MS104.MAR–HC Power–SC
The dos and don'ts of specifying a hot-swap power supply
Careful attention to details will maximize benefits from this emerging
power technology
BY JACK GRAHAM HC Power Systems Irvine, CA
Too often, apparently insignificant components dramatically affect the
success of a finished product. This is never more critical than when
developing or selecting a redundant power system with hot-swap capability.
A hot-swap power system could be the answer to applications requiring a
high degree of fault tolerance, such as communications networks, medical
instruments, and critical control units that must always be on-line. In
these applications, simple or even multiple redundancy of power supplies
is not acceptable. At some point, the system must still be taken out of
service to replace a failed supply. Users must be able to easily replace a
defective power supply while their system is in operation. They must also
be able to service these critical systems using semi-skilled personnel.
Understand shortcomings Before specifying a hot-swap power system, the
user must understand the key criteria in hot-swap technology.
Unfortunately, many hot-swap approaches have been add-ons to existing
systems. They have met with limited success for the following reasons: *
The mechanical design was inadequate or did not take critical alignment
and connector mating factors into consideration. * The connectors used
were not designed for plug-in service. * The power supplies used in the
system could not handle absolute current sharing. * Troubleshooting or
fault identification–in the event of a module failure–was not fully
considered. * Safety considerations for service personnel, as well as
that of the system, were not adequately addressed as an essential part of
the design. * The mechanical aspects of the design did not match the
application.
Self-contained design important The hot-swap power system, ideally,
should be self-contained. All electrical, mechanical, and ergonomic
considerations should be resolved within the power system. This dictates
an enclosure called a power shelf and a series of plug-in power modules
that operate redundantly and in a load-sharing mode. The finished system
should be fully assembled, tested, burned-in, and delivered to the
production line ready to be installed into the host system using simple
hand tools. The box, “Factors to consider in selecting a hot-swap system,
” summarizes the critical design factors to consider when specifying a
hot-swap power system. Of these, the most critical aspects of an overall
redundant power system design often involve mechanical considerations that
must be attended to early in the design cycle. One consideration is the
selection of the connectors. They must be capable of multiple insertions
with minimal degradation. In addition to the signal conductors, they must
incorporate contacts capable of carrying high currents with minimum
voltage drop. The typical voltage drops are:
* 150 A
Even with specially designed female contacts, these connectors will have
relatively high insertion forces for each contact set. As a result, the
total insertion and extraction forces for the connector will be high. A
means should be provided to mechanically aid in inserting and removing the
power module from the power shelf. The connectors should have a mounting
that provides some float. Another important item, yet often overlooked,
is a means to precisely align the mating connectors when a module is
inserted into the power shelf. A special cam-action handle on the front of
each power module provides the required insertion and removal forces. It
also securely locks the module in place when it is fully inserted into the
system (see Fig. 1). Many high-power switching regulated power supplies
have high inrush currents. Removing and installing a power module must be
done while the system continues to operate and the main input power is on.
To prevent arcing and damage to the connectors, it is wise to include a
mechanical device that automatically turns the module input breaker off
before a module is removed or a replacement module is installed (see Fig.
2). Ideally, the power shelf should be designed for EIA standard 19- and
24-in. rack panel widths. (The telephone industry also uses a standard
23-in. rack.) It should be able to accommodate two or four power modules
per shelf. The shelf should be self-contained electrically. It should
require only basic hook-up with simple hand tools when being installed into
the host system. Ideally, the power shelf should be capable of being
configured for either ac or dc inputs, redundant ac or dc inputs, or a
combination of both. Do not bring all the terminations from the plug-in
power modules out to the rear of the power shelf. This will mean extra
work and more chances for errors during installation of the power shelf in
the host system. All the power system wiring should be contained within
the power shelf. The rear panel of the power shelf should be simple and
clean. It should have connections for only the input power, the signals,
and the output power.
Avoiding electrical problems As noted earlier, absolute current sharing
between power modules is needed to maximize reliability and performance in
parallel redundant power systems. Current sharing between supplies should
not depend on voltage setting. Supplies that require special voltage
settings or have only quasi-current sharing will not work in these
systems. Be wary of vendor specifications. Precise current sharing means
that all modules in the system uniformly operate with minimum stress and
that they will only have to accommodate the minimum step load change if a
module fails. Upon failure, the system design should ensure that the
power module will disconnect automatically from the current share
circuitry of the remaining functioning modules. The host system must be
protected during removal or installation of a power module. “Glitching” of
the output bus or buses during module installation presents a particular
hazard and should be avoided by incorporating isolation diodes for each
output in either the power module or the power shelf. Desirable
electrical features that should be incorporated in the power system are
remote sense, remote inhibit, an output current monitor for each module,
and a dc-OK signal. Provision should be made for power shelves to current
share with each other so high power systems can be configured. All
similar power modules should be 100% interchangeable, and no special
adjustments should be required before installation. However, if different
types of power supplies are being used in the system, it is very important
to “key” both power modules and the power shelves to prevent incorrect
installation of a module in the power shelf.
Keep it simple and user-friendly Human factors attendant in real-world
operation of a system demand that the electrical, mechanical, and
ergonomic design be simple, straightforward, and virtually foolproof.
Review your basic design carefully, since problems that may go unnoticed
in a pre-production prototype system can have serious consequences if that
system is committed to production. Careful attention to design factors up
front means that your critical electronic system need never be forced out
of service because of a failed power supply.
CAPTIONS:
Fig. 1. A special cam-action handle on the front of each power module
aids insertion and removal of modules into a hot-swap system's shelf.
Fig. 2. To prevent arcing and damage to the connectors in a hot-swap
system, a mechanical device that automatically turns the module input
breaker off before a module is replaced is desirable.
BOX:
Factors to consider in selecting a hot-swap system * The power modules
should be totally pluggable. No wiring hook-up should be required by the
user during module exchange or replacement. * No special electrical
adjustments should be required on the module before installation. *
Module exchange or replacement should be easily accomplished in 30 seconds
or less without the use of hand tools. * Module replacement should not
require special training. * All power modules should be capable of
absolute current sharing. * All service should be capable of being
completed from the front of the system. * Individual power modules should
be light enough and small enough to be easily handled by one person. *
The power modules and the power shelves into which they are installed
should employ fail-safe technology that will ensure that no power module
can be inserted or removed from the power shelf while input power is
turned on. It will also ensure that no power module can be mistakenly
inserted in the wrong position in the power shelf. * The power system
should provide a logic signal to advise the central processing unit when a
power module has failed. * An easily seen signal should be provided on
the front of each power module to indicate both the presence of input
power and status of the power supply. * Operation on both ac and dc input
power should be considered.
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