Optocoupler (LED-phototransistor isolator) — Failure Modes & Failure Rate

Galvanic-isolation device combining an LED and phototransistor in a single package. Used for crossing isolation barriers in SIS, motor drives, and medical equipment. Failure rate dominated by LED degradation — a wear-out mechanism, not random.

λ typical

1.0×10-8 / h

Range

5.0×10-9 – 5.0×10-8

Source

IEC 62380

Failure modes

LED degradation / CTR drop

Root causes: Crystal defect propagation in the GaAs LED accelerated by current stress and temperature. Current Transfer Ratio (CTR) typically falls 30–50% over 100,000 h at rated current.
Detection: Output-side phototransistor doesn't saturate at the design current; sometimes monitored explicitly via a feedback loop.
Mitigation: Derate LED current to 50–70% of rated; design to end-of-life (low) CTR rather than nameplate; use feedback-stabilised types (linear optos) where the property matters.

LED open-circuit

Root causes: Bond-wire fatigue; metallisation electromigration at high current density.
Detection: Output stuck at the inactive level; no signal transfer.
Mitigation: Current limit via series resistor; bond-wire-fatigue rating for thermal-cycling applications.

Phototransistor degradation / saturation drift

Root causes: Long-term junction degradation; gain (h_FE) drop with stress.
Detection: Measured CTR drops; saturation voltage rises.
Mitigation: Comprehended by the same end-of-life CTR derating as the LED.

Insulation breakdown

Root causes: Manufacturing defect or contamination in the isolation barrier; voltage transients exceeding withstand; long-term stress under sustained DC.
Detection: Hi-pot test at incoming inspection or during periodic SIS proof-test; insulation-resistance measurement.
Mitigation: Reinforced insulation parts (VDE 0884) for safety isolation; voltage transient suppression on both sides; periodic isolation testing in safety cases.

Typical applications

Galvanic isolation in SIS input / output cards; gate-drive isolation for motor inverters and SMPS; status-feedback across 24 V → 5 V boundaries; medical-equipment patient-isolation barriers.

How to model in a fault tree

For SIS analysis, the optocoupler often sits on the dangerous-undetected (λ_DU) path — LED CTR drop is gradual and not detected until proof-test. Model it as a basic event with λ_DU dominated by the wear-out term and a proof-test interval driving the time-averaged PFD. See λ → PFD for the SIL computation.