What's shrinking AC-DC power supplies?

Don Knowles, Director of Engineering, N2Power


Smaller components, enhanced thermal performance, and DSPs enable smaller units with the same power rating.

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, as described in Figure 1. Vendors are shifting the basic resistor and capacitor footprint from 0805 (2.0 mm × 1.25 mm, 0.08" × 0.05") size, to 0603 (1.5 mm × 0.8 mm; 0.06" × 0.03") and to an even-smaller 0402 (1.0 mm × 0.5 mm; 0.04" in × 0.020") size. Diodes, as well, are also available in smaller packages than they previously were. The use of larger parts was a holdover habit from previous generations of supplies. It made a lot of sense for vendors to continue using parts that 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 just 20% 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 squeeze the size down. Traditionally, these components have actually 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 also making better use of the available inductor volume. For example, they are re-orienting 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 packaging of the power semiconductors has also shrunk, primarily for the critical MOSFETs of the output. Chip-scale power MOSFETs such as the DirectFET from International Rectifier (where the metal enclosure which covers the bare die is also the drain terminal) provide significantly more power capability, but in a much-smaller package. This has had a major impact on supply size, since the DirectFET has a 30% larger MOSFET die but with a 60% reduction in PC board footprint compared to the widely used D2Pak package. Compared to the D-Pak package, the DirectFET die can be 33% larger than the D-Pak package yet its footprint is 54% smaller. Of course, power supply size also involves the necessary concerns for thermal loads. To address this issue, engineers at N2Power 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 company redesigned the heat-sink structure to use less of the gap-pad material, thus cutting both cost and size. Finally, by increasing the internal operating frequency for these switching supplies, designers have been able to decrease overall size or, alternatively, pack a higher power rating into the same size unit. For example, the 125W XL125 supply from N2Power has a power factor correction (PFC) circuit operating at 87 kHz and a half-bridge output at 43 kHz. By moving both to 125 kHz in the 160W XL160 design, the volume of the associated magnetics was reduced by one third, thus allowing this 160W unit to fit in the same 3"×5" 1U footprint as the 125W supply, and with a height of just 1.25".

As another dramatic example of the benefits of higher frequency, consider the XL375 from N2Power, Figure 2a, versus the XL275, Figure 2b; despite being rated 36% higher in output, the 3.3"×5.0"×1.5" (1U form factor) 375W unit (power density of 15W/in3) is just 0.3" longer than the 1.25"×3.00"×5.00" 275W supply (12W/in3>).

What about "smarter" supplies? Active components also are playing a major role in the shrinking of supplies in several ways. IC vendors are developing better analog components for power management, especially helpful for complex functions such as PFC. The result is better accuracy and performance at both high and low line voltages. Early-generation PFC circuits could achieve power factor up to 0.99 (nearly a perfect unity value) at lower AC-line voltage of 90/110VAC, but the correction dropped to as low as 0.75 at 240VAC. Using the latest analog ICs, designs can now maintain high PFC across the full voltage range, leading to the need for fewer corrective components and thus smaller supply size. The most dramatic change for supplies architecture is the incorporation of digital control circuitry in the inner workings of the supply. In recent years, processors have been used in the secondary side of the supply to monitor key points and performance, to establish some operating parameters and to manage a communications interface port. But the primary-side control loop still remained analog. With the availability of high-performance, low-cost digital signal processors (DSPs), digital control now extends to the primary loop. This brings additional flexibility in control and operating points, of course, including on-the-fly adaptive control and dynamic operating changes. It also impacts size, by putting more of the control functions in less space, due to fewer needed ICs and passive components to implement the hardware control-loop strategy. For example, a supply using DSP control can do three-phase AC line control with about the same footprint as a single-phase unit. The DSP can also provide the required PFC with no further footprint penalty. Conclusion For switching AC/DC supplies, no single factor is responsible for shrinking physical size. An additive combination of the switch to smaller passives, smaller power MOSFETs and increased availability of customized magnetic components is the starting point. Add the enhancements which analog power IC vendors are offering, plus the radical architecture shift which DSP-based control brings and the result is that users are seeing shrinking supply size, but without penalties in performance or cost. N2Power