IEEE Power Electronics Magazine - June 2015 - 37

Failure Rate (Failures per Unit Time)

The highly accelerated life test (HALT) is a stress testing methodology for assessing product reliability during
the product development process. It is performed to identify design and process weaknesses. By subjecting the
power supply to progressively higher stress levels brought
on by thermal dwells, vibrations, rapid temperature transitions, and combined environments, the power supply is
exposed to limits well beyond operating conditions. These
fundamental technology limits are the data that drive the
improvements on design robustness and development time.
The benefits of HALT to end customers are demonstrated
quality, reduced field failures, and increased reliability.
Highly accelerated stress screening (HASS) or "burn-in"
is the process used to eliminate infant mortalities. These
take place in the first portion of the reliability "bathtub"
curve, as shown in Figure 4.
Infant mortalities are usually attributed to problems
with components, faults, processes, workmanship, or handling. To ensure that these early failures do not occur while
in the end equipment, it is best practice to employ 100%
burn-in for all power supplies. This can be carried out at elevated temperatures and under varying input line and output
loading conditions to simulate various modes of operation.

Early Life
(Failure Rate Decreases
with Time)
Wearout Life
(Failure Rate Increases
with Time)
Useful Life
(Failure Rate Approximately
Constant)
Time (h, mi, Cycles, Etc.)

fig 4 The HASS bathtub curve.

analyze the current bill of materials and suppliers to ensure
compliance. No new components should be approved by the
power supply manufacturer without supporting documentation from the component supplier assuring compliance.
This should be an ongoing activity and requires regular
updating of databases and compliance statements from the
power supply manufacturer.

Environmental Regulations

Conclusions

New environmental regulations now affect medical device
manufacturers, particularly with regard to the restrictions
of use of hazardous materials in their products. The European Union (EU) Reduction of Hazardous Substances
Directive (RoHS) covers electronic and electrical equipment. Since July 2014, RoHS has been in effect for medical
devices. Medical product manufacturers will now have to
anticipate the impact that even small changes in component selection can have on meeting the environmental regulations. In addition, the EU has also implemented the Regulation, Evaluation, Authorization, and Restriction of Chemicals Directive, which identifies a number of hazardous
materials and substances and defines acceptable limits for
the content of these in products.
These EU directives relate to products placed on the
European market, but in the case of global medical equipment companies, these restrictions have major impacts.
Furthermore, the scope of these directives is changing as
additional materials and substances fall under the umbrella
of the overall directives. The selection of appropriate components that meet the current and, just as importantly,
future directives, will continue to be a challenge for product manufacturers and, by direct relationship, the component suppliers.
To comply, power supply manufacturers should analyze
the bill of materials to the single component level and verify
compliance with each supplier based on the current lists
of restricted substances. Ongoing maintenance for component changes, alternative suppliers, and newly restricted
substances should also be undertaken. Best practice
involves the use of a third-party company to objectively

Compliance to safety standards for the medical industry takes
precedence over all other factors. It is incumbent on the
equipment supplier to protect patients and operators to the
highest possible level. To ensure this, appropriate component
selection is paramount. Power supplies, and in particular ac/
dc power supplies, are safety-critical components in any medical system. Medical equipment demands power supplies that
are efficient, cost effective, in compliance with safety and
environmental standards and, above all else, are safe and reliable. OEMS of medical equipment should take care in the
selection of the power supply to ensure they meet the existing
(and possibly future) requirements of the IEC, EN, UL, and
CSA safety standards. This greatly simplifies the system
approval to the safety agencies and the U.S. Food and Drug
Administration reduces development time and costs, and
brings about corresponding reductions in time to market.

About the Author
Dermot Flynn (dermotflynn@excelsys.com) is the director of sales for Excelsys Technologies Limited, an industry
leader in high-efficiency, high-reliability power supplies for
medical, industrial, and high-reliability applications. He
holds a B.A.I. degree in electronic and electrical engineering and a B.A. degree in mathematics from Trinity College,
Dublin, Ireland, as well as a higher diploma in management
and marketing from University College, Cork, Ireland, and
a higher diploma in education from Trinity College. In his
16 years in the power supply industry, he has held a number of senior roles in product development, product marketing, and sales.

June 2015

z IEEE PowEr ElEctronIcs MagazInE

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - June 2015