MS105–Elpac–March 1993
Ten design tips for cost-effective use of external power supplies
Following simple advice ensures required performance and features at a
lower cost
BY HAROLD MYERS Elpac Power Systems Irvine, CA
In the typical design cycle, the decision to use an external power supply
is reached during the concept phase. From the beginning, designers should
try to involve manufacturers of these supplies to reduce power supply
cost. Here are 10 possibilities for achieving cost-effective designs.
1. Specify for usable power that exceeds rating External power supply
manufacturers typically design for maximum power very conservatively. The
result is that most agency-approved units on the market today can actually
perform at much higher power levels than those to which they are approved.
For example, designs incorporating either tape or disk drives typically
require additional power during spin-up of the motors. This booster power
may exceed steady-state needs by 25% or more. External power supply
manufacturers can absorb this extra power need in the external designs
without affecting cost. Designers should clearly define what the actual
steady-state power needs are and understand what the manufacturer's unit
is capable of doing. Together they can come up with a match that is
cost-effective for the application.
2. Trim the number of outputs The number of outputs definitely affects
the cost of external power supplies. The designer should therefore include
only those outputs required for the design. Applications requiring 100 mW or
less can easily be handled using low-cost dc/dc converters. In turn, fewer
outputs must be added to the power supply.
3. Specify looser regulation Most regulated external power supplies sold
today offer a +/-5% regulation tolerance covering all variations of line
and load. Many devices that use externals as the primary power source
require much tighter regulation and achieve this control by incorporating
dc/dc converters at the board level. Dc/dc converters accept a rather
broad range of dc input voltage. Hence, the designer need not specify a
+/-5% tolerance when +/-15% will suffice. Specifying looser regulation for
the external power supply will reduce the cost.
4. Reduce need for holdover storage Manufacturers of external supplies
can reduce their capacitor cost if designers consider carefully the
requirements for holdover storage. This is particularly true in
battery-operated systems. The battery usually takes over instantly if ac
power is unplugged or if a power outage occurs. The battery therefore
virtually eliminates the need for holdover storage by reducing the power
reservoirs. Reducing the need for extended holdover storage allows the
manufacturers to reduce the size of capacitors significantly.
5. Use universal input sparingly Universal input of 90 to 260 V on all
external power supplies may be an attractive feature for those
manufacturers servicing global markets. However, if only a small
percentage of products is to be sold internationally, then a considerable
sum is being wasted on universal input. Products would be much more
cost-effective by using separate outputs for the North American market and
the European market.
6. Consider low-cost connectors Connectors can contribute to the expense
of external power supplies. To reduce costs, manufacturers should consider
low-cost options like DIN connectors or barrel-type connectors when they
are appropriate to the applications. The use of multi-pin connectors or
connectors with locking devices may increase the cost by $4 to $10 per
unit. Units that sit on a shelf forever are not the same as units that are
constantly moved. Remember, the application dictates the type of connector
to be specified. Manufacturers with largely domestic markets can save
money by eliminating IEC connectors and international cord sets with IEC
mating parts. By so doing, they can reduce the costs of transformers and
EMI filters.
7. Correct EMI, save on cable Cable cost has to be weighed against
applications. A zip cord of 50% the cost of s shielded cable, but the
shielded version may be necessary for EMI. Correcting EMI at the source,
particularly in the application, may be a better alternative than using an
expensive shielded cable.
8. Consider wallmounts over tabletops Wall-mounted power supplies are
less expensive than table models. Wallmounts eliminate the ac cord and the
mating connector. In addition, wallmount power supplies simplify control
of RFI, because eliminating the cable eliminates a possible antenna.
9. Eliminate indicators and switches For either wall-mount or tabletop
units, indicators and switches do not appear to serve any useful purpose.
Wallmounts plug into the wall. Tabletop units rarely reside on the
tabletop, but rather on the floor. Indicators and switches increase the
overall cost of the power supply by adding costs for the case design and
the installation.
10. Ship in bulk
If volume product is a consideration, then bulk packaging, egg-crate
style, is much more cost-effective than individual cartons of each power
supply.
Any or all of these design tips can add up to cost savings that will be
reflected in the sales price. External power supply manufacturers are more
than willing to work with designers to help them reduce their costs.
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