||NAVSO P-3641A: More Power For The Dollar
The development and availability of state-of-the-art
active and,passive components offer consistent improvements in cost and
reliability of low voltage power supplies. The application of these
components must remain consistent with the environmental constraints of the
weapon system. There are a number of issues to consider in the selection
of components for low voltage power supplies, as follows,
Component derating is a proven method for
improving reliability and operational life by reducing operating stresses below
the manufacturer's recommended maximum ratings. Depending upon the
specific component type, construction and materials used, stress may include
voltage, current, temperature and power dissipation, as well as other stresses
imposed in the local environment.
Voltage and power derating are separate
and independent considerations. Voltage derating is applied to reduce
failures caused by electrical breakdown, whereas power derating is accomplished
to reduce thermal stress.
The operating voltage of active components,
such as semiconductors, is dependent upon the breakdown characteristics of the
physical properties. The conservative voltage derating of passive
components prevents breakdown, flashover and corona effects at the atmospheric
pressure (altitude) to which the components are exposed. These effects are
dependent upon voltage gradients, configuration of terminals and the nature of
the dielectric path.
The degree of heat transfer from a heat-dissipating
component and the local environment surrounding the component will determine the
surface temperature, junction temperature, core temperature or hot-spot
temperature at a particular power level. For optimum life, junction
temperatures should not exceed + I I OOC under worst-case operating
conditions. The maximum operating temperature for each component in a
power supply should be determined analytically and verified empirically.
In addition, semiconductor and integrated circuit peak junction temperatures,
and magnetic components and capacitor peak core temperatures must be accurately
determined. Appendix E, Parts Derating Tables, presents guidelines for
component stress derating recommended for the design of reliable power
A high density design, using innovative packaging
techniques can result in improved reliability compared to older
technology. There are several components used in power supplies that
require special design and manufacturing considerations to assure adequate
reliability and minimize failure modes in a cost effective
Plastic Encapsulated Devices - The successful application of Plastic Encapsulated Devices
(PEDS) requires that the imposed actual environments and available PEDs
specifications are well matched throughout the product life cycle.
Currently, PEDs are not recommended for some applications, e.g., critical
applications such as man-rated, space, radiation hardened and long term dormant
storage. Currently, there are no validated models for making long-term
dormant storage PED predictions.
Power Semiconductor Isolation and Thermal
Conditions - Since power
semiconductors typically do not have isolated junctions, it becomes necessary
for the power supply manufacturer to provide a reliable method of mounting these
devices with low thermal resistance while establishing high electrical
isolation. Various materials and techniques have been used, each with
their unique problems. Thinner materials, such as mica and mylar film are
subject to damage by burrs and sharp edges. High thermal conductivity
ceramic insulators require thermal grease and, because of their brittleness, are
easily broken. The use of thermal grease should be avoided as it is
difficult to control, attracts contaminants and affects reliability and
Special Procurement Procedures - All components should be capable of withstanding worstcase electrical, mechanical
and thermal stress conditions. Nonstandard components are obtained through strict
procedures maintained by power supply manufacturers in their procurement specifications.
Modified standard components are similarly controlled but only for selected items and/or parameters.
Common procedures used by most power supply manufacturers include:
- vendor evaluation based on prior performance and
- test to acceptable quality levels or I 00%
- sample Destructive Physical Analysis (DPA) and periodic
inspection, including construction and electrical performance to determine
component strengths and weaknesses, and
- documentation of component problem histories to
restrict future usage of known problem items.