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

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In league with today’s “green” movement, power converter designs have become increasingly efficient. Contemporary converter efficiency can be 5 or more percentage points higher compared to just a decade ago. While converters have improved, not much is said about the system interconnects, the means by which high currents are distributed throughout a system – until now.

By Russ Larsen and Forrest Sass, Methode Power Systems Group, San Jose, California

Low loss, high current (>50A) connector design is challenging. The most commonly desired spec is low contact resistance, and for good reason. Lower resistance has the beneficial effect of minimizing IR voltage drop, making voltage regulation easier. Lower resistance also reduces I²R power losses, reducing contact temperature, improving connector reliability, wasting less energy and allowing for a smaller connector.   

In addition, low insertion / extraction force is also highly important. Insertion / extraction force is simply the mechanical force necessary to mate or unmate the connector. Reducing this force has the beneficial effect of minimizing contact surface wear, a major factor in connector failure. Recent connector improvement efforts concentrated almost entirely on developing improved contact surface coatings and materials. Other design efforts modified the pin shape to minimize the insertion force.

Can the common power connector be further improved?

The answer is yes, initiated by a MIT by a professor and graduate students investigating tribology, the science and technology of interacting surfaces in relative motion including the principles of friction, lubrication and wear. The researchers developed a prototype power connector with significantly superior qualities. 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.   

The Tribotek connector performance was 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 were pure copper to minimize resistance.

The group 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.  

Tribotek connector basic elements showing pure copper wire wrapped around Kevlar tensioning fiber and held in contact to a copper mating pin

Seen at a microscopic level, a single conductor weave has at least four points of contact resulting from the four wraps around the Kevlar spring. 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.

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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 and lower power loss.  

Equivalent resistance model of a single weave

Equivalent resistance of multiple weaves  

The connector technology handled almost 500A with very low voltage drop and very low insertion force. However, because the fabrication of this connector is labor-intensive, it tends to be used for high value applications.

Conventional connector design challenges

In order to increase contact surface area and lower resistance, manufacturers finely polish and plate mating contact surfaces. This leads 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 surface is very small. A connector’s polished 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. The percentage of the surface area which actually passes current is very small.

Manufacturers have adapted to this limited amount of contact area using different methods to maintain low contact resistance, including

1.       Increasing the contact mating area, resulting in 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 either mating surface 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 alloys are easily fabricated into a part that serves as both spring and conductor. This solution is widely used today in low cost connectors.

Evolution of the PowerBud connector

Methode acquired Tribotek, the company founded by the MIT researchers, in March 2008 and began to evolve the connector with the goal of making it more manufacturable. The result was a new class of patented power connector named “PowerBud”^TM that successfully overcomes key limitations of conventional power connectors.

Microphotograph of a PowerBud connector showing two rows of contact fingers. Each finger is formed to allow two points of contact. The illustration shows the details of the first and second conductor rows and the dual contact points on each finger

The PowerBud uses two rows of conductors arranged one over the other to create massively parallel points of contact. 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. The conductors have a larger cross-section than those on the Tribotek connector to partially compensate for the alloy’s higher resistivity.

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.   Like Tribotek, 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.