The Basics of Power Integrity in PCB Design

In addition to honesty and strong moral principles, the word integrity also means to be in a state of being whole and undivided. When it comes to power on a printed circuit board, that definition is really true – honestly. The circuitry on a modern PCB needs its voltage and current to be maintained at its required levels within its power delivery network (PDN) with very little change. Power that fluctuates too much can disrupt the expected performance of the signals, and give false responses.

Problems like these can cause you a lot of effort in the debugging stage, as you trying to find and correct them. To avoid these headaches, it is important to create a printed circuit board design with a good PDN in it. But before you jump into the deep end of that pool, you should first start off with an understanding of what power integrity is. We’ll take a brief look here at the basics of power integrity to help you get started.

The basics of power integrity, Getting the Power to the Components

The different devices on a circuit board all need power delivered to them. Many years back the common integrated circuit (IC) device on a circuit board only had a single power and a single ground pin on it. Providing a power network for printed circuit boards back then was easy; you simply created one plane for power and one for ground in your board layer stackup. You would then connect the power and ground pins of the ICs to the appropriate planes, throw in some bypass caps, and presto! You were done. Today however, the power needs of the complex ICs in use on PCBs are much more complicated.

Circuit boards today will still have power and ground planes in them, but managing the power delivery network is much more complex than simply connecting an ICs power and ground pins to them. Each IC will need power supplied to it that is strictly controlled to comply with its power requirements. The goal of a good power delivery network in a circuit board is to provide stable voltage references, and to distribute power to all of the devices with acceptable noise and tolerance levels. This careful management of power to all of the devices on the board to keep a consistent voltage level is what is known as power integrity.

The careful design of a PCB’s power and ground planes is essential for good power integrity

The Challenges of Achieving Good Power Integrity

Back when the devices on a board were simpler in nature, and only had one power and one ground connection, the noise tolerances were not as tight. You could easily control excessive noise with an extra capacitor. The switching speeds in those devices also didn’t create the same problems as the high speed devices of today do.

With the processor chips today on large ball-grid array (BGA) packages, the power requirements are much more intense. There are hundreds of power and ground pins in these devices, and each device may require multiple supply voltages with some of those voltages going down to less than one volt. These pins are also pulling many amps of current for the processor to do its job, and to provide “clean” power requires a huge network of capacitors.

To understand how all of this works and how best to supply the power requirements of these complex devices, the study of power integrity has become very important. To create good power integrity on a circuit board means understanding how the power will propagate through the PDN of the board design. This includes everything from the power supply, to the routing and vias, the power and ground plane pairs, the capacitors, and of course the devices themselves.

Power plane configuration in a board layer stackup is also important for power integrity

Other Power and Ground Concerns

Power integrity is also tied in with some other concerns of the power delivery network. Some of these include:

• Ground bounce: Also known as simultaneous switching noise or SSN, ground bounce happens when a lot of signals switch at the same time. This can happen when a processor writes to memory, and the data signals all switch at the same time. If the signals don’t return all the way to their reference ground level due to the speed of the switching, they “bounce” above it. This ground bounce noise can create false switching and potentially disrupt or shut down the device.

• Power ripples: The switching characteristics of a power supply can cause power ripples. These ripples may create crosstalk in adjacent circuitry compromising the accuracy of the signals. Once again, this can cause disruption of the circuits.

• Electromagnetic interference (EMI): How the power planes in a board layer stackup are arranged can contribute to an EMI problem if not configured correctly. The power and ground planes can help prevent EMI from affecting the performance of signal layers by shielding them, but only if the signal layers are between the planes.

• Return paths: The planes of the board need to be designed in order to preserve the best signal return paths. Unfortunately the power requirements often leave a plethora of holes for connecting vias and stitching vias, and the multiple power supplies can dictate the need for split planes. All of these can put obstacles in the way of a good return path for the signals which will lead to poor signal integrity.

All of these concerns must be considered along with the analysis of the board for good power integrity. Many of the problems outlined above can be taken care of with strategically placed capacitors and resistors as well as carefully laid out power planes.

How Your PCB Design Tools Can Help

Creating a circuit board with good power integrity may seem like a daunting task, but there are a lot of resources available to help you. You can find a lot of information on power integrity and other power delivery network subjects online or in technical classes and seminars. Additionally, PCB design CAD systems today have a lot of analysis and simulation tools that can help as well. These tools will give you the ability to analyze the signal and power integrity of your PCB design, and even help you with an analysis of your ground plane signal return paths.

An example of the PCB design system that can help you with the power integrity design on your circuit board is available from Cadence. Allegro PCB Designer is the design system that you have been looking for that will support your physical multi-board assembly needs.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.