Introduction:

In critical infrastructure — rolling stock, electrical substations, signalling systems, continuous-process plants — the cost of one hour of unavailability far exceeds the price of any electronic converter. Over the last decade, the fraction of total cost of ownership attributable to the acquisition price has fallen below 30% in railway and substation grid applications; the remaining 70% is split between energy consumed, scheduled maintenance, spare parts and, above all, unavailability.

The result is that the purchase decision has migrated from unit price to total cost of ownership. The real question is no

longer “how much does it cost?” but “how long will it last and how will you prove it?”

1. Why Power Electronics Fails — The Physical Mechanisms

Electronic components do not fail randomly: they fail through identifiable, modellable and predictable physical mechanisms. The Physics-of-Failure (PoF) methodology — which Premium SA adopts as primary reference, as opposed to the outdated MIL-HDBK-217F — classifies these mechanisms into four families:

2. The Bathtub Curve — Each Phase Calls for a Different Technique

Electronic components exhibit three recognised phases: infant mortality (high initial failure rate from latent manufacturing defects), useful life (low and approximately constant rate), and wear-out (rising rate from accumulation of physical mechanisms). A mature DFR process attacks all three simultaneously.

3. RAMS — The Contractual Language of Reliability

RAMS stands for Reliability, Availability, Maintainability, Safety — the four metrics with which a railway operator or grid DSO measure equipment suitability over its useful life. EN 50126 is the framework standard; EN 50128 and EN 50129 cover software and functional safety respectively.

4. The Premium SA DFR Process — Four Sequential Gates

The Design for Reliability (DFR) methodology — originated at the Center of Reliable Power Electronics (CORPE, Aalborg University) and adopted as an internal standard at Premium SA — structures development in four sequential gates with quantitative exit criteria:

5. FMEA, FTA and the Analytical Hierarchy

Once derating is established, reliability engineering identifies and mitigates dominant failure modes through three complementary analyses:

• FMEA/FMECA (Failure Mode and Effects Analysis): Bottom-up. RPN = Severity × Occurrence × Detection (1–10 scale). At Premium SA, any RPN >80 triggers a documented action plan; RPN >100 blocks the D4Co gate.
• FTA (Fault Tree Analysis): Top-down. Decomposes boolean combinations (AND/OR gates) that may cause system fault. Quantifies impact of redundancies; allows calculation of system-level MTBF.
• ETA (Event Tree Analysis): Inductive. Models scenarios and responses derived from a single initiating event. Useful for evaluating fault propagation in multi-state systems.

6. Keeping Reliability in Service — PHM, FRACAS and Lean Reliability

PHM — Prognostics and Health Management

The active technical frontier today is the transition from fixed-schedule preventive maintenance towards maintenance based on actual condition. In Premium SA converters:

• Real-time Tj estimation of semiconductors from Vce_sat and current, with case-temperature compensation
• Cumulative power-cycle counting with associated ΔTj, to predict RUL (Remaining Useful Life) by PoF models
• ESR monitoring of capacitors by ripple analysis
• Periodic export of operating data over communication bus to the operator’s supervisor, with P-F (potential-functional) profile

FRACAS — The Continuous-Learning Loop

Failure Reporting, Analysis and Corrective Action System: every field return is analysed at the appropriate level — LRU, board, component — root cause is identified, and the corrective action is propagated to design and production. The consequence is that the population of installed equipment improves over time rather than degrading. The problem detected in unit #150 is prevented in unit #151.