Although the high-tech spotlight rarely shines on power connectors, they're crucial to the performance of many advanced electronic systems. Without the interface they provide between power supply and load, it would be difficult to create scalable power designs. After system configuration, it would also be a time-consuming, laborious operation to replace a failing power-supply module. Naturally, the system would need to be powered down, as hot-swapping would be impossible.
Of course, modularity is at the heart of many high-performance applications. Numerous examples can be found in the telecom and networking areas, where systems rely on modular designs to provide both configurability and very high reliability. Due to their stringent demands for reliable power, telecom and networking applications are driving many of the developments in power interconnects. This requires development of the connectors that interface to the power supply, as well as the associated backplanes, wiring harnesses, and bus bars that provide power distribution throughout the system.
The ruggedness of power connectors, as well as their current- and voltage-handling abilities, distinguish them from their signal-routing cousins. Power connectors are available in a number of mechanical styles, including circular, rectangular, and D-subminiature. They attach to boards and wires via soldered, crimped, or press-fit connections.
The term "power connector" might refer to any component that mates to a supply line. In this context, it means those interconnect devices that can handle several amps or more of current.
Beyond the high-current ratings, the connectors discussed here are designed to allow blindmating and hot-swapping of power supplies.
Power Interconnect Requirements
Given the weight of the power supply they're attached to, power connectors must withstand relatively high mating forces when used in blindmating designs. The power-supply module in blindmating systems is inserted along guides. These lead the unit back in the rack until its connector mates with its counterpart on a backplane or bus bar. Evidence of proper mating may be tactile feedback or the illumination of an indicator.
The force required to mate the two power connectors is typically much higher than that of two signal connectors. For example, it's not uncommon to have 25 lbs of insertion force exerted on a power connector during mating. In contrast, mating two SCSI connectors only takes about 5 lbs of force.
Blindmating imposes a few requirements on the connector. First, the connector housing must serve as a guide that allows for some tolerance in the alignment of the connectors. Also, due to the weight of the power supplies and the forces applied to them during insertion, the connector must be mechanically rugged. Durability is not only needed to protect the connector, but also to safeguard other assemblies. So, the connector's housing should be designed to absorb as much insertion force as possible to prevent damage to backplanes and other assemblies. In addition, closed-entry contacts can help prevent damage to the connection interface.
Naturally, there are tradeoffs in de-signing connectors for blindmating. A connector's ruggedness and tolerance for misalignment affect its size, for instance. In other words, enlarging a connector increases its suitability for blindmating. So, demands for blindmating conflict with the ever-present demands for smaller connectors that have higher pin and current density. Consider two blindmating power connectors offered by Positronic Industries. A 30-contact power connector designed to take up 1 mm of misalignment measures 2.4 (l) by 0.8 (h) in., while a similar connector built for 7 mm of alignment error is 3 by 1 in.
Demand for blindmating, in turn, has created demand for mixed-signal and power connectors. To implement blindmating, designers in many cases have moved from multiple connector arrangements (power in, power out, and signals) to a single connector. Gino Nanninga, vice president of sales at Positronic, says this has led to connectors with an assortment of different size contacts and contact spacings.
Another popular requirement is hot-swapability. This feature is largely driven by demands for reliability and uninterrupted operation. Two methods for implementing hot-swapping exist. The basic method makes a power supply "hot-plugable," while a more complex approach relies on sequential mating.
In a hot-plugable supply, the connector acts as a current-interrupt device. Safety agencies define the term "current interruption" as the connector's ability to confine damage caused by arcing or overload from high inrush currents to areas that don't compromise the integrity of the connection. In addition, the connector's housing must allow proper functioning of the device under arcing or surge current conditions. (More information is available in Connector Design for Hot Plugability, Power, and Signal, by Robert G. Foley, Elcon Products International)
A hot-plugable connector has consistent normal forces between mating surfaces, as well as low contact resistance. Contact and housing design must prevent damage to the contacts caused by arcing and surge currents.
The other approach, sequential mating, depends on the staggering of power and power-enable pins. In the simplest implementation, two-level sequential mating, the connector contains a power-enable pin that's shorter than the other pins. When the connector is inserted, the power-enable pin makes contact after the power-handling pins, and on disconnect, breaks contact before the power contacts do so.
According to Nanninga, the implementation of two-level sequential mating does not have a major impact on the connector design. Yet, additional layers of sequential mating do have some effect. This becomes a concern when customers require three, four, or even more levels of sequential mating for grounds, various power pins, and power enable.
