Power Connectors Beat the Heat

09.02.2008 // Posted by: Bob Hult // Posted in: Articles, Connectors

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They say the world is getting smaller, and that certainly applies to electronic devices. It used to take a large room of equipment to get the computer processing power now available in a laptop. Storage devices, the size of refrigerators, have been replaced with pocket-sized solid state flash drives. Long ago, engineers adopted the mantra “more functions in smaller packages,” which includes the challenge of delivering the necessary power while removing the resulting heat.

As system packaging density increases, so does pressure to reduce the size of every component in the box. Signal connectors have evolved to address higher speed and pin counts on smaller centerlines, but power connectors must deal with Ohm’s Law and the heat generated at the interface.

Several years ago, manufacturers of devices, such as processors, FPGAs, and memory, recognized that the race to faster speed was driving power demands beyond manageable limits. The advantages of quicker processing times were overwhelmed by the complexity of power distribution and heat management issues. Since then, the theme in the industry has become reduced power per processing cycle. Introduction of multi-core processors and low-power memory has slowed power consumption growth per device, but more devices packed into smaller envelopes continue to challenge system designers.

Manufacturers have introduced new power supplies that address the power and density demands of the industry. Modular switching power supplies feature efficiencies greater than 90%, with power density of 25 watts per cubic inch. Power supplies in 1U (1.75”) form factors are rated up to 700 watts. This degree of power density enables the shrinking of product profiles, but increases the challenge of power distribution and thermal management at the system level. Power connectors play an increasingly important role in the solution.

Power connectors are commonly rated by the number of amps of current necessary to raise the temperature of mated contacts no more than 30°C. Power is applied to a mated pair of contacts, and the temperature at the interface is carefully monitored. As the current increases, the temperature rises until a stable 30-degree limit is reached. This becomes the published current rating of the connector.

A number of factors influence the current rating of a power contact, including the gauge of the wire attached to the contact, the number of energized adjacent contacts, the ambient temperature, and the degree of air movement. These are all related to the test conditions, and should be disclosed in the test specification. Connector designers are looking for ways to increase the intrinsic efficiency of power connectors. Solutions to that challenge fall into three main categories.

1. Increase the basic conductivity of the contact materials.
2. Improve the contact design to increase the metallic surface areas that are in intimate contact.
3. Increase cooling airflow circulation around the contacts.

Connector manufacturers are exploring all three avenues to enable the development of new connectors in smaller form factors and greater power density.

Traditional power connectors are typically large, bulky interfaces, primarily because they are not the most efficient conductors. Pure copper wire has excellent conductivity. Unfortunately, separable connectors require some degree of spring characteristic to ensure adequate normal force at the interface.

Pure copper offers very little spring, so many connector manufacturers form their contacts by using a variety of copper alloys that provide the essential compression characteristics. Beryllium copper and phosphor bronze have excellent forming and spring characteristics, but the electrical conductivity can be 40% or less of pure copper. This is not a major problem in low-level signal applications, but mandates larger contacts to compensate for the reduced conductivity in power circuits.

Some contact designs utilize nearly pure copper contacts and add a separate spring mechanism, which is not part of the electrical path, to generate normal forces at the interface. These durable connectors offer excellent conductivity, but extra parts consume valuable space and may limit connector density.

New copper alloys are being introduced that retain excellent formability, yield-strength, and stress relaxation characteristics, while increasing conductivity to greater than 65% of pure copper.

Tighter system packaging and reduced cooling airflow result in higher operating temperatures within the box. Contact alloys, which retain their spring characteristics at elevated temperatures, become highly desirable in this application. The increase in the contact material’s thermal conductivity improves its ability to efficiently transfer heat away from the separable interface. The use of these new materials can boost the current rating of existing connectors by as much as 20%.

New contact designs that feature redundant high-normal force contacts ensure multiple points of contact, which significantly reduces resistance at the interface. The escalating price of gold has stimulated efforts to improve contact reliability with thinner plating in selected locations.

Some existing contact designs have added a louvered spring insert to the female socket. The addition of this band creates a series of controlled mating contact points that lower resistance.

Higher operating temperatures have mandated changes to plastic housing materials. These materials offer excellent dielectric strength and can withstand higher temperature lead-free soldering operations, as well as permit precise molding of thin wall sections required by small centerline interfaces.

Several new connectors use cored-out plastic housings, which allow air to circulate around the contact and result in higher current ratings. Vented housings are becoming a popular feature in power connectors.

Smaller connectors, that are lower in profile, offer less resistance to cooling airflows, improving thermal management for the entire product.

Even time-honored paradigms are being challenged in an effort to develop more efficient power interfaces. The assumed superiority of solid screw-machined contacts has been questioned by a recent white paper published by Tyco Electronics. Its conclusion indicates that stamped-and-formed contacts present much greater surface area than a typical round screw-machined contact.

Larger surfaces radiate heat more effectively, which improves the power rating of the contact by greater than 40%, compared to a comparable screw-machined contact. More efficient use of materials, and the ability to form a variety of shapes such as compliant pin, make stamped-and-formed power contacts an attractive option in many applications.

Connector manufacturers utilize a blend of each of these incremental improvements to provide performance that meet customer expectations.

Power connectors are also becoming designer-friendly. Unlike past power interfaces that were available only in selected sizes, newer connectors are often tooled using laminated molds and programmable contact insertion. This process allows quick and inexpensive modifications, which can produce a wide variety of sizes and configurations with no engineering or custom tooling charges. Engineers are able to design what they need, rather than using what is available from a catalog.

Custom connector configurations that combine unique combinations of low-power, high-power, and control-signal contacts can be produced in a matter of a few weeks, rather than months.

Power connector manufacturers are also responding to the need for more detailed performance data. A failed power connector can be a rather dramatic event, and is something to be avoided at all costs. Designers often build in additional safety factors by reducing the connector manufacturers published power rating by 50%. This practice avoids failed contacts, but often results in larger-than-necessary connectors that cost more. That practice is no longer acceptable in today’s compact systems that must compete in cost-sensitive markets. Specifications that closely match a product’s real-world characteristics can be used to accurately predict performance of a power connector in a specific application. Connector suppliers are defining the test parameters and conditions in much finer detail, including the gauge of terminated wire, thickness of copper layers in PCB-mounted product, ambient temperature, degree of airflow, as well as the number of contacts energized. Environmental tests—such as mixed flowing gas, shock, vibration, mating durability, and salt fog—provide insight into the expected changes in contact resistance that occur in its end-of-life. This data allows the designer to select the most cost-effective interface for each individual application. The result is reduced risk and cost for the equipment manufacturer, and greater product reliability for the consumer.

Bishop & Associates Comments

• The semiconductor industry is making significant progress in power reduction per device, but overall demand continues to grow in every market segment, including computing, telecom, medical, military/aerospace, and automotive.
• Power connectors are undergoing significant changes in design, materials, manufacturing, and testing to satisfy increasing performance requirements.
• Recently introduced power connectors offer configuration flexibility, as well as increased power ratings.
• Many of these products utilize advanced materials and design to enable interfaces with greater power density in smaller profiles.
• The introduction of power connectors manufactured with modular tooling allows suppliers to produce custom configurations quickly, with minimal or no tooling charges.
• Connector manufacturers are blending a variety of incremental material and design improvements to meet the escalating demands for increased power in smaller packages.

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