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NAVSO
P-3641: Navy Power Supply Reliability Design & Mfg
Guidelines

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3.3 Output Power Density
- Low Voltage
Determining Factors - Low
Voltage

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 nonstandard components.

Conduction cooling to a heat exchanger is the
preferred method for heat removal, since it
minimizes the power supply volume and is
applicable for tactical applications.

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 EMI
filtering.

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
C
.

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 associated
circuitry.

(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
circuitry.