It is generally recognized that a number of specification-imposed design
factors adversely affect low-voltage power supply reliability by increasing the
output power density beyond what is inherent in the technology. These factors
may be broadly grouped into mechanical and electrical categories.
Mechanical Design Considerations
Allowable volume should be maximized. Failure to do so will result in a
design which runs hotter, operates with greater component stress and uses more
Conduction cooling to a heat exchanger is the preferred method for heat
removal, since it minimizes the power supply volume and is applicable for
Electrical Design Considerations
Although EMI is always of importance, the allowable level in each case
depends upon the application and system requirements. The specification of "full
MIL- STD-461 compliance" at the power supply level imposes a significant burden
on the designer due to the volume within the supply which must be dedicated to
Two areas should be addressed so as to minimize the impact on output power
density due to EMI requirements: (1) the EMI specification should be tailored to
actual system requirements, and (2) equipment, cabinet or system EMI filtering
should be employed wherever possible.
When tailoring the EMI specification, consideration should be given to
providing up to 10 dB relaxation of the CE-01 narrowband limits between the
first and tenth harmonic of the power line frequency at the system level and
relaxation of the RE-01 requirement at the individual power supply level. The
former allows the power supply designer flexibility in the use of multiphase
rectification and the placement of filter resonances.
Accepting low frequency EMI and line-frequency harmonics responsibility at
the equipment, cabinet or system level will result in significant power supply
output power density improvement. Any equipment/system level EMI power line
filter can cause instability or performance degradation of the power supplies.
The designer of the power supply must specify the characteristics of the
equipment/system level power line filters that, working in conjunction with the
filters internal to the power supply, will ensure equipment/system EMI
requirements are met without degradation of power supply characteristics. This
requires verification that the combination is satisfactory by both an open loop
stability analysis that includes the specified filter and by empirical
measurements. The required techniques are described in MIL-HDBK-241.
Shipboard systems have special requirements, not imposed on aircraft or shore
systems, to limit the harmonic currents flowing in the ship electrical system.
These requirements are imposed by MIL-STD-1399 Section 300 which limits
individual harmonic line currents to 3%. Present techniques for meeting this
requirement employ either multiphase rectification with line-frequency magnetics
or multiswitching techniques with increased complexity and reduced efficiency.
Imposition of the 3% harmonic line current requirement at the power supply
level, with its resultant size, weight, cost and reliability penalties, should
be carefully weighed against the advantages of providing a system level
solution, such as DC prime power.
Two methods of meeting the more stringent shipboard requirement are shown.
The first method approaches within 6 dB of meeting the 3% harmonic current
requirement. This method uses multiphase rectification on the secondary of a
stepdown power transformer and a single section L-C filter. The second method
replaces the filter with an electronic line conditioner to fully meet the 3%
harmonic current requirement. The relative factors of this method could be
greatly improved by using one of the numerous topologies that are available,
whereby the line conditioning and power conversion may be integrated.
As can be seen, there is a substantial weight and
volume impact in meeting the MIL-STD-1399 Section 300 requirement. A "brute
force" EMI filter used to meet the requirement could be larger than the power
supply. Since the impact on power supplies designed to meet the shipboard 3%
harmonic current requirement is both severe and not often addressed by many
program managers, additional information on the subject is provided in Appendix
Some systems, particularly those containing signal processing memories,
require an electrical warning signal in advance of an output power failure. This
warning signal is usually generated at the time prime power degrades below a
predetermined level, whereas the output power continues for a period dependent
upon the energy stored within the power supply. Energy storage requires volume,
so it is advantageous from the standpoint of power density and reliability to
specify the minimum hold-up time required. A desirable technique which should be
considered to minimize stored energy is partitioning system power into critical
(requiring hold-up) and noncritical outputs.
Performance Monitor/Fault Location (PMFL) and Built-In Test Equipment (BITE)
circuitry can reduce reliability by increasing component count. Wherever
possible, output monitoring should be performed at the system level, minimizing
diagnostic circuitry within the power supply.
The following alternatives to requirements which impact output power density
should be carefully considered:
(1) Prime power switching and input/output fusing
Provide prime power from the system circuit breaker. Control output power
with logic signals and protect outputs with electronic current limiting inherent
in most power supply circuit designs. This eliminates the bulky and sometimes
inaccessible switches, circuit breakers and fuses from the supply.
(2) Crowbar overvoltage protection
Retain overvoltage protection, but eliminate the crowbar requirement.
Positive protection for most switching-mode power supplies may be provided by
modifying or terminating the switching action. This eliminates the large,
seldom-actuated, silicon controlled rectifier (SCR) crowbar devices and the
(3) Isolated multiple outputs
Allow all outputs to be connected to a common point within the power supply.
Some systems require isolation to eliminate ground-loop problems, with the
output returns ultimately connected at remote points. In these systems, specify
the maximum voltage difference between returns. This will minimize the number of
internal auxiliary power supplies needed for post regulation