Electronic Design

Power: Introduction/Power Semiconductors

Smarter Devices Spur Savvy Supply Design
Over the years, advances in power semiconductors, especially discrete power transistors and power control ICs, have played a crucial role in the development of power supplies. They have coped with the challenges of every generation meticulously and promise to keep that momentum going in the future. Improvements in power devices have inspired power-supply designers to adopt new topologies and move forward to meet the stringent demands of emerging systems.

In 1974, the emergence of the pulse-width-modulated (PWM) controller chip sparked the transition to switching topologies. It opened the door to using ICs in power-supply designs. The trend to higher switching frequencies also replaced bipolars with power MOSFETs. Together, these two parts have dramatically impacted power-supply design. The evolution of voltage regulators and integrated dc-dc converters has eased the path to distributed architectures in a variety of systems.

Over the last few decades, MOSFETs have undergone rapid evolution to serve a myriad of application needs. From cellular to stripe to trench structures, MOSFETs have taken different shapes and forms to keep their losses low, while keeping switching frequencies and efficiency levels high. MOSFETs have also ex-ploited the benefits of packaging to raise the performance bar to new heights.

Concurrently, power controllers have witnessed a number of im-provements to make designers' lives simpler, while delivering more functionality, higher frequencies, higher efficiency, more current-handling capability, greater protection, and a lower bill of materials. Newer topologies like multiphase have emerged to tackle very high-current requirements at lower voltages. For high-voltage offline applications, integrated CMOS controllers capable of withstanding 700 V have delivered compact low-cost offline switching power supplies.

With board space constraints, and the trend toward complete solutions, suppliers have begun to put MOSFETs with PWM controllers in the same package. Likewise, power-factor-correction (PFC) circuits and offline switchers in one package are coming soon as well.

A more recent device, the insulated-gate bipolar transistor (IGBT), has advanced to challenge MOSFETs in 250-W and higher ac-dc power supplies. In this space, newer and faster IGBTs promise an order of magnitude improvement in power density. Like their MOSFET cousins, IGBTs are heading in the same direction. As they begin to make inroads into this arena, they will be combined with controller ICs and other functions to deliver an integrated package that minimizes the number of necessary external components. For applications that experience unbearable heat, silicon-carbide (SiC) FETs are in preparation. In fact, SiC MOSFETs are being touted for RF amplifiers.

In the future, this challenge is far greater as forthcoming systems require better efficiency, much faster transient response times, smaller size, higher reliability, and lower cost. Moreover, these features must be provided at 1-V and below operating voltages with current ratings of 100 A or more, as dictated by new generations of multigigahertz CPUs. Just a few years ago, such expectations were considered impractical.

Thanks to a close relationship between academic and industry organizations, power semiconductor inventions are more rapidly moving into the commercial world. Expect a new breed of digital power controllers that promise a new level of flexibility soon.

While power-supply vendors exploit power IC advances, they also are acquiring design talent to develop proprietary control ASICs that best serve their performance goals. In addition, they're using synchronous rectification for higher performance.

But semiconductors are simply one piece of the design puzzle. Vendors are seeking out the latest innovations in passive components. Smaller electrolytics, low-profile planar transformers, and surface-mount inductors are reducing space requirements and enabling the very low profiles associated with ac-dc and dc-dc converters.

Still, with all of these advances and the crowding of components on supply boards, designers are resorting to thermal modeling and even wind-tunnel tests to determine how to lay their designs out for optimum heat transfer given the constraints on cooling. Packaging has become a critical factor in power-supply performance. This is seen clearly in the dc-dc converter area, where power and current levels handled by various brick-style formats have continuously risen over time, while package heights have come down dramatically.

Because system requirements are critical, many power-supply developments have been tailored to address system needs. Swapability and configurability have also driven power-supply package design toward modularity with the goal of keeping systems powered up at all times. The development of increasingly powerful modular supplies has, in turn, created a need for better power-distribution components such as connectors, backplanes, and bus bars.

Recently, telecom applications have played a larger role in dictating power-supply packaging requirements, input and output configurations, current demands, and other electrical parameters. The growth in distributed power systems has affected the ac-dc or front ends and on-board dc-dc converters. Other system concerns come into play with UPS designs that must often coordinate their operations with that of other power sources. In the end, considering system-level requirements amounts simply to greater customization, a trend that's steadily blurring the line separating standard power-supply products from fully custom ones.

