CEO, Digital Power Systems
What Constitutes a "Failure" in Power Supplies?
A failure can occur both abruptly and gradually. A clear failure definition depends on the specific application. In some cases, a power supply has failed when it delivers no power at all. In other cases, an output voltage deviation of a few percent is sufficient to constitute a failure.
The Bathtub Curve
The failure characteristics of electronic assemblies follow the so-called bathtub curve with three phases:
- Infant Mortality (Early Failures): Elevated failure rate immediately after commissioning due to manufacturing defects or material flaws
- Constant Statistical Failure Rate: The stable operating region - this is where MTBF/FIT values are measured
- End of Life (Wear-out): Rising failure rate due to component ageing
Mean Time Between Failures (MTBF)
MTBF describes the average time between two random failures in the stable operating region. Electronic circuits easily achieve 500,000 to 1,000,000 hours MTBF - equivalent to 57 to 114 years on paper.
Lifetime
Lifetime describes when a component has aged to the point where the failure rate increases significantly (phase 3 of the bathtub curve). In power supplies, the typical ageing mechanism is the drying out of electrolytic capacitors. By eliminating electrolytic capacitors entirely, this failure mechanism can be completely avoided.
Conclusion: Three Factors to Consider
When selecting a reliable power supply, three factors should be considered:
- Infant Mortality: Minimisable through burn-in testing and quality control
- Statistical Failure Probability (MTBF): Higher is better - but not a lifetime indicator
- Lifetime: Defined by the fastest-ageing component
When evaluating power supplies, focus on the specified lifetime, not the MTBF. MTBF alone says nothing about durability.
Criteria for Long Lifetime
- No fan (mechanical wear)
- No electrolytic capacitors (electrolyte dries out)
- No mechanical components (relays, switches)
- No optocouplers (LED degradation)