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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
Techniques for Higher Output
Power Density

This section highlights some of the
techniques which are currently being used to
obtain higher output power densities. Power
supplies exceeding 6 watts per cubic inch are
available from industry today and even higher
power densities are currently under
development. Increasing switching frequencies
from 20-40 kHz to 100-500 kHz or higher is the
most significant approach to increasing power
density at this time. This reduces the size of
the magnetics and filter capacitors.

Magnetics

A major design effort is required for the
development of magnetic components used in
power supplies. As the switching frequency
increases, the power transformer becomes a
greater design challenge. If the flux density is
restricted to reduce self-heating, most of the
common core materials (i.e., ferrite, powdered
iron, tape core and others) are suitable at
switching frequencies below 200 kHz.

While high-frequency magnetic components can
be wound with standard magnet wire, better
copper utilization is gained with either Litz
wire, multifilar magnet wire, or copper ribbon.
These materials reduce skin effects, which
become increasingly important above 100
kHz.

A challenge lies in winding higher frequency
magnetics, not only to reduce the leakage
inductance but also to produce symmetrical and
predictable secondary voltages.

Several techniques are available, including
interleaving, multifilar windings, twisted
wires and coaxial transmission lines.

Semiconductors and Microcircuits

A significant reduction in power supply size
has been made possible by the introduction of
higher power Field Effect Transistors (FETs).
Power FETs exhibit several advantages over
bipolar devices in switching-mode power
supplies, these include 10 to 100 times faster
switching speeds and reduced drive
requirements.

Low to medium power devices can be reduced in
size by mounting semiconductor chips on
ceramic, beryllium oxide (BeO) or silicon
nitride (SiNi) thick film substrates. The
hybrid technique, including components, yields a
physical package reduction of at least
40%.

There are many monolithic integrated circuits
currently available on the market which help
increase power density. Currently available
monolithic pulse width modulator control
circuits for output voltage regulation are
moderately complex devices containing all the
necessary analog and digital circuitry; they
result in reduced component count, improved
reliability and increased performance compared
to discrete designs.

Capacitors

The selection of capacitors is important in
maintaining power supply packaging efficiency.
Size and cost influences the selection of
capacitors. Capacitor specifications are a
serious problem in the procurement of modern
switching-mode power supplies. Military
specifications lag capacitor technology and
tend to be very conservative. For example,
several military specifications and application
reports indicate that solid tantalum capacitors
cannot be used reliably above 10 kHz: this is
no longer true. Ripple ratings also need to be
revised.

Aluminum electrolytic capacitors have
improved. Recently developed aluminum
electrolytic capacitors are smaller, and the
equivalent series resistance (ESR) has been
reduced. Several types are made especially for
switching-mode power supplies; however, the ESR
increases by a factor of 10 to 20 at -55ºC. Many
of these capacitors are applicable for
operation to +125ºC. They are widely used as
input capacitors for switching-mode power
supplies and often as output
capacitors.

There are many types of tantalum capacitors.
The unreliable wet-slug tantalum silver-cased
types (MIL-C-39006/21, /9) have been supplanted
by tantalum-cased MIL-C-39006/22, /25
capacitors. These capacitors will take a very
high ripple current and can replace several
capacitors operating in parallel. Volumetric
efficiency dictates their use in many
applications; however, ESR also increases at
low temperature. The upper voltage limit is 125
VDC, thus not allowing application as a filter
for rectified AC line voltage. Tantalum
capacitors are often applied as input filter
capacitors for DC-to-DC power supplies and as
output filters.

Solid tantalum capacitors are very widely
used as output filters. Usually several of
these capacitors are paralleled to make up an
output filter assembly for a switching-mode
power supply. The ESR is much more stable across
the temperature range of -55ºC to +125ºC. The
many advantages of this capacitor over the
aluminum electrolytic will keep its usage high
even though it is more costly. As the switching
frequency of power supplies increases, the cost
and size of capacitors and other filter
components (as percentages of total power
supply cost and size) should decrease.

When capacitors are paralleled, the design
must ensure that ripple current rating of any
individual capacitor is never exceeded.

Packaging

Efficient mechanical packaging is critical in
high density power supply design. A second or
third iteration often produces dramatic
improvements in thermal management,
maintainability and reliability. It is important
the program provides sufficient time and
funding to achieve these improvements.

Shape factors of components can increase
power densities. For example, rectangular
tantalum capacitors are available. The shape
factor of the applied magnetics can also
increase power
density.