Power Electronics

High-Frequency DC-DC Converters: The Writing Is On the Wall

In the past few years, the unprecedented demand for handheld electronic appliances and laptop computers with an ever-more impressive list of features and capabilities has meant more demand for processors. This demand directly translates to greater needs for higher current and power at voltages that will soon approach the fractional voltage range from the dc-dc converters in these appliances.

These demands mean that converters must now be more efficient to allow for longer battery life, while fitting into ever-shrinking volumes and pc-board footprints. And, as if this were not a tough enough challenge for power-supply designers, better transient response and tighter load regulation are also required.

To the experts it is clear that high switching-frequency operation — in the 5-MHz to 10-MHz range — is becoming mandatory. This single parameter holds the key to achieving designs that can meet all the demands of the next generation of portable appliances.

Higher switching frequencies will allow engineers to design supplies with considerably wider control-loop bandwidth, which is typically between a fifth and a tenth of the switching frequency. This is important because the wider the loop bandwidth, the fewer output filter capacitors needed, leading to cheaper design and smaller pc-board footprints. At present, a large number of output capacitors is needed in low switching-frequency applications to properly handle the large and fast transient load currents, while staying within transient voltage regulation limits.

As designs move to high switching frequencies, the filter inductor may also shrink, as less filter inductance is needed. This change further reduces the total volume occupied by the output filter and adds the benefit of lower cost.

The transition to 5-MHz to 10-MHz switching frequencies has been slow in coming due to the demands it places on almost all the components used in the design. Also, there is an immediate need for engineers to learn new processes for designing high current and high frequency converters.

For starters, if we really want to reap the full benefits of the high frequency approach, ceramic capacitors used in the output filter must be chosen to handle larger ripple currents for a given value and package at a lower effective series resistance for reduced output-ripple voltage. A variety of small-outline, low-profile, surface-mounted inductors are currently available for designs in the megahertz range and in the right inductance range of 30 nH to 100 nH. But some further reduction in size is still needed for currents in the neighborhood of 5 A to 10 A and even beyond to address the developing needs for the not-so-distant future.

This brings us to the switching power MOSFETs needed in these applications with their two ingredients, namely package and silicon technology. High frequency-power packages have matured in the last few years, and several packages are already available and can be used once this trend comes to fruition, becoming common practice.

Power-switching devices like MOSFETs that are used in these applications must offer low enough switching and conduction losses to achieve total power efficiencies of 85% to 95% at full current. At the same time, the control scheme must be designed to offer similar efficiencies across almost the entire range of load current to maximize the battery life. The demands on the power train create opportunities for the engineering community to develop the present topologies and/or invent new topologies and smarter gate drivers to deliver the performance levels needed.

In my opinion, the best approach would be “complete power modules” that are comprised of the controller, gate driver and switching devices where all the interfaces have been optimized to deliver the highest possible performance for a given application. Few such modules are available today from a small number of suppliers. Nevertheless, these modules open the door for further development to improve dc-dc converter performance in the portable and handheld market.

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