Effective Decoupling Radius


Reading time ( words)

Power distribution networks (PDN) are becoming an important topic. Many engineers are finding that properly designing the power supplies and providing adequate decoupling for devices is a challenge, especially since devices are switching faster and dimensions are shrinking. Engineers often focus on discrete decoupling capacitors placed local to switching devices in hopes of providing the required capacitance for these high current demands. One of the more overlooked items of the power distribution system is the PCB, and how it contributes to the power distribution system’s ability to decouple the switching devices. The following experiment will outline a basic principle that should be in mind when designing a stack-up and PDN.

Basic PDN Model

A basic PDN includes the voltage regulator model (VRM), the discrete decoupling capacitors, the PCB, and any on-die capacitance formed on the IC or device. Each one of these components could be written about separately, but it is the PCB that will be focused on; specifically the effective decoupling radius.[2]

When a device is active, it will require current. The type of device (process size), load on the I/O drivers, and how the device is operated, all have an effect on the current required, among others. When the device demands current, it flows through the complex impedance of the PDN and causes a ripple voltage to appear. This transient current is drawn from a variety of sources including the local on-die decoupling capacitance, the PCB, the discrete capacitors, and finally the VRM.[1] The edge rate of this switching current is extremely important when trying to calculate how effective the PDN will be in suppressing the ripple voltage. The switching edge can be dissected into a variety of harmonic sine waves at decreasing amplitude described by a Fourier series equation. It is here that we discover the importance of the PCB, and its role in the PDN.

The simplest way to represent a PCB is a distributed RLC network. Capacitance is formed by the copper layers and the dielectric between them. Inductance is formed by the loop area between the layers, and the resistance is formed by the cross sectional area and length of the copper planes.

To read this article from the April 2015 issue of The PCB Design Magazine, click here.

Share




Suggested Items

The Practical Side of Using EM Solvers

08/01/2022 | Heidi Barnes, Keysight Technologies
Electromagnetic (EM) solvers based on Maxwell’s equations have proven invaluable in the advancement of digital electronics and wireline communications. Plain and simple, electrical engineers need to know what a circuit or electrical interconnect will do when excited by a dynamic or varying signal. In the signal integrity world, an interconnect that passes a DC connectivity check can completely fail at higher frequencies. In the power integrity world, a power rail that measures the correct DC voltage could easily go into oscillation when a dynamic load is applied. Learning the basic skills to fire up an EM simulator, obtain qualitative answers in minutes, and higher fidelity answers in a few days, can be the difference between sleepless nights of product failures vs. robust designs with wide design margins.

AltiumLive 2022: Power and Signal Integrity—Return to Sender

02/23/2022 | Andy Shaughnessy, Design007 Magazine
I recently spoke with Heidi Barnes and Stephen Slater, both engineers with Keysight Technologies, about their presentations at this year’s virtual AltiumLive. They discussed ways to avoid signal and power integrity challenges later by following simple board design practices early on, how SI and PI are interconnected, and why the return path must be more than an afterthought in high-speed designs.

A High-Voltage PCB Design Primer

01/12/2022 | Zachariah Peterson, NWES
Of all the different boards a designer can create, a high voltage PCB design can be complicated and requires strict attention to safety. If not laid out correctly these boards can be safety hazards or can fail to function on first power up, leaving a designer with wasted time and effort. In the best case, the board will function reliably for a long period of time thanks to correct layout practices. High-voltage PCB design can be as complex as any high-speed digital design. Boards for high-voltage systems can be space constrained and they carry important safety requirements. They also need to be highly reliable to ensure they will have a long life when run at high voltage and current.



Copyright © 2022 I-Connect007. All rights reserved.