OPD October 2000: Laminated Bus Bar Assemblies Improve Power Distribution in High Power Electronic Systems

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Today's electrical and electronic systems are being asked to deliver higher performance, higher quality, and more capabilities, with better reliability, lower cost, and smaller size, than previously thought possible. This is putting increased pressure on the designers of these systems to look for better technologies to replace the “usual way” of doing things.

In systems where high power levels are distributed, such as motor drives, electric vehicles, telephone switching central offices, and large computer cabinets, designers must optimize the way power is distributed within the cabinet or enclosure. This is necessary because power distribution issues can have a major effect on the cost, performance, reliability, and size of systems where tens, hundreds, or even thousands of amps are distributed. A technology that has proven useful in meeting these power distribution requirements is laminated bus bars. To understand why the laminated bus bar is an improvement, we first have to look at the characteristics of traditional power distribution approaches.

TRADITIONAL APPROACHES

The traditional solution for distributing high power within an electronic enclosure has been with cables and/or cabling harnesses. These standard wiring solutions are fabricated with individual conductors that have been individually terminated, then tied together to form assemblies (Fig. 1). For simple systems operating at relatively low power levels, these cable assemblies are acceptable. However, as systems become more complex, and power levels rise, these traditional cabling harnesses have limitations.

As current levels exceed several hundred amps, it is necessary to use heavy-gauge conductors in these cable assembles: 2/0 AWG or heavier. Such a heavy gauge conductor is very stiff, and difficult to bend into the exact shape to fit inside an enclosure.

Also, with more complex electrical signals that have higher-frequency components, the EMI/RFI performance of conventional cable assemblies may be inadequate. System problems can result when one conductor within a cable harness induces an unwanted signal into a nearby conductor.

More complex signals are due to the increasing use of high-speed digital components. These higher-frequency signals are more susceptible to interference, which is often present when system power supplies use IGBTs or other high-power switches. These devices are common in motor controller and UPSs, and they can induce transient spikes in power distribution cabling. Inadequate power distribution cabling then sends these spikes and noise throughout the entire system, causing interference with sensitive data signals. High inductance power distribution cabling worsens switching transients. And, traditional cabling harnesses have high inductance.

As power-distribution topologies become more complex, traditional cabling systems must also become more complex, with each termination individually connected to the proper device or subsystem. The greater the number of such terminations, the higher the probability that an assembly error will occur. Tracking down and correcting such errors is time-consuming, and increases total product assembly costs.

In computer and telecommunications systems, the use of traditional lug-style terminations may no longer be an option. That's because many of these systems are being designed to be “hot-swappable” so that a defective or failed circuit board can be replaced without turning off system power. Conventional cabling harnesses, with their exposed terminations and unprotected live conductors, represent a safety hazard when hot swapping is necessary. The lug terminations of standard cables cannot be used in this environment. For all these reasons, a better solution has been developed for handling the power-distribution needs of today's more complex, high-power systems: the laminated bus bar system (Fig. 2).

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LAMINATED BUS BARS

Laminated bus bars are a power interconnection and distribution system consisting of conductive strips separated by insulating layers. The conductive strips are usually copper, plated with tin, nickel, or gold, depending on the system's requirements system. Aluminum can be used in place of copper where overall system weight is a concern (such as aerospace applications).

A wide choice of insulating material is also available, again depending on the system's requirements. Typical choices include:

Nomex paper, when a high degree of scratch/abrasion resistance is needed.
Polyester, when the ability to reject absorption of ambient chemicals is important.
Kapton, when a high temperature rating, or the ability to withstand repeated thermal cycling is needed.
Epoxy-type powder coating, when resistance to chemicals and a good cosmetic appearance are needed.

The number of layers, as well as the cross-section of each conductor, is a function of the requirements of the system or cabinet into which the bus bar assembly will be installed. The stack of conductors and insulators are laminated together — often with the edges being sealed — to form a monolithic, multi-conductor assembly that can be completely insulated on the outside.

These laminated bus bar systems can also incorporate circuit elements such as capacitors, fuses, circuit breakers, diodes, resistors, IGBTs, and heat sinks, as well as all types of conventional and unconventional terminations and/or connectors. Even flex circuit break-outs can be designed in. Fig. 3 shows a laminated bus bar with additional integrated components.