One benefit of multilevel sequential mating is its usefulness in implementing sequential startup of different subsystems. But with three or more levels of sequential mating, connector tolerances for contacts and housing come into play. Given these tolerances, the connector's depth (its height off of the backplane) must have some minimum length. This will ensure that the different pin levels mate in the desired order. Therefore, designers may need to trade connector size for the complexity of their sequential mating schemes.
Another size tradeoff is in the area of power density. Naturally, the pressure is on connector developers to either shrink the power connector or increase its current-handling ability in the same form factor. One measure of power handling is the connector's rating for current per contact. As might be expected, connector manufacturers are working to develop contacts that can carry higher currents in smaller sizes.
But as efforts to develop connectors with greater current density continue, designers must also consider how much current other segments of the interconnect can handle. In some applications, the connector may be able to support much higher currents than the associated pc boards can tolerate.
As Don Wood, vice president of engineering at Elcon, re-marks, "The question is, how much copper can you put on the board?" Even when a single contact can handle the current being transferred, distribution of current across multiple tails mounted to the pc board may be necessary to avoid the creation of hot spots on the board.
Also influencing the trend toward miniaturization is the shift in power requirements. With supply voltages dropping and currents rising, especially peak transient currents, connector parasitics become a concern. As a consequence, connector vendors have been asked to reduce the parasitic effects from resistance and inductance present in all power interconnects.
Experiments conducted by an R&D team at Elcon show that it's possible to create power connectors capable of delivering 200 A per linear inch (feed and return) with an inductance of approximately 250 pH. These numbers reflect results obtained with engineering prototypes, which have been tested using multiple methods and modeled in Spice (Fig. 1).
In general, concern for parasitics is influencing the design of all power interconnect components. As Wood says, "The same scrutiny that has been applied to high-speed signal interconnects is now being applied to interconnects within the power delivery system." Concerns over I×R losses and heating not only affect the design of the power connector, but also elements such as backplanes and wiring harnesses. In some cases, system designers are moving from backplanes and wire harnesses to bus bars. This is to reduce losses in these power distribution components, and to simplify assembly and troubleshooting. Additionally, bus bars provide a platform for greater system-level integration while increasing reliability (see "Bus-Bar Benefits" in the online edition of this article at www.planetee.com).
Bus bars should find greater use in the future as current levels continue to rise. Mark Wojcicki, director of R&D at Anderson Power Products, says that some customers are now projecting that they will need hot-plugable power connectors that can handle up to 1000 A.
Despite the tougher requirements placed on their products, power connector makers must respond to time-to-market pressure too. While they're frequently considered custom, performance-critical products, there's little time for power connector development. The design of such products often happens very late in the overall design process. "Power design is typically the last thing in system development, and connector development is an afterthought to power," Wood notes.
Connector performance is influenced by numerous factors that leave room for improvement. The materials, mechanical design, and assembly techniques used to produce contacts, plating, and housings all affect connector operation. In particular, contact design is key to performance.
Many commonly used power contacts are stamped and formed from sheet metal, Nanninga says. That's because this technique lends itself to high-volume, low-cost manufacturing. At higher current levels, however, it becomes necessary to use other techniques. He notes that solid machined contacts are needed in applications requiring higher current capability and greater reliability. Positronic implements this technique to fabricate its contacts. Using a high-speed automatic lathe, the company screw-machines its contacts from solid bar stock composed of various types of copper alloys.
Nanninga cites several military and industrial reliability standards that necessitate the use of solid machined contacts. These include the mil std 24308 and IEC 807-2 and 807-3 for D-subminiature connectors, as well as mil std 28748 and IEC 807-7 for rectangular connectors. But current requirements come into play too. "It has been my experience that once you get up to 8 or 10 A per contact on a size 16 (0.062-in. diameter) contact, you need a machined part," Nanninga says.
Positronic offers up to 25-A UL current ratings on a size 16 contact. That number jumps to 40 A on a size 12 (0.094-in. diameter) contact. Plus, the company is now developing a size 8 (0.142-in. diameter) contact that will carry 100 A and crimp onto a size 6 wire. Unlike the size 12 and 16 contacts, which are machined from standard materials, the size 8 contacts are fabricated from proprietary, high-conductivity base materials. All of these power pins go into rectangular-style power connectors.
