Without power, the components on your PCB might as well be inert pieces of metal. In order to light a lightbulb, spin a motor, or otherwise perform some action such as maintaining the optimum temperature on your DIY espresso machine, your board needs a steady supply of voltage to function. In this post we’ll dive into detail about what power planes are and some best practices you can follow to incorporate them into your PCB designs.
What exactly is a power plane?
Often designated as Vcc in a PCB design, the powerplane is simply a flat plane of copper connected to a power supply. Similar to how the ground plane is connected to the ground connection of the power supply. Its purpose is to provide a steady supply of voltage to your board. Whenever a component needs to draw power simply run a trace to a via that makes contact with the powerplane and completes your circuit.
You’ll notice that you will generally only see power planes in boards with 4 or more layers. This is because the best practice for multi-layer stackups are to use an even number of layers. Odd layer stackups don’t bring any cost savings, and the resulting asymmetry can lead to warping, twisting, and other structural defects.
The popular 2 layer board will generally benefit more from a ground plane instead of a power plane, relying on tracks to deliver power from a power source.
Why use power planes?
Power planes come with a number of advantages over tracks and traces when they can be used in a PCB design:
Improved decoupling between circuits. The surface of a power plane can create a parallel plate decoupling capacitor between itself the insulating layer and the following ground plane that prevents noise from propagating through the power supply from one circuit to another.
Shorter return paths. The convenience of following a via down from a signal layer to the power plane to power a circuit. Shorter return paths lead to better EMC performance.
Larger current carrying capacity. Power planes can handle more current than traces or tracks, lowering the operating temperature of your board.
Power plane domain splitting
Having a single well of power for all the components on a signal layer to draw from makes a lot of sense if they all have the same voltage requirements. Increasingly however, more complex high density PCB designs have made the practice of splitting a power plane into multiple domains more popular.
If your MCU requires one voltage while your I/O ports need another, you may need to split the reference plane to accommodate both voltages. This saves space but does put you at risk of higher noise, crosstalk, and other EMI/EMC problems. It’s important to follow EMI/EMC best practices such as separating your analog, digital, and high power circuits.
That said, the most efficient routing would be to accurately account for all EMI/EMC considerations and have separate power rails for each circuit on your board, forgoing the powerplane altogether in favor of efficient routing. In such situations you will usually still have a ground plane to allow for shorter return paths and noise absorption.
Use PCB design software to account for all EMI/EMC considerations
At the end of the day, you’re ability to understand the various EMI/EMC influences that affect your circuits is the key behind designing your power planes. EDA software greatly simplifies the process of designing, simulating, and testing your PCB designs for electrical, thermal, and physical performance considerations. Check out Cadence’s suite of PCB design and analysis tools today.
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