Power Integrity in Interconnect Design

The need for high-current power interconnect solutions in
increasingly smaller spaces continues to rise rapidly. As demand grows for more
power in smaller packages, solving the power equation on new architectures and
system platforms can pose electrical and mechanical design challenges for OEM
system and power engineers charged with specifying interconnect components that
ensure both signal and power integrity.

Unlike signal connectors, which continue to get smaller at higher
transmission speeds, power connectors require a specific amount of conductive
material to carry specific amounts of current or amperage. There are no special
secrets to design that will allow smaller power contacts to carry more current
- as power needs increase, so does the amount of space needed for higher
ampacity interconnects.

Design Density and Power

Even though new system
designs often require more power to travel across a limited amount of space,
several factors still affect the density of a design and how much power it can
actually handle. A clear understanding of each of these elements is critical to
successfully designing power integrity and safety into the system - and will
help streamline the overall design process. These key factors include:

Balancing Space and Power. First, it is necessary to determine how much
space is required for a power interconnect versus how much available space has
been allotted in the finished product design. While saving space is a high
priority for most OEMs, the height, width and length of the connector, and
particularly its copper content, will directly affect the achievable current
density. System architects always want to get more power in the same space,
which can pose a challenge for connector manufacturers.

Power Integrity  in Interconnect Design

However, leading global
connector manufacturers continue to develop new and innovative designs that use
higher conductivity materials and utilize space more creatively to improve
power delivery and electrical performance without expanding space requirements.
For example, in some cases, a lower profile connector may be preferable to
maximize air flow for cooling. In other cases, a taller connector offering
improved contact performance may be the right solution to properly handle the
amount of current generated in less card edge space. What's important is
striking the optimal balance of power and its resultant thermal effects in the
PCB with the spatial design requirements to ensure the end product's safety and

Thermal Management. Thermal issues caused by contact or constriction
resistance and inefficient air flow are always a concern and should be
carefully considered early on in the design process. PCB copper content is one
element of this. Too little copper can restrict current flow, causing
constriction resistance. Appropriate copper trace sizes decreases bulk resistance,
allowing for cooler temperatures and less loss. Otherwise that heat could be
"sinked" to the connector interface, increasing reliability concerns. Power
supply manufacturers are very creative in supplementing PCB structures with
features to alleviate thermal and constriction concerns.

In addition, as systems are
packaged into smaller boxes with more components, it is critical to ensure
proper management of air flow around connectors positioned at the intersection
point (such as between a power supply and server). Ample air flow around and
through the connector helps cool the power contact, allowing for more current
and/or an increased margin of safety. At the same time, connectors are
sometimes located at key points and block airflow. The process of cooling
connectors, however, is often not high on the list of priorities for designers
when considering air flow.

With operational safety in
mind, the designer needs to consider the entire system and its power
architecture to understand what potential may exist - from end to end - for
constriction areas and voltage drops that affect thermal and electrical
performance. Typically, a maximum 30mV drop defines the threshold of thermal
stability for a power contact. Once this threshold is crossed, the probability
of thermal instability increases significantly.

World-class connector
producers are working with their customers to develop improved power
interconnect solutions for safe, reliable operation in smaller spaces at higher
temperatures over long product life. New designs incorporate new alloys and
molding resins, plating, improved contact technology - all to increase current
density without sacrificing safety and reliability.

Risk Mitigation. Connector manufacturers have
traditionally based current ratings on their products' electrical performance
on testing under ideal circumstances. These published ratings, while accurate
for what they measure, rarely tell the whole story because they fail to take
into account the various conditions and interactions that will affect the
environment in which the connector actually will be operating.

Power Integrity  in Interconnect Design

As a result, a common practice among OEMs has been to derate the
connectors in order to build in a thermal safety margin over product ratings
published in the connector manufacturers' literature. Many use a simple
approach, testing a smaller circuit count along with a longer one and charting
a range of T rise versus current showing lower current carrying capability as
the circuit count increases. In addition, some customers assign yet another
arbitrary percentage, so if a connector supplier submitted a product rated at,
say, 100 amps, the user would automatically derate it by 30 percent to ensure a
built-in safety margin against the possibility of overheating.

Today's leading connector providers understand this and will work
closely with OEMs and their design team to match their connector selection to
the specific application, based on scientific testing and performance analysis
under real-world application conditions.

To provide accurate ratings, top manufacturers conduct extensive
testing and predictive modeling, such as Joule Heating FEA (finite element
analysis) and CFD software (computational fluid dynamics) with inputs
pertaining to the connector and PCB geometry and material properties, current,
contact resistances (actual test data) and air flow. In this way, they can
estimate the performance of each of their interconnect products and provide
reliable counsel to customers as to which of their products would be the best match for the application requirements. It is not practical to simulate and/or test every
possible environment but these models and analyses can help guide designers to
smarter choices in a shorter amount of time. This is important in the
fast-paced design cycles required in the electronics industry.

Power Integrity Planning Yields
Better Results

With compact size,
transmission speed, signal and power integrity being paramount in electronic
device technology - the benefits of proactive power integrity engineering
simply cannot be overstated. Increasing demand for computing power is driving
the demand for more raw power. Meanwhile, product design cycles continue to
shrink, giving power engineers less time to make critical decisions.

Gaining a clear understanding of all the requirements early in
the design phase, before specifying interconnect components, can help ensure
the right decisions and avoid costly missteps. Most important, high-quality
power integrity engineering enables OEMs and their product designers to
maximize the performance, reliability and safety of their products.

Ken Stead is global new product development manager for power
products at Molex Inc.

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