Moving to Integrated Magnetics

May 1, 2004 12:00 PM
David Morrison, Editor

The goal of higher power and current density in power supplies is being achieved largely in silicon. Improvements in power semiconductors are increasing their efficiency and power-handling capabilities. At the same time, functional integration is producing power supply controllers capable of higher performance while shrinking board-space requirements for the supply.

There's also ongoing progress in bringing power controllers and power MOSFETs together on the same piece of silicon. For example, this month Texas Instruments is introducing another member in its SWIFT family of dc-dc (stepdown) controllers with integrated FETs. The new SWIFT chip (TPS54010) is rated to deliver 14 A of continuous output at voltages as low as 0.9 V, while operating from a 3-V to 4-V supply. This current rating is impressive considering that, up until a few years ago, monolithic buck regulators with on-chip FETs were limited to just a few amps of output.

Naturally, some tradeoffs have been made to achieve this high current rating in a monolithic chip. In the case of the TPS54010, the FETs' low on-resistance comes at the expense of their lower-voltage ratings. Nevertheless, this chip demonstrates the high level of performance that's possible when all of the active circuitry for a synchronous buck converter is integrated on one die.

For now, such highly integrated converter chips occupy a narrow niche in the spectrum of dc-dc converter designs. They fall somewhere between multichip designs and functionally complete modules in terms of their performance, ease of use and cost. But in time, as performance improves and costs fall, monolithic dc-dc converter chips should find greater use in point-of-load (POL) power applications.

Beyond integrating just the active circuitry, the logical next step is to integrate the passives as well. One vendor, Enpirion, has gone this route in developing the 3-A POL converter described in this month's Product Innovation department (see page 62).

Enpirion's POL co-packages a monolithic buck converter with a MEMS-based inductor. The two silicon die are placed side-by-side in a 12.5-mm × 8.1-mm DFN package that also allows room for ceramic decoupling capacitors. The POL then requires just a few external capacitors to complete a synchronous buck converter that is said to be significantly smaller than comparable designs.

This part represents just a beginning in the integration of active circuitry and magnetics. The potential exists for building active circuitry and magnetic components on the same piece of silicon. However, such a space-saving approach may be a few years away from commercialization.

Although researchers continue to work out the details of designing and fabricating magnetics on silicon, the success of this technology hinges on the development of power converter circuitry capable of operating efficiently at higher switching frequencies. The target is in the 5-MHz to 10-MHz range rather than the 1-MHz and below range that is typical of most PWM converters today. The higher frequencies are needed to limit the size of the inductors and transformers so they can be fabricated as silicon-based structures.

In the case of the Enpirion converter, the company developed proprietary circuitry and process technology to allow satisfactory performance at a typical operating frequency of 5 MHz. This accomplishment is achieved with chips fabricated in mainstream CMOS processes.

This ability to build magnetics in silicon using common semiconductor manufacturing equipment will be critical to the commercialization of integrated magnetics. That applies to fabless companies such as Enpirion, as well as to the more-established power chip suppliers who rely on their own fabs. Some of these power IC manufacturers are already moving in the direction of monolithic converter designs with efficient multi-megahertz switching. So it may not be long before more chip makers develop or adopt some form of integrated magnetics in their converters. Such developments will further blur the lines separating power chips from power modules as well as the distinctions between the companies who supply them.

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