Structural improvements and packaging advances have helped slash the on-resistance of low-voltage power MOSFETs to a new low of 3 mΩ in a miniature package like the SO8. Using trench technology, a further cut in on-resistance is expected. Other technologies besides the trench design are also being explored. Developments in drift-region engineering at the power semiconductor research center of North Carolina State University are heading to the commercial world. This is expected to provide a multifold improvement in the on-resistance for the same breakdown voltage. It also breaks the old rule that defines the breakdown voltage as a function of doping concentration. However, on-resistance isn't the only hurdle. With switching frequencies approaching 2 GHz, gate charge is becoming equally important. On-resistance times gate-charge is the figure of merit for a new class of power MOSFETs, with the focus on improving this parameter.

Clever modifications to high-voltage processes have enabled power IC makers to integrate a complete offline switcher, including a high-voltage power MOSFET, PWM controller, fault protection, and other control circuitry, on a single CMOS die. One such device is being extended to handle up to 250 W of power. More players are expected to jump into this fray. Also, offline switchers will soon be combined with power-factor correction (PFC) circuits in a single-chip solution.

Lateral-diffused MOSFETs have made substantial progress in gain, efficiency, peak power, linearity, and reliability to make inroads into RF power applications. Also, the input and output impedances of these transistors are being matched to make their implementation easy. In addition, they're now going into inexpensive plastic packages to deliver low-cost/W performance. Taking these improvements into account, RF LDMOS FETs are ready to displace traditional silicon bipolars from RF power amplifier sockets in wireless infrastructure equipment, where competition is aggressive and suppliers are many. Although present advances are aimed at 800 MHz to 2.4 GHz, work is in progress to push the RF envelope of LDMOS to 3.5 GHz. There, it intends to compete with GaAs FETs.

Faster IGBTs are tough competition for high-voltage MOSFETs in power supplies above 250 W, where the switching frequency is normally below 125 kHz. Major features are higher current density, lower conduction losses, and an inherently faster antiparallel diode. Further improvements are expected as the demand for these devices grows. Like MOSFETs, they too will come in miniature packages and be combined with other functions in the same package.

Smart power devices have made significant strides in the last few years to "super-smart" power. As they implement on-chip microcontrollers and DSPs with logic, memory, control/protection, and high-voltage power MOSFETs, or other high-power switching circuits, they're migrating to 0.35-µm and finer CMOS design rules for system-level integration. As the functional integration rises with time, so will the voltage-handling capability of the super-smart power solution. Newer materials like silicon-on-insulator (SOI), in conjunction with clever isolation techniques, will enable suppliers to push the voltage-handling capability of such devices to 700 V and above, while permitting low-voltage circuits to perform reliably, side by side, on the same chip.

While low-dropout (LDO) voltage regulators are moving into smaller SOT23-5 packages, they also are supporting higher output currents at lower voltages. Now the trend is moving toward sub-1-V output at higher current (>1 A) with ultrafast transient response, while requiring only a small inexpensive ceramic capacitor at the output. Many portable applications are using multiple LDOs, so suppliers are putting multiple such devices in a single package. Likewise, PWMs have been integrated with multiple LDOs. But as the pressure for miniaturization mounts, designers are asking for more functions on-chip. Toward that end, suppliers are investigating the merger of battery charging functions with several LDOs and dc-dc converters on a single CMOS chip.

As switching regulators move up the frequency scale, they also are squeezing more power from smaller packages. Driven by the needs of portable electronics, the pressure for more and more in terms of efficiency continues. Also, for longer battery life, quiescent currents will continue to drop. Advanced CMOS processes will permit multiple such regulators in a single miniature package.

SiC FETs can now compete for RF power amplifier sockets in the 2-GHz spectrum. These newer transistors promise higher density with better thermal conductivity. Meanwhile, developers are refining the process to squeeze more power out of a smaller die.

As power MOSFETs move into smaller packages with higher performance, they're being combined with PWM controllers in the same package. Likewise, for many other applications, drivers are being merged with MOSFETs on the same substrate. For instance, for automotive applications, suppliers plan to combine FET drivers with low- and high-side MOSFETs in a single QFN type package, which is about half the size of an SOT23-5.

While thermal- and current-sensing for protection has been available with some MOSFETs, the use of so-called smart or intelligent discretes is growing. More suppliers are now jumping on this bandwagon.

See associated timeline.

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