Busbars are simple in principle, complicated in practice: part 1

Bus bars appear to be simple and low glamour in comparison to many other active and even passive components, and in some ways, they are. However, they are also sophisticated structures that require an understanding of voltage drop due to conductor resistance, materials science, thermal issues, mechanical joining, insulation, coating chemistry, and electrical safety and integrity tests.

The function of the bus bar is direct and clear: to convey power (as high current and/or high voltage) from the source to the load with an acceptably low voltage drop and power loss. This means using solid bars of copper (sometimes aluminum) with a cross-section size that keeps resistive losses and associated self-heating below a defined threshold.

Modern systems present an ironic dilemma. On the one hand, the power dissipation of individual circuit functions, such as transistors, gates, drivers, and amplifiers, has decreased by orders of magnitude over the decades, allowing designers to accomplish tasks that were previously inconceivable just a few years ago.

At the same time, the power demands of many systems have increased dramatically. For example, while dissipation of a few kilowatts per rack was common a few years ago, many racks are now pushing ten, twelve, and even fifteen kilowatts, and higher numbers are also occurring, as seen in Figure 1.

Figure 1. Per-rack power dissipation and thus delivered power requirements have increased dramatically and are forecast to continue this rapid increase. While the numbers vary depending on the data source, the trend remains clear. (Image: Semiconductor Engineering)

Why is this happening? Power demands have increased at a far higher rate than per-element power decreases, while functional density demand has also increased dramatically. The phenomenon is not limited to racks, as individual PC boards in many other applications routinely demand hundreds of amps. Of course, it’s not all discouraging news: a desktop PC that used to require several hundred watts now offers far more performance, but at one-half to one-third the power of its predecessor.

The reason for this exponential increase in power demands is the usual one: as we can do more, we want to do more than just more, we want to do much more of it. In plain English, demands have expanded to fill and exceed previous limits, and no good deed goes unpunished.

Much of the attention in power-related design has been centered on the challenges of extracting all this power (meaning heat) from the rack or system using advanced convection, forced air, active cooling, and even various forms of liquid cooling. The objective is to get that heat to that magical place called “away,” where it becomes someone else’s problem.

However, all this deserved thermal attention is really the second stage of the overall power problem. Before you encounter the dissipation challenge, you face the problem of distributing all that power, whether from an AC line, high-voltage DC, or low-voltage DC, to where it is needed.

Bus bar basics

That’s where busbars play a crucial role, as illustrated in Figure 2. The applicability of busbars goes far beyond data centers and server racks. They are used in solar- and wind-power installations, switchgear, large factory motors, aircraft, ships, and even hybrid and battery-electric vehicles (BEVs) — essentially anywhere higher levels of current, often at high voltage as well, need to be transferred reliably with minimal losses and at a low cost. Even modest-sized products benefit from their ability to deliver current with low losses, and do so safely and efficiently.

Figure 2. Busbar installations come in an infinite variety of arrangements, ranging from small to large, but they all share a dramatic, no-nonsense appearance. (Image: Red Seal Electric Company)

Bus bars do not necessarily have to be large, highly visible, sometimes intimidating components. Physically small bus bars are often used between PC boards and even within boards to carry power to various subassemblies and subsections. We’ll look at these small bus bars later.

There’s a “back to the future” aspect when talking about bus bars. They have been around since the earliest days of electricity, when low-power applications simply didn’t exist, “electronics” had not yet been developed, and electrical power was used primarily for industrial motors and heating.

This power was often generated and delivered at lower voltages, resulting in higher currents due to technical issues, which led to significant resistance-induced voltage drops and dissipation in the power delivery. Today, large amounts of electrical power still need to be delivered, while the laws of physics and Ohm’s Law remain in place, and the busbar solution is still valid and viable.

Why use a busbar rather than a wire cable and connector? Although the copper (or aluminum) cross-section area for a given current is nominally the same for busbar and cable, the reality is that busbars are easier to install, offer multipoint pickoff, are more rugged, have better thermal characteristics, and don’t need high-current plug-in connectors (that latter point can be a benefit or a detriment, depending on the situation). Both cables and busbars have their place in the designer’s solutions menu.

Sizing the bus bar for IR drop and temperature rise

How much cross-sectional area does a busbar require? As is usually the case, the answer is “it depends.” The two dominant and closely related factors are the amount of IR drop that can be tolerated at maximum current levels, and also the acceptable temperature rise due to I²R dissipation.

Some suggestions for copper bus bars in one specific applicable class are shown in Figure 3, with “ampacity” (carrying capacity in amperes) as the starting-point parameter. There are many similar and more highly detailed tables associated with bus-bar applications in specific applications, from both industry associations and safety organizations. The temperature rise can be significant, as the table shows – a consideration that many designers are unaware of, especially those with experience in low-power design.

Figure 3. These dimensional numbers (in inches) for 100-A and 500-A busbar nominal ampacity (ampere capacity) using copper shows the effect of allowed temperature rise. (Image source: Copper Development Association)

The minimum cross-sectional area for the bus bar is not determined solely by the designer’s judgment, calculations, and modeling simulations. Every industry that uses bus bars has well-defined standards that define how big the bus bar must be to limit temperature rise and voltage drop to specified values under different circumstances.

Keep in mind that a temperature rise has several negative consequences. First, it impacts nearby associated electronics and contributes to reduced life. Second, bus bars obviously expand due to the temperature rise, and this affects their mounting brackets, connections or “splices” to other bus bars (called jointing), and the electrical integrity of the connections to cables. Stress due to heat expansion can spur microfractures, which eventually become failures.

Furthermore, if the bus bar heats and cools in response to changes in current, repeated thermal cycling can accelerate crack development and growth, ultimately leading to premature failure. This affects not only the metal-to-metal connection points but also insulating coatings (if any).

Part 2 will continue with perspectives on other critical bus-bar issues.