Bus bars offer independent DC rail path as an easy add-on to the PCB to address voltage drop and low voltage at the load.
Despite the widespread use of low-power ICs—or counterintuitively, because of it—DC power-rail currents on PCBs keep increasing. It’s not unusual to have a modest-size board drawing high tens of amps and even more. Getting rid of the resultant I2R heat is a well-known thermal problem, but there’s an issue that comes even before that consequence: delivering the nominal rail voltage to the load without excessive voltage drop due to PCB track resistance.
This phenomenon is not a piece of news to electrical engineers, of course, 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 loss is far greater with a 1-V rail versus a 15-V rail for a given drop.
Voltage drop and low voltage at the load is more than just a nuisance. It can cause circuits not to function at all, which is not good, or function erratically when the voltage is at the edge of the allowed specification for the various ICs, which is often worse, as the typical rail tolerance window ranges from ±1% to ±5%, depending on the component. That’s why it’s critical to analyze the drop between the power supply and load and deal with excessive IR drop.
Voltage drop workarounds
How much drop can be expected? Again, it’s a simple calculation. A PCB trace that 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 shown in Figure 1 (left) is 1.74 × 10-8 Ω-m at 20°C.
At a modest current of 10 A—which is 500 mV of drop along the supply rail alone—it may even be twice if the DC ground-return runs 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 calculate temperature rise as well.
Figure 1 The calculation of IR drop in PCB traces is a direct application of Ohm’s Law; various online calculators are available to make it a simple task. Source: Trance Cat
To address this problem, designers can use thicker, more-costly PCB cladding—2 ounce/70 μm is common—but this only cuts the loss in half. Another popular option is to use an intermediate bus converter (IBC) architecture 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 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.
Fortunately, there’s another potential solution if you think “out of the box,” or at least out of the plane. No, it’s not adding additional PCB layers, although that can certainly be done, again at a cost in board material, layout issues with buried, blind, through vias, and more. The formal name of through vias is vertical interconnect access.
Instead, it’s the use of “old-fashioned” bus bars. These are often used in industrial settings to deliver power to motors and more, but are an option on a much-smaller physical scale for PCBs. The bus bar concept and implementation are simple. Bus bar is an insulated strip of copper with protruding connection pins along the long edge, and it’s available in many standard lengths. Custom lengths are also available at negligible or no up-front non-recurring engineering (NRE) cost and minimal lead-time impact.
Figure 2 Bus bar goes across the PCB and is placed into board holes, which then connect to the various areas of the board that need the power it conveys. Source: Storm Power Components
How bus bar works
Bus bar fits onto the board like any other through-hole component and adds an independent DC rail path. It can be wide and thick enough to provide a current path with sub-milliohm resistance. It’s a no-headache, easy add-on to the PCB approach that provides a higher degree of freedom for the layout distribution of those high-current rails. It also facilitates better non-power signal integrity since the previous wide surface rail is no longer in the way of the preferred signal path.
Bus bar has other benefits as well. There are dual- and even triple-layer versions, so the same bus bar can carry one or more power rails as well as the ground. Bus bars also offer a free secondary benefit of stiffening the PCB against flexing and vibration, as shown in Figure 3, which becomes a concern as circuit boards get larger.
Figure 3 A secondary and free benefit of bus bars is that they stiffen the board against flexing, thus reducing vibration-induced stress cracks, which can lead to failure. Source: Storm Power Components
Bus bars are available in hundreds of standard configurations from dozens of vendors. Before you look to a more complex solution to the problem of lowering IR loss to an acceptable level such as wider traces, thicker PCB copper, another board layer, or use of an IBC and local DC/DC regulators—all of which have their valid place in the solutions toolkit—consider the 3D technique and component that literally takes your PCB to another dimension.
This article was originally published on EDN.
Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.
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