By Tom Johnson
 Figure 1. Early modular connectors evolved into robust, cost-effective, one-piece, drawer-type interconnects. |
Blind-mate, pluggable, high-current power connectors as they are known today are a result of a 15-year-long evolution driven mostly by the requirements of telecommunications and computer applications in the pursuit of higher reliability and better performance at a lower cost. Prior to the appearance of commercial computer and telecommunications applications, power connectors referred mostly to battery disconnects that were used primarily in aircraft, both military and otherwise. The most typical example was the ground power connector, which consisted of a rugged battery disconnect that was hand-plugged together with a jack screw or just a stick to jumpstart airplanes and helicopters.
As the early commercial computer applications started to appear in the market, most of them mainframes and minis, power distribution relied on nuts-and-bolts-style interfaces. This was an unreliable method and repairs always meant service interruption to the end user. Computer manufacturers needed more reliable power systems if they wanted to provide uninterrupted service to the final end user at all times, but this required a radical change in the way power was distributed to the circuit boards.
The solution was the fault-tolerant N+1 redundant power system with pluggable power supply units (PSUs) that could be replaced in the field without service interruption. This concept materialized through the transformation of the power interface from the old nuts-and-bolts type to the pluggable power connector of today, without which the N+1 power systems could have not been achieved. These were the first customer-replaceable units (CRUs) which, as opposed to field-replaceable units (FRUs), could be easily replaced by the end user without the help of a high-priced technician and without having to bring the system down.
Modular Power Connectors
Interestingly enough, and probably contrary to general believe, modular power connectors were used in some of the first pluggable CRUs. Considering the low-volume requirements of the computer systems of those times, it would have been very costly to tool a complete connector just for a single design. Also, different systems had different design requirements that had to be met.
 Figure 2. High-performance contact elements. |
The modular interconnection system furnished interchangeable modules for power contacts of different sizes with current-carrying capabilities ranging from 15 A in the low end to 200 A or more, and signal contacts for system control purposes as well. With the modular approach, system design engineers were able to mix and match the right combination of modules in order to design the power connector they needed without incurring high non-recurrent engineering (NRE) costs. Plus, they had the flexibility to try several module combinations before deciding on a final design.
As the high-tech industries entered the 1990s, demand for mission-critical computer and telecommunications applications climbed, and so did the need for pluggable power connectors. Volumes then reached critical mass and modular interconnect systems started being replaced by one-piece molded units, which provided the same functions but in more robust and cost-effective design (see Figure 1).
Contact Technologies
Contact technologies also evolved with time, mostly to satisfy the trend for lower voltages and consequently higher currents. Contact elements with multiple contact points designed to provide lower contact resistance, higher reliability and more consistent insertion/extraction forces were developed (see Figure 2). Power connectors using these more sophisticated contact technologies were, and still are, able to achieve lower millivolt drop and temperature rise than similar connectors using blade-to-blade contact elements that have only one point of contact.
Hot-plug Contacts
Another breakthrough in contact technology was the development of true hot-plug contacts. The terms "hot plug" and "hot swap" are presently used interchangeably, but in the early days of PSU designs, hot plug meant plugging and unplugging under load. PSUs were then designed around true hot-plug contacts to allow for a hot bus plug-in.
 Figure 3. A true hot-plug contact. |
True hot-plug contacts have a unique contact design that gives full protection to the contact surface area from the destructive forces of arcing in true current-interruption applications (see Figure 3). These contacts comply with safety regulatory agency standards for current interruption under load.
In contrast, hot-swap applications refer to situations where high-current interruption is completely avoided by using a first-break/last-mate technique. In this case, a very low-current signal contact is used to trigger the flow of current from the PSU at the last sequencing stage, only after all the other contacts are fully mated, thus avoiding current interruption on high-current contacts. The hot-swap technique relies on contact sequencing triggering the function of O-ring diodes and/or circuit breakers, therefore adding to the likelihood of failure. A true hot-plug system would never bring the system down.
Board-to-board Power Connectors
 Figure 4. Board-to-board power connectors with multiple levels of sequenced mating. |
While the important role of power connectors in fault-tolerant N+1 power systems with front-replaceable CRUs has not changed in essence over time, the pursuit for better and more cost-effective interconnection solutions has continued. By the mid-1990s, the road was open for improvements in connector performance, cost and, perhaps most importantly, the repercussion of connector design in assembly costs at system level. It was not long before the big original equipment manufacturers (OEMs) realized how costly and time consuming it was to wire drawer-style connectors. The truth was that, in some cases, the cable ended up costing as much if not more than the connector itself!
