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Miniaturisation of power converter packaging

Posted: 22 May 2014     Print Version  Bookmark and Share

Keywords:Electronic systems  FPGAs  micro-BGA  chip-scale-package  power MOSFETs 

These days, electronic systems, especially those in areas such as telecommunications infrastructure, networking and data management, incorporate increasing number of functionalities into smaller and smaller profiles. The microprocessors and other highly-integrated ICs on which they are based require correspondingly greater levels of power, often at voltages that are lower, and currents that are higher, than ever before.

Power provision has evolved along with every other component and sub-system: regulation, distribution and voltage conversion have had to migrate on to the main system circuit boards and can no longer be designed as a separate entity. Advanced packaging designs shrink power-conversion functions enabling greater power integration with main board functions; dramatically improved efficiencies cuts losses to levels that make heat removal from those smaller packages, a practical endeavour.

Many of the trends in today's systems were evident as long ago as the early 1980s a convenient point in the context of this article, as it marks the period during which linear power supply topologies were being replaced by switching methodologies for the great majority of designs and also saw the appearance of the brick, of which, more later. State-of-the-art design rules for chip fabrication were around 1.5 microns; parts already fabricated that you bought off the shelf, would likely be larger still. The change of semiconductor geometries that Moore's Law has charted, from 1.5 microns to today's leading-edge 22 nm, represents an increase in functional density of over 4600:1.

Reduced silicon geometries drove integration, packing more and more functional blocks on to each chip. Beyond that, the chip integration story is largely one of packaging. The economically-optimal die size has not changed much other than in the case of the largest and fastest processors, FPGAs and systems-on-chip. But compare an earlier-generation 40-pin DIP with today's chip-scale-package, micro-BGA offering and the trend is clear to see.

Conversion efficiency enables high power density
Power supplies for electronic products have followed a parallel path, although not always in a smooth progression of diminishing size. The story is also one of increasing density over time and with it, efficiency. Ever-higher figures for Watts delivered from a given volume have gone hand-in-hand with efficiency; the losses (heat) dissipated by the typical supply of three decades ago could not be extracted by any practical technology from the volumes occupied by todays power conversion blocks.

Figure 1: Traditional open- and closed-box power sub-systems, including the ATX12V silver box power supply shown here, provide low power density and limited scaling potential.

One small reference point can be judged by comparing a single device from then with a complete function now. A workhorse bipolar junction transistor of the 1970s, still widely used in the early 80s, was the 2N3055 or in its plastic-packaged form, MJE3055T. In its TO-220 form it would occupy a footprint of 16 x 10 mm (just one switch), not allowing for its through-hole terminations, and it was a 60 V, 10 A maximum-rated device. Today, we can draw 1.2 kW continuously from a single package on a footprint of 64 x 23 mm and that is for the complete function, not just a single transistor.

While improvements in semiconductor switching performance (higher frequencies, lower device on-resistance), magnetics and architectures drove the early part of this trend, more recent improvements have also depended on advances in thermo-mechanical design, particularly of power management components.

Some formats from decades past remain in use, largely unchanged, today; for example, the silver box supplies that power desktop computers. A 400 W ATX12V features a largely discrete design (figure 1). Individual heat sinks cool power MOSFETs and output rectifiers but the overall thermal design results in large thermal gradients, which are problematic at high ambient temperatures. With a typical efficiency of 80%, the 138 x 86 x 140 mm form factor provides a power density of only 0.24 W/cm3. High-quality ATX supplies designed to meet the 80-plus Platinum specification endorsed by the US Energy Star programme can almost double that number to 0.42 W/cm3 but as a design approach, they fall far short of the density needed for telecommunications racks or server farm/data centre applications.

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