Resonant technology will spur power-supply design developments
Power supplies for the 1990s will require a new type of technology to become faster and smaller like the VLSI devices they support. A new power conversion concept, resonant technology has twin goals of increasing the power density of supplies and improving performance by operating at higher frequencies than current pulse-width modulation (PWM) supplies can handle.
The trend is also away from large centralized power supplies to distributed supplies designed as modular or board-mounted types. Board-mounted supplies will need to have the same low profile as ICs, necessitating improvements in component technology,
For most of the 1970s and 80s, switching power-supply design was based mainly on pulse-width modulation (PWM) circuits. But the 1990s will require a new type of technology. Because VLSI advances make electronic systems faster and smaller, future power supplies must follow suit.
VLSI technology shrinks everything except the demand for power. Over the past five years, a new power-conversion idea called resonant technology was under development in private companies and universities. Its aim is smaller and more efficient power supplies than what’s possible with PWM methods. In the long run, perhaps by the end of this decade, resonant technology and new component and circuit fabrication techniques could reduce certain types of power supplies to commodity products, much like ICs.
All resonant designs share common goals. First is to increase the power density of supplies. A second goal is to improve performance by operating at higher frequencies than is possible under PWM technology. With PWM techniques, 100-200 kHz is about the upper frequency limit. Resonant converters will operate at several MHz and above. A commercial 1-MHz resonant converter is now available.
The major trend is the move away from large centralized supplies to distributed supplies. Distributed supplies are more suitable for the smaller, faster systems resulting from the rapid advancement of VLSI technology. They’re designed as either modular supplies or board-mounted types. Increasingly decentralized supplies will be built with resonant technology.
Resonant technology and distributed power aren’t new ideas, but rather ideas whose time has come because we’re now gaining the technology to implement them. In addition to work at the Virginia Power Electronics Center at Virginia Polytechnic Institute and State University, MIT, AT&T, Bell Laboratories, General Electric, and Unisys are working on resonant converters.
Whereas conventional PWM supplies for computer systems have power densities of 1 or 2 [W/in..sup.3.], experimental resonant technologies are at least an order of magnitude higher. A design target at GE and at VPI’s Center is 50 [W/in..sup.3] intially, and 100 [W/in..sup.3] before the end of the decade. Bell Labs has demonstrated a resonant supply that can deliver 50 W and runs at 20 MHz. But most 50-W output, [50-W/in..sup.3] power converters operate from 2 to 4 MHz. Some of these types of supplies will be practical by the early 1990s.
Much of the impetus for developing high-density, 50-W supplies comes from the military. VHSIC requirements call for compact distributed supplies that mount directly on a logic board, deliver up to 50 W of 5-V power, with superior transient response to power logic that can be running at 100 MHz and greater. These converters must have a power density of 50 [W/in..sup.3] and operate at several MHz. The long-term objective–towards the end of the decade–is to build supplies in chip form and mount them on pc boards.
It should be no surprise that units with power densities of 50 [W/in..sup.3] must be highly efficient to reduce the amount of head produced. A 1-MHz resonant converter should be able to operate at 90% efficiency, considerably higher than the 80% figure of today’s PWM supplies. It’s almost an axiom in the design of high-frequency resonant supplies that you don’t have a technology until you can deal with the heat.
In addition to high power density, the coming generation of board-mounted supplies will have to have extremely low profiles, much like ICs. This means that improvements in component technology are necessary to flatten out the package. Magnetic elements in particular are receiving lots of attention in an attempt to reduce their size. A thin magnetic plate or substrate can have windings printed on it, and then be covered with a low-profile core to make a transformer. Multilayer pc-board techniques are already being applied to inductors and transformers. Capacitor technology is also advancing. It’s now possible to integrate many capacitors on one chip in a dual-in-line package.
Another reason to reduce magnetics to an IC-like technology is to take most of the labor out of the manufacturing process. If a transformer or inductor can be built like an IC, the process can be automated. In fact, this is the ultimate goal; manufacture a power supply just like you make an IC, and reduce it to a commodity product.