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What is Power Integrity? Understanding Power Distribution Networks (PDN)

Updated: Apr 9

A lot of folks often ask the question, "What is power integrity?"


When I think of power integrity, there are a couple of definitions that come to mind. My favorite definition is "power integrity is all about the quality of power (& noise) seen by the circuits on the ASIC die [1]."

-Larry Smith and Dr. Eric Bogatin - Principals of Power Integrity for PDN Design.


My other favorite definition is "power integrity is the science of getting power to the devices that it likes. Our job in power integrity is to ensure we have optimized the power for the devices that we are powering so that they can work at the peak performance."

-Steve Sandler, Picotest.com


What both of these definitions tell us is that in order to be successful with power integrity design, the entire system must be accounted for in the simulation model. That means including the load (which could be an ASIC, FPGA, or even a microcontroller), the package, the printed circuit board (PCB), and the power supply model. When we talk about the PCB and the package, this means providing electromagnetic (EM) extracted models of those respective power planes. A lot of engineers who do power electronics design often think their job is complete after designing the power supply. The opposite cannot be more true. When designing the power supply, what matters is how the power supply performs when integrated as part of the system, as part of the power distribution network (PDN), which includes the PCB, package, and load. Many times, I've seen a power supply that was supply but when the PCB effects were added to the design as part of the simulation, the power supply become unstable and created more noise at the load.


So, including all aspects of the system that make up the PDN will ultimately be the best path to a successful power integrity model.


As a colloquial to one of Dr. Eric Bogatin's quotes on signal integrity engineers, we can instead say there are two types of power integrity engineers, those who have power integrity problems and those who will.


To wrap up this discussion, we can finish our thoughts on power integrity with two final quotes.


"Power delivery is AC, not DC," - Heidi Barnes, Keysight Technologies


"Noise follows impedance, always!" - Ben Dannan, Signal Edge Solutions


These last two quotes emphasize the true AC nature of power integrity. As shown in Figure 1 - Impedance is not just resistance; it is made of combinations of resistance, inductance, and capacitance. This implies that we have to shift our mindset in terms of how we look at power delivery. As an example, with reference to Figure 2, the time domain voltage ripple shown is improved by merely improving the impedance in the PDN. So, with that being said, to achieve quiet power, power delivery and impedance must go hand in hand.


Depiction of Impedance vs. Frequency

Figure 1 - Depiction of Impedance vs. Frequency


Figure 2 - Full PDN Voltage Ripple [2]


Two recommended books on power integrity that every engineer should own are:


These books can be found in the Signal Edge Solutions company store.


If you want to learn more about power integrity topics or are looking for power integrity training to support your team. We can be reached at info@signaledgesolutions.com


References:

1.Smith, L. D., & Bogatin, E. (2017). Principles of Power Integrity for PDN design -- simplified: Robust and cost effective design for High Speed Digital Products. Prentice Hall.


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