

Drag each term onto its definition, or click a term and then click a definition.

Placed: 0/6

Part D — Engineering Scenarios

Scenario 1 — The brighter LED that shortened life
To gain margin, a designer doubles IF from 10 to 20 mA in a PC817-class input running at 70 °C. Is that a good trade?
Analysis. Today’s CTR×IF roughly doubles, but ageing accelerates super-linearly with current. In the teaching model, time to 50 % CTR at 70 °C falls from about 27 years at 10 mA to about 5 years at 20 mA. The higher current also brings the LED closer to its derated limit. Better: keep IF near 5–10 mA and gain margin with a higher CTR rank, a smaller required IC (larger pull-up, if speed allows), or a threshold-type (logic) part.
Scenario 2 — A 1000-hour test and a 10-year claim
A supplier shows 1000 h of CTR data at 100 °C with < 5 % loss and claims it proves 10-year stability at 55 °C. Check the claim (Ea = 0.5 eV).
Analysis. AF(55→100 °C) ≈ 8.4, so 1000 h ≈ 8400 h ≈ 1 year at 55 °C. The data supports roughly one year of equivalent field use, not ten. Ask for longer data (several thousand hours at several temperatures and currents) or use a conservative EOL factor.
Scenario 3 — Which rating binds?
At TA,max = 85 °C the fictional part allows IF ≈ 20 mA and Ptot ≈ 80 mW (derated). The design uses IF = 12 mA and dissipates 18 mW. Does it meet a 50 % derating policy?
Analysis. Power: 18/80 = 22.5 %, which passes. LED current: 12/20 = 60 %, which fails the 50 % limit. Fix: reduce IF to ≤ 10 mA (choose a higher CTR rank if needed) and record the derated limits and their sources in the report.

Additional Resources

Data to request

Vendor reliability reports and long-term CTR degradation curves (current and temperature matrix), FIT data with confidence level, and qualification test summaries.

Handbooks and standards

SN 29500 (component failure rates) • IEC 61709 (reference conditions and stress models) • JEDEC JESD47 (qualification) • AEC-Q101 for automotive discretes.

Keep learning

Module 12 explains the degradation physics; Module 13 shows how to measure CTR drift on the bench; Module 10 applies these margins in complete designs.