The principle of busbar is simple, but the practice is complex: Part 1

Compared to many other active or even passive components, busbars seem simple and sluggish, and in some ways, they do. However, they are also complex structures that require an understanding of voltage drops caused by conductor resistance, material science, thermal issues, mechanical connections, insulation, coating chemistry, and electrical safety and integrity testing.

The function of the busbar is direct and clear: to transmit power (such as high current and/or high voltage) from the power source to the load with acceptable low voltage drop and power loss. This means using solid copper bars (sometimes aluminum) with cross-sectional dimensions to keep resistance losses and related self heating below specified thresholds.

The modern system presents an ironic dilemma. On the one hand, the power consumption of various circuit functions such as transistors, gates, drivers, and amplifiers has decreased by several orders of magnitude in the past few decades, enabling designers to accomplish tasks that were unimaginable just a few years ago.

At the same time, the power demand of many systems has sharply increased. For example, although several years ago it was common for each rack to consume several kilowatts of power, now many racks consume 10 kilowatts, 12 kilowatts, or even 15 kilowatts of power, and even higher numbers are emerging, as shown in Figure 1.

Figure 1. The power consumption of each rack and the resulting power requirements have increased sharply and are expected to continue this rapid growth. Although the numbers vary depending on the data source, the trend remains clear. (Image: Semiconductor Engineering)

Why is that? The growth rate of power demand is much higher than the decrease rate of power per component, and the demand for functional density has also increased sharply. This phenomenon is not limited to racks, as a single PC board in many other applications typically requires several hundred amperes of current. Of course, this is not all discouraging news: desktop computers that used to require hundreds of watts now offer much higher performance, but only half to one-third of the power of their predecessors.

The reason for the exponential growth of electricity demand is usually that we can do more, we want to do more than just more, we want to do more. Simply put, demand has expanded to meet and exceed previous limitations, and no good thing goes unpunished.

In power related design, most attention is focused on the challenge of extracting all of this electricity (i.e. heat) from the rack or system using advanced convection, forced ventilation, active cooling, and even various forms of liquid cooling. The goal is to bring the heat to that magical place called 'away', where it becomes someone else's problem.

However, all of these deserved hot concerns are actually the second stage of the overall power issue. The problem you face before encountering dissipation challenges is to distribute all power (whether from AC lines, high voltage DC, or low voltage DC) to where it is needed.

Basic knowledge of busbar

This is where the busbar plays a crucial role, as shown in Figure 2. The applicability of bus bars far exceeds that of data centers and server racks. They are used in solar and wind energy installations, switchgear, large factory motors, aircraft, ships, and even hybrid and battery electric vehicles (BEVs) - basically anywhere that requires reliable transmission of higher levels of current (usually high voltage) with minimal loss and low cost. Even medium-sized products can benefit from their ability to provide current with low loss and achieve this safely and efficiently.

Figure 2. There are various ways to arrange busbar devices, from small to large, but they all have a striking and serious appearance. (Image: Red Seal Electric Company)

Busbars are not necessarily large, highly visible, and sometimes intimidating components. Physically smaller busbars are typically used between PC boards, even within the board, to deliver power to various sub components and sub sections. We will take a look at these small busbars later.

When discussing bus poles, there is a 'back to the future' aspect. They have existed since the early days of electricity, when low-power applications did not exist, "electronic products" had not yet been developed, and electricity was mainly used for industrial motors and heating.

This type of power is typically generated and transmitted at lower voltages, but due to technical issues, higher currents result in significant resistance induced voltage drops and dissipation during power transmission. Today, a large amount of electricity still needs to be transmitted, and the laws of physics and Ohm's law still exist. The busbar solution is still effective and feasible.

Why use busbars instead of cables and connectors? Although the copper (or aluminum) cross-sectional area of busbars and cables is nominally the same at a given current, the reality is that busbars are easier to install, provide multi-point picking, are more robust, have better thermal characteristics, and do not require high current plug-in connectors (the latter point may be an advantage or disadvantage, depending on the situation). Cables and busbars have their own positions in the designer's solution menu.

Determine the size of the busbar based on IR voltage drop and temperature rise

What is the required cross-sectional area for the busbar? Usually, the answer is' depending on the situation '. The two main and closely related factors are the tolerable IR voltage drop at the maximum current level and the acceptable temperature rise due to I2R dissipation.

Figure 3 shows some recommendations for copper busbars in a specific applicable category, with "current carrying capacity" (in amperes) as the starting parameter. In specific applications, there are many similar and more detailed tables related to busbar applications in specific applications, from industry associations and safety organizations. As shown in the figure, the temperature rise may be significant, and many designers are not aware of this, especially those with experience in low-power design.

Figure 3. The dimensions (in inches) of the nominal current carrying capacity (ampere capacity) of the 100 A and 500 A busbars using copper demonstrate the effect of allowable temperature rise. (Image source: Copper Industry Development Association)

The minimum cross-sectional area of the busbar is not only determined by the judgment, calculation, and modeling simulation of the designers. Every industry that uses busbars has clearly defined standards that define how large the busbar must be to limit temperature rise and voltage drop to specified values under different conditions.

Please remember that there are several negative consequences of rising temperatures. Firstly, it will affect nearby electronic devices and lead to a shortened lifespan. Secondly, the busbars will obviously expand due to temperature rise, which will affect their installation brackets, connections or "joints" (called joints) with other busbars, and electrical integrity with cable connections. The stress caused by thermal expansion can stimulate microcracks, ultimately leading to failure.

In addition, if the busbar heats and cools according to changes in current, repeated thermal cycles will accelerate the development and propagation of cracks, ultimately leading to premature failure. This will not only affect the metal to metal connection points, but also the insulation coating (if any).