Power Electronics

Despite Designers' Best Efforts, Passives Persevere

Thirty years of progress in materials and processes is keeping demand for passive components strong despite new designs that eliminate them by the dozens.

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Almost every day, it seems, semiconductor devices are introduced that reduce the need for capacitors, resistors and other passive components in power electronics equipment, but manufacturers of passives say that demand for their products remains strong. That's because of the size reductions and performance improvements they've made over the past 30 or more years, and because lower passive components counts per pc board are largely offset by the huge increase in the number of boards being shipped.

The passive components industry, however, has changed in a couple of significant ways. For one, its center of gravity has shifted to Asia, and for another, passives suppliers and IC vendors work more closely together than they did in the past.

“Thirty years ago, there was a robust passive components industry in the United States, and it stayed healthy into the late 1980s, when business began flowing to Asia because of the manufacturing cost benefits,” said Rich Hannon, a senior engineering fellow at Astec Power (Carlsbad, Calif.).

Advances in silicon integration also have made an impact, often reducing the need for passive components in power supply designs. For example, Hannon cited the ADM1041, a secondary-side power supply controller that Astec worked closely with Analog Devices (Norwood, Mass.) to develop. The IC replaces hundreds of discrete devices, eliminating the need for manual calibration and adjustment and lowering a power supply's component count by up to 70%.

However, despite the power IC designer's ability to eliminate some passives, they still play vital roles in most power-management applications. And power chip developers now must pay greater attention to the passive components that will be used with their components.

“We keep hearing about the death of passives, but they keep reappearing,” said Jim Williams, staff scientist at Linear Technology Corp. (Milpitas, Calif.). His firm, like other integrated circuit developers, keeps in close contact with passive component suppliers.

As recently as the early 1990s, Williams noted, “Our customers' design engineers knew a lot more about their end applications than we did, and they asked very specific questions about line items on our data sheets.”

By contrast, designers today want solutions that include passive components as well as semiconductors. “They'll tell us what the power and size constraints are, and what performance is required, then they rely on us to produce a board-level solution. That's been a huge sea change,” said Williams. “The days of selling an IC based on its specs are long gone, and it means that we have to be aware of what passive component suppliers are offering, so we can recommend what particular passive devices are needed to make our parts work in each application.”

“The market for passive components is growing and will continue to grow,” said Dave Valletta, senior vice president, worldwide strategic sales, at Vishay Intertechnology (Malvern, Pa.). “Capacitor usage is up tremendously,” he added, citing the power proximity requirements of higher-speed microprocessors as one contributor.

Acknowledging the trend toward integration of passive functions into integrated circuits, Valletta noted that most integrated devices still require support from passive components.

“Historically, the semiconductor and passive components industries have tended to grow in parallel, with semiconductors leading and passives trailing by approximately six months,” said Valletta.

There have been two major trends in passive components technology, especially for capacitors and resistors,” Valletta continued. “The first is to make components smaller, and the second is to increase their power-handling capacity or other performance characteristics.”

“When I entered the business in 1972, we'd make a 104 (0.1-µF) capacitor — a common building block — in a 1210 case size. Today we can make it in an 0402 or, at lesser voltage, in an 0201, or about a magnitude smaller than it used to be,” recalled John Denslinger, executive vice president of sales and marketing at Murata Electronics North America (Smyrna, Ga.).

The size reduction re-sulted in part from the use of ceramic materials such as barium titanate, which offer a higher dielectric constant than most other materials. “Ceramic sheet thickness used to range from 10 to 30 microns, but now we can produce film or cast sheet dielectric materials in the submicron range. The semiconductor industry has gone to nanotechnology, but so have passive components,” said Denslinger (Fig. 1).

“Years ago, a 1-µF capacitor might be half an inch long. Today, it's about the size of a grain of rice,” said Jimes Lei, application engineering manager at Supertex (Sunnyvale, Calif.). “There's been a huge improvement. A 1-F capacitor the size of a garbage can is now the size of a nickel. It's the same with resistors; what used to be 0.6 in. is now 0.03 in., or about 20 times smaller.”