What came out of this realization was the next step in the metamorphosis of the power interface: the development of board-to-board power connectors. These connectors were designed to be mounted directly to the board on the PSU and the backplane, and therefore did not require any wiring. The elimination of cables reduced the cost of the power system by approximately 50 percent. Other benefits brought in by the board-to-board power connectors were in terms of higher packaging density: the use of flat contacts allowed more current in less space compared with the designs using round contacts. Apart from that, all the other key features were still required: integrated contact assembly with alternating current (AC) in, direct current (DC) out and signal, true hot-plug contacts; guides for blind mating and improved gatherability; and contact sequencing (see Figure 4).
The first generation of board-to-board power connectors was modular, providing design flexibility and cost-effectiveness for even low-volume projects. This allowed design engineers to combine power modules with different voltage spacing and pin lengths, true hot-plug modules, signal modules and guide modules to match any design requirement exactly. Dedicated layouts were tooled over time and, at present, there is a wide array of robust one-piece board-to-board power connectors that would certainly satisfy the needs of any power distribution system.
Card-edge Power Connectors
A unique and more recent landmark is the advent of card-edge-style power connectors. Although card-edge connectors have not typically been associated with power, they have been occasionally used to deliver low-to-midrange currents by busing multiple contacts, a somewhat unsafe and unreliable technique. Card-edge power connectors do exist, and they are capable of providing reliable, high-current delivery plus signal capabilities (see Figure 5). The advantages to low-to-midrange power supply design engineers are several, including lower cost (mates with a board, so no pin connector is needed) and the ability to reach higher currents in less board real estate.
Power Interface Specifications
 Figure 5. Modular card-edge power connector with high-current contact technology. |
In recent years, organizations such as the PCI Industrial Computers Manufacturer's Group (PICMG) have made attempts to regulate the power interface for applications like CompactPCI. Standards have been defined as a result, the PICMG power interface specification being one of them. The design proposed by this standard is adequate for low-to-midrange power supplies but falls short of addressing the higher end of the spectrum, specifically high-current power supply designs that cannot afford to have two power connectors (as proposed by the standard) because of space constraints.
As a result, alternative designs have been developed to meet and even exceed the standard performance levels within the same footprint requirements of CompactPCI spurring, perhaps ironically, the development of the next generation of high-density, high-current power connectors (see Figure 6). Right now, power supply, backplane and system design engineers have two choices: a standard interface that is somewhat inadequate and restrictive, or alternative designs that can provide higher levels of performance within the prescription of the standard specification. Which direction will prevail remains to be seen.
What the Future Holds
 Figure 6. Next-generation, high-density power connector. |
A look at the present trends in power system design requirements tells a lot about what the future holds for power connectors. Lower voltages, higher currents and increased packaging densities continue to be the driving forces for smaller and more powerful connectors. However, the effects of inductance on future power distribution designs may well be the single most significant factor driving research and development (R&D) for the next generation of high-current power connectors. With processor speeds in excess of 1 GHz, the effect of inductance has become more and more critical at every stage of the power distribution chain, including the power connection interface. R&D is already underway to develop the next generation of power connectors capable of delivering high current reliably and with flexibility as they do now, but that will also meet the more stringent power requirements of high-speed digital circuits.
Telecommunication services, the Internet and computer networks — just to name a few — have all become part of modern society. In business, an electronic transaction aborted by a power failure can mean millions of dollars in lost profits. For an individual, not being able to access a bank account due to a power failure on the bank's servers represents an inconvenience that the demanding modern consumer is not willing to put up with. People expect all of these services to be continuously available, always working, no matter what.
Continuous availability is not an option but a must, and because of that, services run on equipment with advanced fault-tolerant N+X power systems designed to ensure absolute zero downtime. The general perception from this bigger context is that the power connector is just another of many components. However, the power connector has always played a vital role in the realization of the ultimate goal of service without interruption to the end user, and it still does. This is not going to change anytime soon.
TOM JOHNSON is senior vice president, New Business Development, Elcon Products International, a part of Tyco Electronics Corp., a unit of Tyco International Ltd., 42700 Lawrence Place, P.O. Box 1885, Fremont, CA 94538; (510) 490-4200; Fax: (510) 490-3740; Web site: www.elconproducts.com.
SPEC SHEET
End Applications: Computers and telecommunications
Related Products: Board-to-board, card-edge and modular power connectors
Main Point: Power connectors have had to evolve to meet the requirements of high-tech telecommunications and computer applications. As the high-tech industries entered the 1990s, demand for mission-critical computer and telecommunications applications climbed, and so did the need for pluggable power connectors. The power interface has developed from the early nuts-and-bolts style to a blind-mate, hot-plug interconnect system.
