Busbars are simple in principle, complicated in practice: part 3

Not every design needs large bus bars; some only need smaller, localized ones or PC board-mounted bus bars. This part looks at these situations, as well as testing of high-current/voltage bus bars.

The previous part explores additional bus-bar considerations. Part 1 can be found here.

Smaller busbars also fill a need

Voltage drop is well known to electrical engineers and is defined by Ohm’s Law and the simplest of equations: V = I × R. The voltage drop is a function only of the current value and the path resistance, and is independent of the rail voltage. Although the percentage of loss is obviously far greater with a 1-V rail versus a 15-V rail for a given voltage drop, the voltage drop itself is unaffected.

Typical DC rail tolerance ranges from ±1% % to ±5% %, depending on the component and circuit. Voltage drop and low voltage at the load are more than just a nuisance; they can be a significant issue. It can cause circuits not to function at all (not good) or function erratically when the voltage is at the edge of the allowed specification for the various ICs (often worse). That’s why it’s critical to analyze the drop between the power supply and load, and address excessive IR drop.

How much drop can be expected? Again, it’s a simple calculation. A PC-board trace, which is 1 mm wide and 10 cm long on standard “1-oz” copper cladding (actually 35 µm thick), will have a resistance of about 50 mΩ (the resistivity of copper is 1.74 × 10-8 Ω-m at 20°C), shown in Figure 1 (left). At a modest current of 10 A, that’s 500 mV of drop along the supply rail alone – and it may even be twice that if the DC ground-return is via a similar trace rather than a wide ground plane. There are many handy online resistance calculators for this loss, as shown in Figure 1 (right); many of them also calculate the temperature rise.

To address this problem, designers can use thicker, more costly PCB cladding (2 ounces/70 µm is common), but this only cuts the loss in half.

Another option is to use an intermediate bus converter (IBC) topology for power distribution, where a higher voltage (and thus lower current), such as 24 VDC or 12/15 VDC, is distributed throughout the board, and then regulated locally as needed for the IC or a subcircuit. This works well technically and is often the “best” solution, but there’s a cost in additional DC/DC regulators as well as board real estate.

Bus bars are not limited to “freestanding” runs of a meter or more, or hundreds of amps. While many busbars are custom-shaped and sized to fit the unique needs of the application, there are also smaller busbars that are used directly with a PC board, shown in Figure 2; these also act as board stiffeners.

PCB busbars

There are also bus bars for designs where power distribution on the board itself is a concern. The obvious approach is to incorporate additional PCB power and ground layers; however, this comes at a severe cost in terms of board material, layout issues with buried, blind, and through vias (also known as “vertical interconnect access”), and reduced production yield.

Instead, a viable option is to use bus bars on a much smaller physical scale for PC boards. The bus bar concept and implementation are simple: it’s an insulated strip of copper with protruding connection pins along its long edge, as shown in Figure 3. It is available in many standard lengths, with custom lengths also available at negligible or no up-front non-recurring engineering (NRE) costs and minimal lead-time impact.

These bus bars fit onto the board like any other through-hole component, adding an independent DC-rail path. It can be wide and thick enough to provide a current path with sub-milliohm resistance. It’s a hassle-free, easy-to-add component for the PC board, providing another degree of layout freedom for distributing those high-current rails. It also facilitates better non-power signal integrity since the power tracks are no longer in the way of the preferred signal path.

There are other options and benefits provided by these bus bars. There are dual- and even triple-layer versions, allowing the same bus bar assembly to carry one or more power rails as well as ground. These “across the board” bus bars also offer a secondary benefit of stiffening the PC board against flexing and vibration, as shown in Figure 4, which is a concern as circuit boards become larger.

Busbar testing

Rigorous testing, as defined by various regulatory and industry standards, is performed on each laminated busbar to ensure both its electrical properties and mechanical integrity. The three primary tests that are performed between physically adjacent layers of the completed assembly are:

1. The high potential (hipot) test, formally known as a dielectric strength test, stresses the insulation and ensures that no current flows from one lamination to another, thereby verifying the insulation’s integrity (it is the opposite of a continuity test).
2. The partial discharge (PD) test finds small electrical “sparks” – localized dielectric breakdowns – that occur within the insulation of medium- and high-voltage conductors. These discharges result from electrical breakdowns in air pockets within insulating layers, which may be due to a void, crack, or partial delamination.
3. An insulation-resistance test (often called a megohm or “megger” test) uses a megohmmeter to apply a high DC voltage between conducting layers, then measures the associated very high resistance.

Each test has a well-defined role. The megger measures insulation resistance while a hipot test measures the closely related parameter of leakage current. The hipot test stresses the insulation’s weak points at higher voltage levels than the megger does, so if a fault is seen with the megger, it will also be seen with the hipot test.

Connecting to the bus

Connecting the power source to the bus bar or connecting the bus bar to the load is a complicated subject. It typically involves bolting a heavy, yet somewhat flexible, cable with crimped-on ring termination to the bus bar. It’s a topic with its own complexities and subtleties, with too many aspects to cover here.

Summary

When you need to route large amounts of current and power, don’t assume a heavy-duty cable and connectors are the only way to go; instead, step up to the (bus) bar and see if they are a better solution in terms of cost, performance, regulatory demands, and more.

References

PCB trace properties calculator, MustCalculate
TraTrace Width Calculator, Bittele Electronics Inc.
PCB Trace Resistance Calculator, Trance-Cat
Spreadsheet modeling tool helps analyze power- and ground-plane voltage drops to keep core voltages within tolerance, SLYT23, Texas Instruments
IR-Drop Analysis, Advanced Layout Solutions Ltd
PCB Trace-The Importance of PCB Traces In the PCBs, OurPCB
What is a Busbar and Other FAQs, Starline Holdings LLC
Busbar Technology, Rogers Corp.
What Is a Busbar?, Aptiv PLC
Advanced Busbar Systems for Electrical Engineer Contractors, Electronic Power Design
Enabling Smaller, Smarter Busbar Designs that Support Higher Power Densities, Ennovi/Interplex Medical
What is an Electrical Busbar: Types, Applications, & Simulation, Simscale
Ampacities and Mechanical Properties of Rectangular Copper Busbars, Copper Development Assoc., Inc.
The Role of Busbars in Electrical Systems, Red Seal Electric
Copper vs. Aluminum Busbars: 8 Key Differences and How to Choose Which One Is Right for Your Application, Approved Sheet Metal
What is PCB Busbar or PCB Stiffener Busbar in Electronics?, Rayming PCB and Assembly
2023 Newest Guide to PCB Busbar and Design it on PCB, PCB Online
Circuit Board Stiffeners, Storm Power Components
Technical Library, Storm Power Components
Laminated Busbar Market by Material…Forecast to 2030, Markets and Markets

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