Although high-conductivity base materials provide greater current ratings for power contacts, they're typically more expensive than other, standard contact materials. Therefore, designers are cautioned not to overrate their contacts to allow use of more standard materials whenever possible. Fortunately for designers, high-conductivity materials aren't the only means for increasing the contacts' current-handling ability in a given size contact. The mechanical design of the contact also is significant, and contact designs vary from vendor to vendor.
Positronic uses its large-surface-area contact mating system to accommodate power contacts ranging from size 8 to 16 (Fig. 2). This system separates the connector's mechanical and electrical functions. To generate the high normal forces required for a reliable connection, the contact contains a spring retention member. For high electrical conductivity, the contact pin is precision machined from either standard or proprietary materials.
Another approach is Elcon's crown band socket design. In this contact system, multiple contact points and current paths provide high electrical conductivity and generate high normal forces, yet minimize the required insertion force (Fig. 3a). Concurrently, the crown band structure absorbs much of the connector's mating forces. This socket works with both round and flat pins. The latter afford increased contact density. To permit hot-plug operation, the socket's housing is designed to route the arc away from the crown band interface (Fig. 3b).
When developing new products, power connector vendors not only seek to improve the current-carrying capacity of individual contacts, but also the overall current density of the connector as a whole. To this end, they strive to decrease the spacing between pins, while maintaining current ratings.
Consider Elcon's recently unveiled MINIPAK connector. It offers up to 65 A (UL rating) on a single contact and contact pitches as tight as 4.5 mm versus the 7.5-mm pitch in the company's FLATPAQ product. Elcon claims MINIPAK achieves better current density in amps per linear inch than anything else on the market.
Elcon has just enhanced this product line with the ability to combine power contacts, signal contacts, and a new alignment guide. On the dc output side, contact spacing can be 4.5, 6.0, or 7.5 mm, or a combination of the three. Rather than offering designers a fixed set of connector variations, Elcon will configure the MINIPAK to meet the designer's actual electrical and safety requirements.
Future enhancements of this product will include DualBlade contacts and active guides. The DualBlade feature, which combines two isolated poles in the space of one power contact, makes it possible to have multiple output voltages with tight contact spacings. The new guides extend the connector's blindmating ability, allowing up to 3.8 mm of misalignment.
Advances in connector design not only encompass changes to the mating area of the contacts, but also changes to the terminals on the other end. Positronic has developed power connectors with compliant press-fit contacts. Although this has been done with stamped signal contacts for years, Positronic claims it's the first manufacturer to offer this feature for machined power contacts. Having press-fit terminals on the power connector can make pc-board assembly considerably easier. In particular, as pc boards grow thicker with greater numbers of layers, it takes considerable amounts of heat to solder connector terminals to the board.
Some recent improvements in power connectors involve enhancements to established products. Greater current handling and new features are among the improvements. One example is Molex's SSI series of backplane connectors, introduced a year ago. Designed to carry both power and signal, the connectors use blade-style terminals to achieve current ratings of 30 A per power contact. The company expects to introduce higher-current versions sometime this year. Molex also plans to enhance its Micro-Fit 3.0-mm pitch series of signal and power connectors by offering blindmating in its wire-to-wire, wire-to-board, and board-to-board versions.
Certain industry trends seem likely to have an impact on future connector developments. One that's already taking hold is the shift to gold plating on power pins. Besides lowering contact resistance, gold allows for a greater number of mating cycles because it requires less contact force. That's partly because the gold acts as a lubricant between contacts. But also, gold plating doesn't corrode. Therefore, the contacts don't have to break through an oxide layer as they do with tin plating.
Down the road, other issues may come to the fore. For example, as current levels continue to rise in the future, power contacts may not only be re-quired at the power supply interface to the system but also at the interconnect to individual cards.
The approach to rating power contacts for the standard 30°C temperature rise may also come under scrutiny. Mike O'Connell of Molex notes that when designing servers, engineers are coming up with new ways to cool the backplane. So rather than being concerned about the 30°C temperature rise, they might instead emphasize the importance of having an electrically stable power interface to ensure balanced current sharing across multiple pins.
The move to lead-free design is another issue that will need to be addressed, says O'Connell. Redesigns of connectors with the new, lead-free solders will require the requalification of these components by connector manufacturers and their customers. Like many other changes in the connector world, such developments will probably be overshadowed by innovations elsewhere in the industry, but are nevertheless crucial to the success of cutting-edge designs.
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Anderson Power Products
Elcon Products International Co.