PMBus – what is the value anyway?


I will never forget the first time I demo’d the PMBus. About eight years ago, I was visiting with a power-supply design engineer and giving a PMBus buck-controller demo. With a stroke of a PC key, I could change the power supply soft start, switching frequency or output voltage. This left the designer literally with his mouth hanging open, because typically, changing these design parameters would have required a trip to the lab and time soldering new resistor/capacitor values, then measuring the performance.

While it was amazing then, PMBus is even more popular now. More and more companies in wired and wireless communication, enterprise server and storage, and even industrial segments use PMBus power supplies. It has become so popular because of its:

  • Ease of design.
  • Reduction in design time.
  • Ability to monitor power supplies to screen out bad boards.
  • Ability to optimize power stages when using new application-specific integrated circuit (ASICs) (measuring the real current drawn and not having to over-design the output inductor and output caps).
  • Ability to collect data for use in routines to perhaps improve data-center efficiency and power utilization.

There are even more benefits:

  • PMBus makes it easy to create a new power-supply design without external analog components, so you don’t have to breathe in smoke from the soldering iron while you try new resistor and capacitor values on the bench.
  • PMBus enables programming, sequencing, configuration, control, voltage margining, output-voltage adjustment, and parameter and fault monitoring through a graphical user interface (GUI), which allows for quick redesigns and a more intelligent response to the parameters and faults is observed.
  • PMBus ICs can eliminate external hardware monitors, supervisors and temperature sensors, as well as discrete logic for delays.

Figure 1 – Turn On/Off PMBus Programming (multiple of the Soft Start time)

  • PMBus power-supply ICs include nonvolatile memory (NVM), which stores the new design values in the IC. These values then become the new “default” values once the power supply is powered down and powered back up. So in a matter of minutes, you have a new power-supply design to get you to market faster.
  • PMBus ICs eliminate external-voltage margining circuits, especially if there are multiple voltage-margining levels.

Figure 2 – Voltage Margin High and Low through PMBus

  • The telemetry of the power-supply output voltage and current can be used for intelligent-system power management such as in cloud applications where many servers, storage boxes, base stations and switches are part of the operating environment. You can use this information to improve the power-usage efficiency (PUE), which is a measure of how efficiently the computing equipment in a data center uses energy. There’s also thermal telemetry as well as temperature information about the hottest parts of the board, which can predict faults before they happen.
  • PMBus telemetry can also help optimize your design by providing you with the right power level of new ASICs or field programmable gate arrays (FPGAs) so that you can choose the best output inductor, capacitors and power-stage components.

Figure 3 – Output Voltage, Current, and Temperature Monitoring through PMBus

If you are interested in finding out more about how to program and monitor a PMBus power converter, visit the TI booth (No. 1001) at the Applied Power Electronics Conference (APEC) today. We will be demonstrating a unique design that minimizes the X-Y area on the board by placing the inductor on top of the IC, with a gap in between.

Missing APEC this year? Then check out this video, “How to create a PMBus inductor-on-top buck converter design” and take a look at our new PMBus Power Solutions Guide. Also, read “Synchronous Buck Converter Delivers 12 A With Fast Load Step Response” in Power Electronics magazine for more information about TI’s new TPS53915 synchronous-buck converter and “PMBus Gives Designers New Options for Meeting Adaptive Voltage Scaling Requirements” in How2Power.