A new technology for high current, low insertion force, low resistance and long cycle life power connectors

Transcription

1 A new technology for high current, low insertion force, low resistance and long cycle life power connectors Abstract Methode has developed a new class of patented power connector named PowerBud TM that successfully overcomes key limitations of conventional power connectors. The PowerBud technology lowers both contact resistance and contact normal force without increasing connector volume, a feat that counters conventional wisdom. The resulting connectors exhibit lower insertion force, lower temperature rise, lower power loss and higher cycle life than conventional high current connectors. Introduction High current (>50A) connector design presents many design challenges. At the top of the list is the need to minimize contact resistance. Lower resistance has the beneficial effect of lowering I 2 R power losses, lowering contact temperature, which in turn leads to higher reliability. The lower temperature rise also allows the connector pairs to be positioned closer together in a connector housing, minimizing the connector size. Minimizing power loss is important because of the growing green initiatives intended to reduce human s adverse impact on the earth. The energy used which is ultimately burned up as connector heat is wasted energy. Low insertion force is also highly important. Insertion force is simply the mechanical force necessary to mate or unmate the connector. Reducing insertion force has the beneficial effect of reducing contact Page 1 of 9

2 surface wear, a major contributing cause of connector failure. Previous efforts concentrated almost entirely on developing improved contact surface coatings and materials. Other design efforts modified the pin shape to minimize the insertion force. This paper discusses the design tradeoffs and innovative methods to optimize current rating, contact resistance and insertion force, and how those tradeoffs led to the development of an innovative new power connector technology. Conventional connector challenges Manufacturers finely polish and plate mating contact surfaces in order to increase contact surface area, leading to a general misconception that current flows through the entire mated surface area. However, the actual percentage of that area which actually makes contact with the mating connector is very small. Fig. 1 shows that a polished gold-plated mating surface, viewed on a microscopic level, consists of peaks and valleys called asperities. Electrical current is concentrated and passes through the asperities which are in actual contact. Manufacturers have adapted to the limited amount of contact area using different methods to maintain low contact resistance, including Figure 1: Surface topography showing asperities on a polished, gold-plated berylliumcopper contact surface 1. Increasing the size of the mating contacts, which yields many more microscopic points of contact. The result is a larger, more costly connector. 2. Increasing the normal force pressing the two mating surfaces together, which slightly deforms the asperities thereby increasing contact surface area. The result is a connector with high friction force that is more difficult to mate, or an expensive connector mechanism to provide the additional force after mating. 3. Using a manufacturing process to reduce the surface asperities The need to mechanically force the mating surfaces together has led to many design compromises. Since copper is one of the very best reasonably-priced electrical conductors (excluding gold, silver and other exotic materials), it would be a good choice for the mating parts of the connector. However, copper has poor mechanical spring properties. If both mating surfaces were pure copper, the connector would also require an additional spring to maintain copper-to-copper contact. In the connector world, that yields an expensive product. A more practical solution is to choose a material with both spring and conductive qualities such as beryllium copper or tin copper. While less conductive than pure copper, these copper alloys are easily fabricated into a part that serves both as spring and conductor. This solution is widely used today in low cost connectors. Page 2 of 9

3 Microscopic look at plated contacts The actual points of contact between mated surfaces can have a relatively high resistance and therefore a relatively high voltage drop. Each of these points of contact has a finite resistance. One way to minimize the overall contact resistance is to have many points of contact. By placing lots of contacts in parallel, junction resistance is reduced. Figure 2. First Contact x/z cross-sectional view taken across Au contact interface showing potential values represented as gray scale and height. Tribotek technology developed by Massachusetts Institute of Technology researchers Methode s original WovenBud was developed by a MIT professor and graduate students investigating tribology, the science and technology of interacting surfaces in relative motion. It includes the study and application of the principles of friction, lubrication and wear. The company name Tribotek originates from the word tribology. The researchers developed a prototype power connector with superior qualities compared to conventional, commercial power connectors. The Tribotek connector, as it was later called, had very low contact resistance as well as very low insertion force, without a commensurate connector volume increase. Figure 3: Illustration of Tribotek connector basic elements showing pure copper wire wrapped around Kevlar tensioning fiber and held in contact to a copper mating pin. Tribotek connector performance is based on maximizing the number of discrete points of contact rather than attempting to increase the contact surface area or polish the mating surfaces to a finer degree. Furthermore, the conductors are pure copper to minimize resistance. Tribotek fabricated a socket-style connector by weaving pure copper wire around a Kevlar non-elastic cord held under tension to produce a connector with many points of contact around the circumference of the mating pin. Page 3 of 9

