Operational Reliability: A Key Metric for Industrial Power Supplies

Designing for the application environment is key to maximizing the operational reliability of the power supply. The harsh industrial environment features a wide variety of conditions that can spell doom for poorly designed power supplies. Climate-related stresses can include extreme hot and cold temperatures, temperature cycling, dust, humidity, and moisture including salt spray. Mechanical stresses can include shock and vibration. And electrical stresses can voltage surges, dips and transients.

Let’s look at some of these stresses in more detail.

• Electrical stresses include frequent or extreme variations in input voltage such as surges, sags, or spikes. These can cause stress on power supply components, particularly capacitors and semiconductors, resulting in premature failure. In AC/DC power supplies, poor quality input AC power can cause harmonic distortion, electrical noise, and transient disturbances in the input power leading to stress and degradation of power supply components over time.
  
  Many industrial environments have high levels of electromagnetic interference (EMI) that can come from numerous sources. Typical EMI generators include switching equipment such as variable frequency drives (VFDs) and power inverters; industrial equipment such as welders, plasma cutters; industrial generators; or even other power supplies. High levels of EMI can significantly affect the reliability of power supplies.
  
  Other electrical stresses include missed cycles where one or more AC cycles are not delivered to the input of the AC/DC power supply. This can happen due to temporary faults, interruptions, or switching operations in the power distribution network. Missed cycles can lead to DC power dropouts that accelerate aging and degradation of components.


• Temperature extremes. A common cause of failures is excessive temperature or temperature cycling. Most are a result of thermal stress brought on by differences in the coefficients of thermal expansion (CTE) of different materials. Temperature extremes affect both electrical and mechanical components.


• Shock & Vibration are common features of industrial environments. They can cause mechanical stress in electronic and mechanical components. Common results include cracks in solder joints, loosening of screws and other fasteners, and metal fatigue.

Measuring the operational reliability of industrial power supplies involves assessing their ability to consistently perform under the specified conditions over time. There are several methods to assess operational reliability, but statistical reliability measurements such as MTBF (Mean Time Between Failure) are not a good indicators of true operational lifetime.

MTBF only provides a general indication of reliability and does not specify the types of failures or their causes. More importantly, MTBF is typically calculated under controlled test conditions that may not fully replicate the diverse and harsh environments in which power supplies operate. The application environment can significantly affect actual reliability.

Based on the experience of RECOM’s customers, most early failures are due to input voltage surges, not due to component aging, so they are not adequately accounted for by MTBF and similar calculations.

A high level of operational reliability is a key criterion in a power supply for industrial use. This blog has reviewed some of the factors that affect operational reliability and some of the design features that RECOM builds into its power supplies that ensure a long operating life in the challenging industrial environment.