Rectifier ICs and Thermal Packaging Enhance SMPS

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The personal computer has significantly evolved over the last 30 years, with performance driven by Moore's Law and cost pushing downward due to component standardization and multiple sourcing. As PCs have evolved, adding a DVD drive, more memory, impressive graphics and sound capabilities, the switched-mode power supply (SMPS) has significantly contributed to this progress by providing more power in a smaller space and at a lower cost.

The computer casing has changed from an uninspiring beige box to sleek new designs where the CPU motherboard slides into the rear of the LCD monitor, or where the whole PC is contained in a box with a desk area only a little larger than a DVD. The key to this increase in power density has been the revolution in power management, which includes new topologies, silicon platforms and assembly techniques to maximize efficiencies.

Traditional 200-W PC designs used the forward topology with 800-V primary field effect transistors (FETs), and a plethora of ultrafast and Schottky diodes, to provide current at various output voltages. These designs used large fans to blow air across inefficient components bolted to large heatsinks and assembled onto large single-sided through-hole printed circuit boards.

Advances in MOSFETs

Several changes have taken place in the industry since then. For example, global power factor correction requirements mean that a stable, lower voltage is now applied to the primary stage. Where voltage spikes were once 800 V, they are now approximately 400 V, allowing for the use of lower-voltage primary FETs with lower R_DS(ON) and allowing cost-effective, alternate topologies. Metal oxide semiconductor field effect transistor (MOSFET) technology itself has also improved, and with 500-V to 600-V nonplanar devices well established, trench technology has now clearly surpassed planar platforms below 300 V.

Another change is that MOSFET-based synchronous rectification for the secondary side has eclipsed diode rectification. Furthermore, new secondary control techniques and stepdown converters have allowed a move from single-sided printed circuit boards to dual-sided, surface-mount assembly for maximum power density.

The secondary-side options available today can enable a high-performance, low-cost power supply without a heatsink. Before examining multi-output power supplies, it's beneficial to understand how synchronous rectification (SR) using MOSFETs has taken over from diodes in single-output converters with a focus on the technically challenging, enclosed laptop adapters. Here, SR takes advantage of the much lower MOSFET I²R power losses compared to the V_F × I_F losses for diodes, and enables smaller, lower-cost SR converters when the current for each output is more than 3 A.^[1] Typical schematics appear in Fig. 1.

In 2003, a commercially available 120-W laptop adapter served as the reference for a new synchronous design. The example result shows that a 1% higher efficiency is achievable at this power and current level while reducing the rectification-system cost by almost 25% (Fig. 2). Today, most new laptop adapter designs above 75 W have SR converters.

New Control Techniques

In 2006, the next advance in SR has been an upgrade in control techniques from current-transformer (polarity-detection) topologies to a new voltage-level-sensing technique^[2], which minimizes secondary reactive currents that cause losses in the secondary MOSFETs, increasing efficiency and lowering system costs even further (Fig. 3).

With a different adapter, the Sony Vaio, serving as a reference, upgrading the secondary stage from a current transformer to the voltage-level-sensing architecture provides the performance and cost results that appear in Fig. 4.

The first three entries used the existing mechanical/thermal design with a large heatsink and TO-220 MOSFETs. The last entry shows the performance of a redesigned secondary circuit using surface-mount SO-8 MOSFETs and no heatsink. Fig. 5 shows the physical differences between this converter and the original unit having a heatsink. Re-arranging the output filter capacitors and slicing around ¾ in. off the length of the adapter would result in a further increase in the new adapter's power density.

Turning to typical multiple-output converters, the typical power level for mini-PCs, whether in “tiny towers” or “panel PCs” where the motherboard is inserted behind an LCD screen, is 180 W to 200 W. Traditionally, multi-output power supplies require multiple transformer-secondary windings or taps with Schottky diodes providing rectification (Fig. 6). Here, the diodes will be large TO-220 (possibly TO-247) through-hole devices bolted to heatsinks.

This arrangement had the advantage of easy design and implementation, but provided low power density, added cost to the transformer and relied on large heatsinks to dissipate the power lost through the diodes.

As in the laptop adapter example, we can replace the diodes with a synchronous circuit and introduce a high-power-density surface-mount layout. For the proposed design, the objective is to take the veritable flyback converter topology and use SR to increase the efficiency, thereby extending the feasible power range to 200 W. As in the laptop case shown in Fig. 3 on the right, a 12-V line is created using the SmartRectifier scheme. From there, stepdown buck converters could generate the lower-voltage outputs (Fig. 7).

Following the 120-W laptop-adapter design without the heatsink, the same thermal/mechanical design based on surface-mount technology is applicable here with SO-8 controllers and FETs wave-soldered on the underside of the main SMPS printed circuit board. Furthermore, all of the secondary control and rectification MOSFETs can fit on a modular, reflow-soldered daughtercard, thus opening up the use of high-efficiency DirectFET-packaged MOSFETs. A broad range of stepdown buck converters are available — single, multiple, with or without extra LDO control pins — to customize and optimize the new design. In this type of configuration, a tightly controlled 12-V output automatically improves the tolerances on the lower-voltage outputs.

The proposed design achieves high power density without a heatsink and could slip easily into the new smaller form-factor PC outlines. As an extra bonus for the designer, the use of IC controllers and MOSFETs aids the creation of circuits that meet efficiency regulations. For example, many control ICs feature enable pins and have micropower standby losses to aid 1-W standby-power compliance, while the improved efficiency under load helps with observing the 80%-plus standard.