Passive component packaging also has evolved. Caddock Electronics (Riverside, Calif.) is credited with pioneering the use of semiconductor packaging, introducing resistors in TO-220 packaging (Fig. 2). Other vendors soon followed suit.

Passive suppliers are continually moving to new materials. In the resistor area, carbon composition resistors have given ground to metal or thick film (Fig. 3). Similarly, there have been material changes in familiar capacitor types such as the electrolytics. “Capacitor makers are moving from manganese dioxide (MnO2) to an organic semiconductor (Oscon) electrolytic material that offers lower impedance,” said Astec's Hannon.

“The biggest trend over the past few decades has been the migration from leaded to surface-mount devices,” said Vishay's Valletta. “Leaded parts are still out there, and we sell a lot of them, but they were huge compared with today's higher-end surface-mount parts.”

Hannon noted that the migration from through-hole to surface-mount resulted in denser, lower-cost products, in large part because there is so much less material in surface-mount parts. “Surface-mount was a turning point in terms of cost,” Hannon said.

“With components automatically inserted, as opposed to manually inserted, there's been a 5-to-1 reduction in resistor sizes for the same power-handling capacity,” added Sarkis Nercessian, chief engineer at Kepco (Flushing, N.Y.). “In capacitors, there's been a 10-to-1 size reduction,” he said.

“Since the 1980s, when surface-mount technology caught on, passive components are much smaller and offer lower inductance and higher frequency performance,” observed Ken Scott, vice president of engineering at the Westcor Products division of Vicor (Andover, Mass.). “Power density has increased tremendously,” he added, “from a handful of watts, back in the days when power supplies were linear, to over 1000 W/cu in.”

Surface-mount technology “crept in over the years,” according to Scott, and while it's the dominant assembly technology, applications remain for through-hole components. He cited inexpensive, silver-case power supplies for computers as one example, suggesting that if there is no significant cost advantage to be gained from moving to smaller components, manufacturers are likely to keep the larger parts with which they are familiar. Precision increased as part of the migration to surface mount, according to Joseph G. Renauer, principal designer at Texas Instruments Plug-In Power Solutions (Warrenville, Ill.). “Thirty years ago, you had to pay a premium for 1% tolerance resistors. Now there's no cost premium — 1% is standard,” Renauer said. “A resistor with 0.1% tolerance [used to] cost anywhere from $0.30 to $1, but now you can buy it for a nickel. The price of precision has come down. Manufacturers are using laser trimming or precision abrasive trimming in volume, cost-effectively.”

Renauer added that the use of newer materials in passive components enabled higher switching frequencies. “In the mid-1970s, 50 kHz was pretty much the standard,” he said. “When everything was through-hole, designers were limited by the stray inductance of the wires and the leads. As things got smaller, there was much less stray inductance, so 500 kHz doesn't raise any eyebrows now.”

Not every product family benefits from new materials or processes. “The [wirewound power] resistors we make are exactly the same as they were in 1932. They haven't changed at all. Neither the materials, nor how resistors are made, or how they are being applied has changed,” said Kirk Schwiebert, director of marketing at Ohmite Manufacturing Co. (Rolling Meadows, Ill. “More options are available today than were available 70 years ago, but wirewounds are probably still the best option for high-current applications” (Fig. 4).

“Back in the 1970s, for requirements above 1 W, you had to use wirewound resistors, which are inductive by nature,” recalled TI's Renauer. At that time, he said, the market was dominated by linear power supplies, and switching regulators were scarce.

“Now the situation is just the opposite,” Renauer continued. “The threshold is 5 W to 10 W, with linear supplies used below that, though there are some switching regulators that can perform cost-effectively at the 1-W to 2-W level. Thirty years ago, linear supplies were used at the 100-W level and beyond. You could warm your office with that on a cold winter day.”

International Components Corp. (Hauppauge, N.Y.) still sells a series of miniature polyester film capacitors that it launched in mid-1977. The units were said to have “leads welded to the foil for positive contact, minimizing contact resistance and intermittent connections.” The capacitors offered “a multiple epoxy coating that ensures resistance to humidity and soldering iron contact damage.”

“We still have customers who prefer this product, which is inductively wound and very compact — smaller than a lot of other products with the same capacitance values,” said Irwin Friedman, president of International Components. “Tantalums are smaller, but quite a bit more expensive.”