Compared with traditional cabling, laminated bus bar systems offer a number of features and benefits for today's complex electrical/electronic systems. They provide better electrical performance. They handle high currents with less voltage drop, thereby doing a better job of maintaining the regulation of the dc voltage across the system. The laminated design also reduces the amount of noise on the power buses because the conductors can be interleaved with the right sequence of voltage, polarities — or even ground planes - to minimize noise. Laminated conductors also act as capacitors, providing a degree of EMI filtering. And, they also havelower inductance than conventional cables, which minimizes spikes and ringing when switching transients from devices such as IGBTs are present.

In addition, laminated bus bar systems offer better mechanical characteristics than traditional cabling. They weigh less. They virtually eliminate the problems of routing conductors through cramped and labyrinth-like interiors, thereby reducing assembly time and cost. Because they are precision-engineered components, they fit precisely in their allotted space, no matter how small it is. Laminated bus bar systems greatly reduce the probability of incorrect assembly, as some parts are built right onto the bus, and the bus terminations end up in exactly the right locations. They are typically designed to be completely rigid and monolithic, so they are much less susceptible to problems created by flexing, shock, or vibration. And finally, laminated bus bar systems can be manufactured from such a wide choice of materials, so they can be made impervious to virtually any chemical, solvent, salt, or contaminant that may be present in the equipment's operating environment.

Other benefits that are a direct result of the features described above include: higher reliability with fewer failures and fewer service problems, a lower installed cost, and predictable thermal performance. And, laminated bus bar systems can be fitted with any connector, so they can be used with existing subsystems and components, with redesign needed for a new or special connector.

APPLICATIONS

Application environments for laminated bus bar systems are virtually unlimited, but they typically meet the following scenario: they are low voltage (3V to 48V), high current (>100A) in which tightly-regulated dc must be distributed. Often, this dc is converted via local dc-to-dc or dc-to-ac converters to provide the correct power for the specific circuit boards, motors, or subsystems.

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Laminated bus bar systems can be configured to work well in many applications, including three categories:

Electromechanical systems, including digital drives, motor controls, elevators, locomotives, all types of vehicles, turbine power plants, and machine tools. In these applications, the distribution of high levels of current is a key issue. Fig. 4 shows a laminated bus bar for an IGBT controlled inverter for a medium horsepower motor drive. The bus bar is populated with various components creating a compact and efficient power inverter assembly.
Large electronic systems, such as telecommunication switching systems, computer network routers, and mainframe computer systems. In these applications, key issues include the ability to provide a large number of interconnects, to allow safe “hot-swapping,” and to preserve the “cleanliness” of the dc power by minimizing noise and spikes that might otherwise appear on power buses.
Military and aerospace applications. Here, minimizing the weight and space required by the power distribution system is very important, as well as maximizing system reliability. Many aerospace and military systems can be subject to high levels of acceleration (“g-forces”) and vibration, making high reliability even more of a challenge.

The designer of a piece of sophisticated electrical or electronic equipment today must deal with so many considerations, that the issue of power distribution may take place late in the design phase. While traditional cabling and wiring harnesses may provide acceptable performance for many systems, experience has shown that if any of the items on the following checklist are “true,” it's time to consider using a laminated bus bar power distribution system instead:

Current requirements result in a power cable of 2/0 AWG or heavier.
The number of terminations becomes too expensive or too difficult to perform.
There is a need to reduce assembly cost of complex systems or cabinets.
EMI/RFI is anticipated to be a problem.
Power distribution wiring must fit into a very tight space.
The power distribution system must be as lightweight as possible.
The finished system needs the highest reliability, quality, or consistent performance.
It is difficult to meet UL, CSA, or ISO requirements.

POTENTIAL SERVICES

It is important to consider which services, in addition to just products, might be useful or even necessary, for the successful design and integration of a laminated bus bar into an electrical or electronic system. For example:

Will you be designing the bus bar system yourself, or will you need engineering help from your vendor?
Will you want the vendor to perform a complete “turnkey” design of your bus bar system?
Will you want components (capacitors, fuses, etc.) installed on the bus bars?
Will you install the components yourself, or will you want your bus bar vendor to install them?
Will you want the bus bars painted, etched, or silk-screened with identification markings?
Will you need just prototypes, full production quantities, or both?
Will you want your vendor to perform any electrical, mechanical, or environmental testing of your bus bar systems?

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