AC-DC Power Supplies Are Shrinking

Power-supply design trends are clear and dramatic: additional features, improved efficiency, and smaller size for the same power ratings, for both battery-powered portable units as well as larger, line-powered ones. For the AC-DC supplies, these changes are due to multiple factors combining to yield these more-compact designs. Some of the factors are obvious, but others are not.

Start with the most apparent change, which is the use of smaller passive components. Vendors are shifting the basic resistor and capacitor footprint from 0805 size (2.0mm by 1.25mm) to 0603 (1.5mm by 0.8mm), and to an even-smaller 0402 size (1.0mm by 0.5mm). Diodes, as well, are also available in smaller packages. The use of larger parts was a holdover habit from previous generations of supplies. It made sense for vendors to continue using parts, which were fully known and characterized in their supply chain and inventory, and all of the same size.

The pressure to reduce supply size, along with the electronic industry's growing use of smaller parts in consumer products, has made them the body size of choice. The result is a significant reduction in required PC board space -- the change from 0805 to 0402 cuts a component's immediate footprint to 20 percent of its previous value -- and there are many such devices on a supply's PC board.

Changes in the supply magnetics (inductors and transformers) are also helping reduce size. Traditionally, these components have been the most complex "simple" passive ones in the supply's implementation, often custom-designed by the OEM to get precisely the desired and often subtle combination of primary- and secondary-tier parameter values (including inductance, DC resistance, size, form factor, winding type, insulation, orientation, volume, and cost).

In addition, supply designers are making better use of the available inductor volume. For example, they are reorienting the inductor to take advantage of the supply's available height-dimension headroom, in return for a smaller footprint.

On the active component side, the power semiconductors' packaging has shrunk, primarily for the critical MOSFETs of the output. Chip-scale power MOSFETs such as the DirectFET from International Rectifier (where the metal enclosure covering the bare die is also the drain terminal) provide significantly more power capability, but in a smaller package. This has had a major impact on supply size, since the DirectFET has a 30 percent larger MOSFET die but with a 60 percent reduction in PC board footprint compared to the widely used D2Pak package. Compared to the D-Pak package, the DirectFET die can be 33 percent larger than the D-Pak package, yet its footprint is 54 percent smaller.

Power-supply size also involves the necessary concerns for thermal loads. To address this issue, engineers are making extensive use of thermal gap pads for more efficient heat transfer. Since this thermal material can be a significant part of the bill of materials (BOM) cost, the heat-sink structure has been redesigned to use less of the gap-pad material, thus cutting both cost and size.