They can also be dangerous. Astec's Hannon said designers are shying away from tantalums. “They can't be sold in Japan because of the danger of explosion.”

“Back in the 1970s, sometimes a capacitor might be labeled wrong and connected wrong, and when you'd turn on a product, it would explode, and a smell would fill the area, if you didn't burn up the place,” recalled Bob Leonard, product marketing manager at C&D Technologies (Datel) Inc. (Mansfield, Mass.). “A tenth of a penny capacitor in a telecommunications product catches fire and destroys a lot of expensive equipment,” Leonard said. He added that the temperature performance of capacitors was also a problem in the olden days. “The quality of capacitors and other passive components has evolved,” he said.

The raw earth materials from which passive components are made have been improved, according to Vishay's Valletta, as have the processes used to manufacture them. One result has been a dramatic improvement in product quality. “The difference between the passive components available today and those of 10 to 20 years ago is day and night,” he said.

“It's the process improvements that have allowed us to make parts so much smaller and so much more reliable. Normally, when you shrink the size of a device and add more functionality, reliability can suffer, but processes and materials have made a difference.” Performance-wise, Valletta noted that a 50-layer MLCC was ample 10 years ago, whereas 100- and 200-layer devices are common today.

Beyond smaller size, improvements to passive components benefiting power supply manufacturers include better thermal characteristics and increased control over parasitics, both resulting from planar magnetics technology, according to Paul Greenland, director of marketing for power management products at National Semiconductor Corp. (Santa Clara, Calif.). “In the past, it was necessary to design for worst-case conditions,” he said. “Now, manufacturing processes are more consistent, and components can be designed for better efficiency.” He cited the move from hand winding to automatic winding of transformers as another sea change to benefit power supply makers.

“There have been major developments in dielectrics for electrolytic capacitors — like the migration to organic semiconductor electrolyte — and we've seen refinements in multilayer ceramic capacitors,” Greenland continued. “Capacitors today offer higher capacitance values for a given case size and voltage, [and] also higher ripple current. Manganin alloy and other new resistor materials have produced better temperature coefficients and stability.”

Innovation in the passive components industry has been both evolutionary and revolutionary, according to AVX applications manager Chris Reynolds. “In the last two or three years, we've seen an evolution to higher capacitance and higher voltages in ceramic and tantalum capacitors. In ceramic capacitors, the difference is due to the spacing between electrodes, the number of electrodes that can be stacked and the dielectric constant of the ceramic material,” Reynolds observed. According to him, reducing the thickness of dielectric layers by half results in a quadrupling of capacitance, while reducing the separation between ceramic layers results in a doubling of capacitance.

AVX said its medium- to low-voltage, high-capacitance multilayer ceramic capacitors are designed to support the trend toward smaller capacitors with higher-capacitance values as well as to conserve board space and minimize component counts. Reynolds said AVX used advanced manufacturing technologies to produce thinner dielectric layers, and also benefited from enhanced material performance. The capacitors, which target output filter and decoupling applications, feature a base metal electrode system.

Other passive component industry landmarks, according to Murata's Denslinger, include a widespread shift away from precious metals to base metal for cost reduction; Restriction of the Use of Certain Hazardous Substances/Waste from Electric and Electronic Equipment (RoHS/WEEE) compliance and multilayer substrate packaging.

AVX's Reynolds said RoHS requires the most significant change in soldering since Roman times. “It's more a political issue than a technology issue,” he said. “Manufacturers will be required to replace materials considered hazardous with materials that are usable, but may not be as good. Then they have to be sure that their parts are compatible with the old as well as the new processes.”

Reynolds said that for the past several years, customers have been investing in new equipment for higher-temperature reflow soldering.

Customers have dealt with soldering issues before. “The tendency in factories today is to want everything in surface mount,” said International Components' Friedman. “The components are placed on a board, and the board only has to go through the solder process once. But manufacturers didn't have that option in the early days of surface-mount technology. Some components were surface mount; others had to be inserted by hand, and there were different temperature requirements in production for surface-mount versus leaded devices, which meant two different soldering processes.”

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