14 power SupplieS
Take a top-down approach to simplify PSU selection
Stephen Dodson looks at the latest issues in selecting the right power supply.
F
or many systems designers, the number and variety of parameters that require consideration can make selecting a power supply
a daunting task. This is especially true when environmental and regulatory issues add to the list of specification requirements. Even board-level dc/dc converter modules can be challenging to select, as factors such as cooling requirements often complicate an apparently simple application. With planning, it is not difficult to find the solution that best suits the application. It is the application that drives this top-down process, starting with understanding the power requirements and how to integrate the power solution.
Most offline ac/dc converters offer universal input voltage capability that typically spans 90–264 VAC at 47–63Hz. This wide range allows configuration-free use anywhere in the world, relieving the system designer from the concern that end-users may incorrectly set their equipment. This is a desirable feature, but remember that the optimal input- protection fuse ratings are rarely the same for 110V and 230V operation as these voltages mean a 2:1 spread in steady-state input current. The input fuse needs to be sized for the lowest operating voltage. Some applications use the ac/dc converter to drive load circuits directly rather than via further dc/dc conversion stages. As a result, power-supply vendors offer a wide range of output voltage and current options, such as the 5V and ±12V that traditionally power systems built from discrete logic and analogue circuitry. Despite the trend towards ever lower voltage operation in systems that use complex digital logic ICs, many industrial designers continue to favour these levels as they offer better noise margins in electrically hostile environments. Ac/dc supplies are readily available with output voltages from 3.3–48VDC, in single and multiple output combinations. Single output supplies are often used in DPA (Distributed Power Architecture) and IBA (Intermediate Bus Architectures), where POL (Point of Load) converters are used
to provide the various necessary voltages locally. In terms of output current capability,
it is tempting to over specify the supply to ensure it runs safely within its ratings under all conditions. Inappropriately over specifying a supply can be a costly mistake both in terms of initial purchase cost, and in running costs. Most converters operate at their maximum efficiency towards the higher end of their operational range, say 80–85 per cent, and they are not anywhere near as efficient below 50 per cent of full load. Some multiple output supplies also require a minimum load to maintain regulation.
The output power and number of output
rails greatly influence a power supply’s physical dimensions. A glance at various manufacturers’ catalogues reveals that encapsulated or open- frame board-mount ac-dcconverters typically have 1–3 outputs capable of delivering 5–30W. For example, 20W board mounted supply measures just 67 x 40 x 18.6 mm. At the
Fig. 1. The ECP20 compact 20 Watt unit measures 67 x 40 x 18.6 mm.
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