IEEE Power Electronics Magazine - September 2014 - 33

PMBus:
A Decade of Growth
An open-standards success
by Robert V. White

en years ago, in September 2004, at the
Darnell Power Forum
in San Jose, California,
I made the presentation that introduced the PMBus power management protocol. This was an important moment for the commercial
power supply industry. The PMBus protocol was the first
time the commercial power supply industry came together
to cooperate and create an open standard. The success of
the PMBus protocol laid the groundwork for later open
standard efforts, such as the Point-of-Load Alliance (POLA)
and the Distributed-power Open Standards Alliance
(DOSA). The fact that the PMBus standard has not only succeeded but also continued to grow and evolve is the result
of the great foundation laid by the first PMBus specification
working group.

Short History of Digital
Power Management
Digital power management is the use of digital signaling
to control, configure, and monitor a power supply. Control actions include turning the output on and off or
changing the value of the output voltage. Configuring a
power supply is generally done at the time of manufacturing, installation in the end equipment, or at power on.
Configuring a power supply includes actions such as setting the overcurrent protection threshold, the switching
frequency, or the characteristics of a digital compensator in the feedback loop. Monitoring signals can be binary status signals that indicate whether the power supply
is operating normally. Monitoring can also refer to sigDigital Object Identifier 10.1109/MPEL.2014.2330492
Date of publication: 5 September 2014

nals that report parametric
information, such as the output
voltage or the temperature of a
critical component.
It is important to note that
digital power management does not refer to real-time cycleby-cycle control of the switching action-this is usually referred to as digital control of a power supply.
Digital signals have been used in commercial power supplies for decades. The most common signals were, and are,
an output on/off control signal, a signal indicating that the
power supply is operating normally and that the output voltage is correct (typically called DCOK ), and signal indicating that the input power is present (typically called ACOK ).
These signals are typically individual signal lines that form
a simple parallel bus.
Simple digital signals work well when the system is small
or self-contained, such as in a computer system. When the
system is large with many power supplies, such as in a telephone central office battery plant, a serial bus works better.
Serial buses, such as those based on the RS-485 standard,
provide the ability to transfer more commands and information than individual digital signals. For example, commands
to precisely program the output voltage can be sent easily
over a serial bus in any number of bits. In comparison, using
ten signal lines to send voltage programming information to
a power supply is simply not economical.
Serial buses were being used in telephone central offices to communicate between a battery plant controller and
multiple rectifiers by about 1980. By the mid-1980s, the computer industry was using serial bus communication within
large mainframe computer systems. By 1990, even smaller
computer systems were using the I²C bus for digital power
management. By 2000, the use of serial bus communication
was typical even in smaller server power supplies.

September 2014

 IEEE POWER ELECTRONICS MAGAZINE

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - September 2014