The PMBus standard for digital power management is the result of collaboration between power supply and semiconductor companies. There are more than 50 members in the PMBus organization. In terms of hardware, PMBus “devices” may be ICs, power converters, or power supplies. No device is required to support all of the available features, functions, and commands, but all must meet certain requirements. Part I of the standard addresses transport, the electrical interface, and timing for hardwired signals. Part II describes the command language.
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What It’s Not
The PMBus standard does not address direct device-to-device communication such as analog current sharing, real-time analog or digital voltage tracking, or switching-frequency clock signals. It is not a standard for ac-dc power supplies, nor does it specify attributes such as form factor or pin-out, which are addressed by industry alliances such as the Point-Of-Load Alliance (POLA) and the Distributed-power Open Standards Alliance (DOSA). And, it doesn’t deal with communication between one power source and another. This is an important point. That kind of communication remains the domain of semiconductor and power supply manufacturers.
PMBus Roots
PMBus was released in 2003. By that time, the power supply community was adapting to new customer needs, particularly in the data center. While chipmakers continued to work on improving efficiency, they also had to respond to system designers who had embraced distributed power architectures (DPAs). DPAs are based on multi-level power distribution, where it isolated dc-dc converters, followed by loosely regulated intermediate bus converters (IBCs) and non-isolated point-of-load (POL) converters.
DPA components were developed because the latest complex microprocessors and ASICs had multiple power rails for core, memory, and I/O. Each of these rails had different voltage requirements (often down to 1 V) and, for cores in particular, extremely fast di/dt swings. There were many power management issues as well. As a minimum, on power-up and power-down, a systemic approach was needed to sequence those devices to avoid back-biasing transistors on the die.
In addition to application in the final product, this kind of control was deeded in design and manufacturing where the use of voltage margining to test design robustness had increased. Meanwhile, the monitoring of converter output currents and temperatures became common practice to improve system reliability.
As needs grew, different power-IC designers responded in unique ways. Sequencing might be accomplished with “output enable signals,” margining by switching trim-resistor values in and out. Elsewhere in the design cycle and in operation, currents and temperatures were sensed and digitized so FPGAs or microcontrollers could be programmed to manage all those processes.
The result was a kind of Tower of Babel. In 2004, Artesyn Technologies (now part of Emerson Electric) proposed the PMBus initiative. Early the following year, it was formalized as a special interest group (SIG) and opened to global membership. The protocol is now in the public domain, and the SIG, known as the System Management Interface Forum, is responsible for further developing and promoting the standard.
Artesyn framed the need for an industry standard at that time by noting that such an approach would fill a need for broad consensus on digital power management. At that point, several power supply manufacturers including Power-One, Zilker Labs (now part of Maxim Integrated), Primarion (now part of Infineon), and Silicon Labs had already announced digitally programmable point-of-load (POL) converters based on their own proprietary architectures and silicon—though just as PMBus was gaining traction, this situation would lead to a crisis that would bring PMBus implementations to a standstill for several years
(see “Power-One’s Z-Bus And Patents.”)
PMBus Basics
The PMBus protocol defines an open-standard digital power management protocol that facilitates communication with a power converter or other device by fully defining the transport and physical interface, as well as the command language needed to accomplish these definitions.
Fundamentally, PMBus is about low-cost real-time control. Its transport layer is based on the SMBus (System Management Bus), a version of Philips’ I2C serial bus that adds packet-error checking and host notification. Importantly for PMBus, SMBus also provides a third signal line, SMBALERT, that allows slave devices such as POL converters to interrupt the system host/bus master. This is where PMBus has an advantage over polling systems.
Under PMBus, slave devices must store their default configuration data in nonvolatile memory or use pin programming. This means they power up without any bus communication, minimizing system startup times compared to schemes in which the bus master configures all slave devices on power-up. Slave physical addresses are defined by means of dedicated pins on the device package.
In addition to the SMBus clock, data, and interrupt lines, PMBus specifies two hardwired signals: a control signal used in conjunction with commands received over the bus to turn individual slave devices on and off, and an optional “write protect” signal that can be used to prevent changes to memory (Fig. 1).