In a previous blog, Digital Power Supplies Are Getting Easier, where I discussed digital power supplies, space did not permit a discussion of a related topic, the PMBus (Power Management Bus). While not exactly a proper analog topic, it's worth taking a quick look at the PMBus, so please permit a brief foray into the digital realm.
The PMBus is a fairly simple, low-speed bus (around 400kHz, top speed) that is used to allow smart power supply subsystems (with either digital or analog control loops) to communicate with one another. Since analog design engineers work with power supplies, it seems to be a topic for which it is worth spending a few minutes.
According to the PMBus standards organization:
The Power Management Bus (PMBus) is an open standard power-management protocol with a fully defined command language that facilitates communication with power converters and other devices in a power system. The protocol is implemented over the industry-standard SMBus (System Management Bus) serial interface and enables programming, control, and real-time monitoring of compliant power conversion products.
Additional information from the standards site mentions that leading power supply and semiconductor companies were involved in creating the bus standard. You would therefore see the bus used both on an Eval board from a semiconductor manufacturer and from an OEM of power supplies (e.g., the “silver box” supply). For a list of PMBus adopters, click here.
You would be more likely to see the PMBus used on a digital power supply (i.e., containing a digital control loop) than an analog supply, but only because the digital supplies offer more control capability in general and because the designers are already working in the digital world. Thus, adding a communications link between supply subsystems is a logical extension of the existing design circuitry.
Here are a few details just to give an overview:
- Similar to I2C — low cost
- Electrically compatible with I2C, mostly
- But more robust than I2C
- More features than I2C
The screen-shot below of a slide from a presentation from System Management Interface Forum shows a bit of detail. The bus structure looks familiar:
As noted, the bus can zip along at 400kHz (maximum), so that is probably fast enough to monitor for over- or under-voltage conditions; and over-current conditions. You can also sequence power supplies and adjust output voltages as needed. Note that you can't do cycle-by-cycle current limiting of your SMPS, but that is not what the PMBus was designed for.
So, as an analog design engineer, that's mostly all you need to know. Have you designed with the PMBus? Let us know about your experiences below.