||NAVSO P-3641A: More Power For The Dollar
Key points to consider when designing and evaluating a
switching power supply:
- Incorporate voltage transient protection on the
input power lines.
- Include a controllable soft-start circuit to
relieve the component stresses during tum-on.
- Provide an internal
housekeeping power supply to isolate sensitive circuits from the hostile power
line, improve human safety and allow control of power-up and power-down
- Incorporate a crossover interlock circuit in the power
stage that is connected across the power line so two devices can never conduct
simultaneously across the power line.
- Incorporate input EMI filtering having
characteristics compatible with the variable negative input resistance
characteristics of the power supplies so that the combination is stable.
- Incorporate a fast-attack latching current
limiting circuit to protect the power devices when the control logic fails
intermittently due to random noise or other abnormal circumstances.
- Design the power inductors, transformers and
magnetics to keep them out of saturation during peak load and transient
conditions for worst-case core temperature.
- Sequence the tum-on/tum-off logic in an orderly
and controllable manner.
- Analyze and measure worst-case peak currents, peak
power, peak voltage and ripple currents in all devices and under all worst-case
static and dynamic conditions and compare the measured results with the rated
limits of all components.
- Perform worst-case thermal and hot-spot analyses.
- Design printed wiring boards and packaging for the best
- Plan the circuit so that it will not be subject to
common-mode and/or differential-mode noise
- Circuit designers must be responsible for the electrical
and printed wiring board layouts.
- Package magnetic devices and other
heavy-cuffent-carrying conductors with thermal interfaces adequate to meet
hot-spot temperature requirements.
- Heat sinking should be provided for capacitors that
have significant core temperature rise resulting from ripple currents.
- Capacitor heat removal is facilitated by form
factors that exhibit large surface to volume ratios.
- Parallel capacitors must have provisions for ripple
- Avoid ground loops and potential crosstalk and
interaction by developing a grounding technique that is appropriate to the
- Analyze and measure loop stability to ensure that
there is adequate phase and gain margin under all line, load, temperature and
component tolerance variations.
- Measure torque values required, when thermally
conductive plastic impregnated fiberglass isolators are used, to determine
whether they are adequate for the thermal interface and will not cause any
- Do not use transistors beyond the forward/reverse
bias Safe Operating Area (SOA) limits adjusted for junction temperature under
any transient condition.
- Use of a "snubber" or load-line control circuits
is recommended. Increased reliability has been observed in power circuits
where semiconductor load-line excursions are constrained to approach the static
operating power limits.
- Components that have a shallow base are sensitive
to ESD and transients.
- Well documented power supplies are usually well
thought out and exhibit fewer latent design flaws. Power supply
documentation should include performance specification, physical description
with interfaces, functional block diagrams, all circuit schematics with theory
of operation, magnetic and unique subassembly specifications and acceptance
criteria, and thermal design data.
- The following requirements should be considered
for power supply components:
- assure that the
supplier performs adequate electrical parametric testing to ensure component
quality for the design application environment. Devices failing during
HALT testing should be returned to the component manufacturer for failure
analysis, corrective action and report of corrective action
maintain an approved vendor list relating to known component quality and
performance. Note: Components of a given type may not perform the same
eletrically in a different application and thus may require a specification
For designing and evaluating modular power supplies, the
following additional considerations apply.
- The interconnection of the high density modules
generally requires common mode filtering to prevent coupling switching frequency
noise to the input prime power source and other associated modules.
Differential mode filtering is required to decouple the power supply from noise
and modulation on the input power bus. If the power supply is to be
integrated into a system where the conducted switching frequency noise modulates
the output voltage, the system performance could be jeopardized. In this
case, a constant conversion switching frequency, synchronized to a system clock,
should be considered. This topology allows the designer to narrow band
attenuate the switching frequency content on the output voltage, thus minimizing
any affect of producing false signals within the weapons
- Ancillary features that are often standard on high density
DC/DC converter modules include over-cuffent/over-voltage protection,
over-temperature inhibit, enable/inhibit output voltage control, remote sense,
output voltage trimming, Built-In-Test and inter-parallel communication of
similar modules to permit current sharing.
- EMC management, in
compliance with either the FCC requirements or MIL-STD-461, may be accomplished
through the use of available standard encapsulated filter modules. These
modules are universally offered for interfacing with DC, single-phase, and
three-phase prime input power. Shipboard power requires that the
individual harmonic current content remain below 3%. Power Factor
correction modules are available that provide power factor management; however,
most do not provide prime power return isolation. These modules cannot be
utilized for three-phase operation and special techniques should be applied when
interfacing with ungrounded, single phase, II 5 volt shipboard power. When
selecting the power factor correction module, specific harmonic current test
data should be obtained from the manufacturer to assure MIL-STD-1399 Section 300
- To enhance power system reliability, the DC/DC converter
output modules may incorporate parallel diodes or'd in an N + M
configuration. Monitoring the output voltage prior to the dedicated
decoupling diode provides a cost effective means to determine the health of each
DC/DC converter to indicate the need for maintenance. Each module within
the power supply should incorporate a plug-in electrical interface. The
mating surface of each module pin and associated socket should be gold plated to
minimize corrosion during long ten-n application in a tactical
- Use of available high density modules results in
minimizing the power supply development time since the effort is primarily
focused on physical packaging and electrical interfaces. The selection of
identical modules throughout a weapons system reduces the TOC when consideration
is given to logistics support.
- Switching power supply circuits should be enclosed
within a grounded conductive shield to prevent radiated