What We Have Here is a Failure to Communicate - CUI Discusses Digital Power

altFor years now we have all read that digital power is the wave of the future for system power design, with report after report forecasting impressive compounded growth rates in this space. I believe, along with many of my colleagues, that we are beginning to see an inflection point for digital power designs, with growth quickly coming as design cycles reach completion. However, this is not another editorial on digital power taking over the world. Instead, my intention is to share recent revelations obtained in the development process for CUI's digital POL products that will hopefully allow designers to better understand some important considerations in implementing a fully digital power system.

Interoperable is defined by IEEE as the ability to exchange and make use of exchanged information between 2 or more components or systems. In 2004, this was a key area of focus for the founding members of the Power Management Bus (PMBus) group, the standard by which the majority of digital power companies and OEMs adopt. PMBus was developed to insure that devices within a power structure could communicate regardless of the manufacturer or type of power device. PMBus is broken down into two areas, the physical layer and the command layer. PMBus commands are communicated over the widely used SMBus/I2C per the specification (www.pmbus.org). The command language is currently on rev 2.0 and has a working group of more than 40 organizations that represent power supply companies, semiconductor companies, and OEMs actually implementing the digital power system.

This seems great in theory— an open standard, clearly defined, and widely supported by all. Unfortunately, it isn't as straightforward as that. PMBus does not ensure interoperability. PMBus does a great job of defining the structure for a majority of the functions from a system level, but it does not define other critical features.

The semiconductor companies designing digital power controllers have begun to implement a separate proprietary bi-directional serial bus to handle functions like active current sharing, fault management, and in some instances, even PMBus commands that also reside on the SMBus, such as sequencing. The primary purpose of this serial bus is to reduce the traffic on the already congested SMBus, as well as provide a higher speed bus to support some of these critical functions. It is something that many designers view simply as an added feature, but this serial bus has transformed into a critical portion of the latest digital controllers on the market. As a result, the ability to have a truly interoperable system at the POL level has been eliminated.

In July 2011, CUI announced a cooperation with Ericsson regarding the second sourcing for POL products. In order to achieve true second source modules, we needed to thoroughly examine the required PMBus commands, the physical layer that supported each of the commands, and physical and command structures of the additional module features.

This seemingly simple task began to take on a life of its own as we broke down the datasheets between the Ericsson POL modules and our existing POL products. Each product is based on a different digital PWM controller. Through our analysis we began to realize, to our surprise, that we had an interoperability issue. The digital controllers that we each used supported the same PMBus commands, but the physical communication of those commands would not support a second source. What we have here is a failure to communicate. We had been so focused on our development of modules that would hit the smarter, faster, smaller targets, searching for the required controller that would assist in those targets, that we didn't focus on our ability to work with others within a power management system. We assumed that PMBus would cover our assumptions— shame on us.

In order to achieve a true interoperable digital system that includes fault management, active current sharing, and other power management functions, designers will need to pick a company/controller family of choice. The existence of the proprietary serial bus will prevent designers from using more than one company's controller. At first glance, this doesn't appear to be a big deal for companies. A designer will more than likely select a controller company of choice and utilize that controller company for their discrete designs, similar to their usage of MCUs or FPGAs. The issue arises when designers want to have a mix of higher power POL modules and down/discrete designs, or design 100% of their power rails utilizing modules. These companies will need to align their designs to ensure that the module company selected uses the same controller as the discrete portion of the design, or, in the case of a power system exclusively utilizing 100% module power system, that the power module companies use the same controller.

Because a majority of companies utilize a mixed solution of discrete designs and modules in their systems, CUI is moving forward with a mixed controller solution platform. While other power supply companies typically offer a portfolio based on only one controller, CUI has developed a roadmap that will provide customers with the ability to pick a controller family that best fits their needs. Today CUI offers 2 different controllers in our Novum® Advanced Power POL product family: Intersil/Zilker Labs and Powervation. Going forward it is our intent to become a controller-agnostic company. As a power supply manufacturer, it is our job to provide our customers with the best digital controller technology available in our modules, allowing them to make the decision that fits their specific needs.

Digital power provides a number of benefits to optimize complex systems or provide integration of simple management tasks, but we are finding that communication has become a critical barrier to taking full advantage of this technology. A design engineer needs to consider the future usage when choosing a digital controller platform. At CUI, our approach is to create multiple platforms that break down this barrier and offer designers true interoperability regardless of their controller choice within their digital power system.

Mark Adams will be speaking in greater detail about this topic at the Applied Power Electronics Conference (APEC) held on February 5-9, 2012 in Orlando, Florida. His seminar, titled "Overcoming Interoperability Challenges in Digital Power Systems," will take place Tuesday, February 7, 3:00-3:30 pm in Coronado room E/F.



Download article written by By Mark Adams, VP Advanced Power, CUI Inc. as published in Bodos Power Systems, February 2012.