Understanding the dominant field failure mechanism for DC power supplies

Billions of units of electrical equipment are deployed world wide. Recent work has shown that, for the vast majority of these devices, the input power supply can operate well outside the design safe operating area under line conditions that are frequently encountered. Discussions with several major equipment manufacturers, as well as tests with commercial equipment have confirmed that input power stage failure is the leading cause of field failure. Input stage failure has typically been attributed to voltage surges, and has led to the UL 1449 standard and test protocol being widely adopted by industry. This paper shows that current-inrush surges, and not voltage surges, are the leading cause of equipment failure. These current-inrush surges occur at the end of frequent voltage sag events, when the inrush current limiting circuits are bypassed and offer minimal protection. The paper presents definitive data on the statistics of power disturbances, detailed analysis of the failure mechanism for various types of input stages, and a methodology for predicting the susceptibility of equipment. The paper also proposes the concept of a Power Disturbance Safe Operating Area (PD-SOA) specification for electrical equipment, and the possible need for a new standard specifying equipment behavior under voltage sag conditions.