Top-Down Approach Simplifies AC-DC Power Supply Selection

May 1, 2010 12:00 PM
Stephen Dodson Senior Applications Engineering Manager, XP Power, United Kingdom

For 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. Yet, with a little planning, it is not difficult to find the solution that best suits your application.

Let's start with considering input and output criteria. Most offline AC/DC converters offer universal input voltage capability that typically spans 90 to 264 VAC at 47 to 63 Hz. 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 characterize power systems built from discrete logic and analog circuitry. Despite the trend towards ever lower voltage operation in systems that use complex digital logic ICs, many industrial designers continue to favor these levels as they offer better noise margins in electrically hostile environments. AC/DC supplies are readily available with output voltages from 3.3 to 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 overspecify the supply to ensure it runs safely within its ratings under all conditions. Inappropriately overspecifying a supply can be costly, 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 to 85%, and they are not anywhere near as efficient below 50% of full load. Some multiple output supplies also require a minimum load to maintain regulation.

PHYSICAL FORM FACTOR

The output power and number of output rails greatly influence a power supply's physical dimensions. A glance at various manufacturers' catalogs reveals that encapsulated or open-frame board-mount ac-dc converters typically have one to three outputs capable of delivering 5 to 30W. An example of a board-mounted supply is the ECP20 from XP Power (Fig. 1). This compact 20 W unit measures just 67 × 40 × 18.6 mm.

At the opposite end of the scale, a rack-mounting mainframe may accommodate more than 20 plug-in modules of differing output voltages and currents that deliver in total more than 2 kW from a single-phase ac input. For some applications, forced-air cooling will be essential, which introduces restrictions such as minimum clearances between the supply's chassis and other hardware for air entry and exhaust paths.

By contrast, the desktop supplies that most often power IT & medical equipment are necessarily totally enclosed and self-cooling, which normally restricts their power capabilities to about 150W. Most such units are single output, but multiple output versions are available. If the equipment that you're building uses a DIN rail format, a typical power supply unit provides just one rail at power levels from 5W to around 1kW. Most often, space is not a crucial consideration in DIN rail environments, so if you need another output voltage, simply add another converter.

EFFICIENCY

Always check the converter's efficiency curves for the best fit for your application. Energy savings apart, it is also worth remembering that as power supply efficiencies approach 90%, a 1% increase in efficiency reduces dissipation by as much as 10%, with obvious implications in not only cooling the power supply, but also in the heat generated within the end equipment. For example, the EMA212 single output 212 Watt AC/DC power supply from XP Power achieves an efficiency of 90% (Fig. 2), providing a power density of 10.6 Watts per cubic inch in a standard 3×5 inch format.

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