Welcome to the first installment of the Speeding Edge Blog—No Place Like Ohm.
Our goal with this blog is to present technical content that will be helpful in the current-generation PCB/System design process. And, we will have different authors who will be doing blog postings on No Place Like Ohm. These authors are our various technical partners each of whom brings insight into a specific aspect of the overall PCB/System design process.
The first topic we want to address is one that has taken the industry a bit by surprise. For the last several years, the PCB design struggle centered on solving a difficult routing job. Now, due to various changes in technology, the struggle centers on solving a difficult power delivery system (PDS) design problem.
A little bit of a history lesson will help to illuminate this shift in technology focus. In the past, successfully designing a PCB was dominated by having enough wire space for all the nets. DDR and PCI address buses are examples of the very wide parallel buses that consumed that wire space. And, as an example, the original PCI bus consisted of 137 different nets or wires all of which required careful layout and routing.
Technology has now evolved to the point where the routing of wide parallel buses is no longer a major concern. The wide parallel buses in the original PCI specification have been replaced by differential signaling. Thus, PCI Express, the successor to PCI, only has two wires. Further, all of the parallel bus architectures, with the exception of DDR 3, have been converted to differential signaling. Layout and routing tasks have become much easier. This is the good news.
In the foregoing designs, PDS design was a fairly straight-forward process because there were just a few power supply voltages that needed to be taken into account. By contrast, today’s designs are characterized by a high number of complex parts hooked together with differential signals. It is not uncommon for these complex parts to require four or five different voltages with many of them running at high current.
To delve into this a little bit further, in 2007 Speeding Edge provided technical consulting services for the design of a 48-port router in a pizza box chassis. We only needed to address five different voltages for this design. In 2013, we provided similar consulting services for the design of another pizza box router with 16 40-gig ports and 29 different supply voltages. The stackup for this PCB was dominated by the power supply planes with only a modest amount of time and energy required to address the signal layers issues.
PDS being the dominant design problem will continue to be the bane of current and next-generation products. The quicker that we as an industry can identify and address all of the PDS issues, the easier it will become for all of us to create designs that work “right the first time.”
We hope you find our blog postings topically informative and relevant to current design issues. We welcome your input and questions which we will address in an on-going basis in No Place Like Ohm.
Lee Ritchey, President