How Large Should Power Rails Be in a PCB?
All conductors have finite electrical conductivity, and they will heat up when they carry a large amount of current. Copper traces in a PCB are no different, but many designers will often not notice whether the design has appreciable temperature rise. This is often because so many designers have a habit of oversizing the traces or pours that are used for PCB power rails.
While it’s not really a “bad” thing for a power rail to be too large, it could take up excessive space that could be used for routing and placement of components. It’s easy to draw out large rails as pours and then allow the clearance rules to open up gaps around pads and components, but one could argue it’s better to estimate an upper limit on trace width to ensure the current capacity requirement is satisfied.
Two Tools to Size PCB Power Rails
There are two industry standards that can be used to size power rails based on a temperature rise target. These two standards are the IPC-2221 and IPC-2152 standards; both provide a standardized approach to designing PCB interconnects that is thermally aware. The designer selects a temperature rise above ambient, and based on this temperature rise the designer can calculate or determine a current limit based on their power rail sizing.
Designers who build power systems should know that the typical approach is to draw out large rails to deliver power. Large rails are needed when current is high because they will have low DC resistance. The problem with this approach is that designers will sometimes draw out similarly large rails in any other design, where arguably the large rails will not be needed. A simple application of the IPC-2221 formulas or data in the IPC-2152 nomographs gives a good estimate of the trace size requirement.
4-layer PCB stackup with two internal planes.
The above points should be considered in small digital systems or small analog/mixed signal systems. For a simple rule of thumb, there is a 100-10 rule, meaning a 100 mil wide trace will start to reach a very high temperature once it carries 10 Amps of current. Based on IPC-2221 and IPC-2152, we can determine what the expected temperature rise would be in each case:
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IPC-2221 temperature rise: 54 °C on 1 oz. copper
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IPC 2152 temperature rise (with adjacent plane): 41 °C on 1 oz. copper
What About a Nearby Ground Plane?
IPC-2221 is not capable of considering ground planes in the PCB, it only accounts for internal versus external trace routing. IPC-2152 considers a multitude of situations, including the presence of a ground plane adjacent to a large power rail.
When the ground plane is present on the next layer, this plane acts like a big heat spreader, so it allows for a higher current limit than you would calculate with IPC-2221. This is the reason for the discrepancy in the values above. The other factor that is affected by the presence of the ground plane is the formation of a capacitor, and so the rail has some capacitance. This is important in high-speed digital PCBs, which need the rail capacitance and low rail inductance in order to provide stable power to large processors.
A good estimation of the rail capacitance is with the following formula:
Based on the dielectric insulator’s thickness, we see that a high-speed stack up with thinner laminate can give higher rail capacitance. It may be worth comparing total rail capacitance with the current demand when sizing a power rail to deliver power to a large processor. For example, large rails may be needed because current demand for iOS is very high, but if the signal Edge rate for these iOS is slow, that high capacitance is not necessarily needed. This is an important consideration in thermally-aware power integrity and is often addressed in power-aware/thermal-aware multiphysics simulation.
Summary
In summary, the easier tool to use for sizing traces in a PCB based on temperature rise limits is the IPC-2221 standard. The formula in the IPC-2221 standard is very simple and does not require a large set of tables to determine the appropriate match between the trace size, temperature rise, and current. If you use the IPC-2221 standard, you will be oversizing your traces somewhat, but it will not be the same magnitude of oversizing you might see from some large pours used in circuit boards.
The other extreme has to do with power integrity in high-speed design, which supports processors potentially drawing hundreds of Amps of peak current. These amounts of current are pulled through multiple rails, so it is often the strategy to cut up a power plane into large rails that each support their share of the total current.
Whether you need to design large power electronics or compact high-speed digital PCBs, use the best PCB design features OrCAD from Cadence. If you’re ready to take even more control over net logic and board layout, you can graduate to Allegro PCB Designer for a more advanced toolset and additional simulation options for systems analysis. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.
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