4 The connector technology handles almost 500A with very low voltage drop and very low insertion force. Because the fabrication of this connector is labor-intensive, it tends to be used for high value applications. Contact Termination Body Metal spring Conducting wire Tensioning Fiber (Kevlar) Mating pin Figure 4: Photograph of practical Tribotekstyle connector showing wound copper wire, Kevlar cord and tensioning spring. How the WovenBud lowers contact resistance A single conductor weave has at least four points of contact and possibly many more depending on the number of asperities on each mating surface. Each electrical path can be modeled as a resistor in a matrix, which results in four parallel paths. In addition, the resistance contributed by the copper wire itself is 4 times lower than the more frequently used beryllium copper. The Kevlar spring need only provide enough force to assure surface intimacy. The resulting lower insertion force means less sliding resistance to mate the contact and therefore less wear over time. Figure 5: Equivalent resistance model of a single weave. Figure 6 (left): equivalent resistance of multiple weaves By arranging all individual weaves in a circular assembly results in massively parallel contact points, significantly lowering overall connector resistance. Low contact resistance means less heat generated under high current loads and less power loss. Page 4 of 9

5 This design approach has yielded a new type of connector with exceptionally low contact resistance as well as very low insertion force. The only drawback to this approach is the cost of assembly, which limits the use of this connector to high value applications. Methode develops the PowerBud technology Methode acquired Tribotek in 2008 and continued to develop the Tribotek connector. The result was the PowerBud, an evolution of the WovenBud technology. It embodies the same design approach massively parallel points of contact but using a mechanical design that is adaptable to mass production. The resulting connector has many of the same electrical qualities of the WovenBud with a lower production cost. Figure 7 (above): PowerBud mechanical construction showing dual contact points on each finger. Figure 8 (below)photomicrograph of a. The PowerBud uses two rows of conductors arranged one over the other. Instead of using pure copper for contacts, the PowerBud uses a proprietary performance-engineered copper alloy material that is substantially better than the more commonly used beryllium copper alloy. The conductors have a larger cross-section than those on the WovenBud connector to minimize resistance. The high performance copper alloy is easily fabricated using automated processes. In addition, each copper alloy beam includes a slight indentation in the finger tip to create dual contact points, adding to the massively parallel contact points. The easily fabricated fingers are primarily responsible for lowering the cost of the connector compared to the WovenBud (Tribotek) design with little sacrifice in performance. Page 5 of 9

6 This figure shows the relationships between three connectors with respect to contact resistance and insertion force. Competitive connectors have a higher contact resistance and higher insertion force than either the PowerBud or WovenBud. PowerBud performance Pow erbud vs. competition voltage drop, 6.4mm or equivalent pin mv Amps Competitor Vdrop (mv) Pow erbud Vdrop (mv) Page 6 of 9

7 Pow erbud vs. competition temp rise, 6.4mm or equivalent pin Centigrade degrees Amps Competitor temp rise ºC Pow erbud temp rise ºC PowerBud vs. competition mating / unmating force, 9.1 mm pin Mate Unmate PowerBud 4N (0.9 lb) 4N (0.9 lb) Competitor 21N (4.7 lb) 13N (3 lb) PowerBud vs. competition cycle life PowerBud Competitor 10,000 cycles 1,000 cycles est Page 7 of 9

8 PowerBud versions The PQ panel connectors feature four PowerBud contacts in a molded thermal plastic housing designed for blindmate applications. Each connector half floats under shoulder mounting hardware and self-aligns to the mating connector half. This connector uses a 9.1mm pin rated at 400VAC per contact. The MQuad is similar to and slightly smaller than the PQ panel connector. MQuad panel connectors are also designed for blind-mate applications. Each connector half floats under shoulder mounting hardware and self-aligns to the mating connector half. Contact terminations can be crimp wire, wired lugs or busbar attached. The MQuad is smaller than the PQ, and uses a 6.4mm pin rated at 600VAC per contact with a 30ºC temperature rise. The Slim Latch is designed for compact applications in which connector current must be optimized while limiting connector volume. The Slim Latch connector includes an integral locking mechanism, eliminating the need for additional means to mechanically secure the connector. It uses a 2.4mm pin designed for 10 AWG wire and is rated at 600VAC per contact. The embedded s have knurled outer side walls allowing direct press-fit insertion into busbars, printed circuits boards and FusionLugs TM. Installation or press-in is accomplished using any flat surface and does not require any special tooling. The current rating is dependent on the physical size of the pin and the heat-sinking capability of the mounting medium. Page 8 of 9

9 Conclusion The PowerBud is a new class of power connector offering lower voltage drop resulting in lower temperature rise and lower insertion force than competitive connectors and much less than conventional connectors. The unique mechanical construction evolved from the Tribotek connector, a pioneer design which delivers a quantum improvement in power connector performance. PowerBud allows more current to pass through a connector that occupies a small volume, potentially reducing package footprint. The lower voltage drop can eliminate the need for a local voltage regulator module. The lower temperature rise reduces system thermal load. The PowerBud is suitable for systems that require connectors capable of handling hundreds of amps of current. It is particularly suitable for systems that require the connector to be mated and unmated frequently. Page 9 of 9