PMBus Revision Expands Spec To AC-DC Power Supplies

This week at the Applied Power Electronics Conference (APEC) in Anaheim, members of the Power Management Bus (PMBus) Implementers Forum introduced version 1.1 of the PMBus protocol, which introduces new commands that enable control of ac-dc power supplies. These PMBus representatives also revealed that a working group within the consortium is developing lists of recommended PMBus commands for specific applications. In doing so, they set the stage for more formal definitions of PMBus compliance and pave the way for PMBus certification on power conversion products.

The PMBus 1.1 spec simplifies the design and implementation of the PMBus protocol into servers, storage systems, routers and other systems. While the first version of the specification targeted point-of-load, dc-dc converters with its command set, this second version includes generalized commands that are applicable to ac-dc power supplies.

Enhanced fan and cooling management features plus improved fault management capabilities allow designers to have better control of the power system. The PMBus v1.1 specification’s query and capability command features reduce demands placed on the host-system, and new clarifications make the standard easier to understand and implement (see the table). In addition to new commands, the revised spec supports an optional 400-kHz clock using the I²C fast-mode ac specification.

“The latest version of the PMBus specification extends the 1.0 spec, enabling OEMs to take advantage of the system power management capabilities provided by this open communications standard. These changes are a direct result of continued collaboration among the PMBus adopters and industry stakeholders,” said Dave Heacock, director of the PMBus-Implementers Forum (PMBus-IF).

Version 1.1 of the PMBus spec is compatible with version 1.0 except for the implementation of direct mode translation, one of two formats used for reporting data. Literal mode, the other data reporting format, retains compatibility between the two versions of the spec.

One of several working groups within the PMBus Implementers Forum, a working group focused on PMBus Command Profiling is developing sets of recommended PMBus commands that PMBus implementers will be encouraged to adopt in particular application areas. For example, this working group is building consensus on a recommended command set of approximately 20 to 30 commands needed in servers.

By supporting those recommended commands, suppliers of power converters will be able to claim PMBus compliance for server applications. That compliance will be certified by a third-party testing laboratory to be designated by the PMBus Implementers Forum.

That approach to compliance differs markedly from the existing criteria for general PMBus compliance. Currently, suppliers of power conversion products can claim PMBus compliance if they meet the electrical specifications for timing and implement at least one, nonmanufacturing-related command. That’s just one command out of the 100+ commands in the specification.

For more information, see www.pmbus.org. This website was recently redesigned to enable easy access to technical specifications and the latest information on all PMBus-based solutions.

Table. New Commands in PMBus Version 1.1