Telemetry in power systems has been around for quite a while, however it usually just showed up in high power environments like server farms. Power system telemetry is the communication between your system processing and the power supply devices. This communication can be used to control and / or monitor details about your power system. PMBus, Power Management Bus is an open standard for digital communication between power devices. The standard was created and is maintained by the System Management Interface Forum, LLC. For more details on the standard itself you can visit www.pmbus.org.
For the purposes of this blog, I’m going to talk about how it caught my eye for some of the Avnet designed products as well as provide links to a couple of videos that show the communication features in action.
Why PMBus?
As I mentioned, telemetry in power systems isn’t a new concept. What has changed, especially recently, is that adding the capability is no longer as cost prohibitive as it once was for lower cost systems. There is also growth in lower power level converters featuring this interface. Infineon for example has 2 devices that I’ve recently used, the first is the IRPS38060 which is a single channel 6A converter that was used on our Xilinx Kintex UltraScale KU040 development board.
Avnet KU040 Xilinx Kintex UltraScale board featuring Infineon IRPS38060
Our latest SOM (system on module) is the UltraZed, based on the Xilinx Zynq UltraScale+ family. This design features the IRPS5401 which is a 5 channel PMIC featuring 4 switching outputs and an LDO.
The most appealing part besides the technology is that you gain this functionality with a nominal cost adder. In the past when these features meant a serious cost premium it didn’t make as much sense for the masses. That roadblock has been seriously reduced with newer devices on the market. Why would you want this capability you may be asking? There are several benefits that may appeal to you.
Active margining
Active margining is typically more beneficial in high current systems. There are cases where margining (reducing the voltage by some small percentage, generally 5% or less) could save considerable power in your system. Margining can be made simple by sending commands to the regulators to lower or raise the voltage in question.
Real time monitoring
The most common reason a device fails from a hardware perspective is generally that the power system is compromised somehow. Whether it’s due to temperature, component aging or some other kind of degradation, a capacitor, magnetic component or a switching component are typically the weakest link. Maybe not in all cases, but in many cases you may be able to predict failures by monitoring the load profile of your system. For example in a motor control system, if you see the current demand increasing on a supply that is driving a motor maybe that is a flag for a technician to go and replace that board or motor before a failure occurs. Real time monitoring can be very valuable for high up time (always or nearly always powered) systems. Along with voltage and current measurements you can also monitor temperature. As the load and / or temperature increases, maybe you use that information to throttle back performance to lower the power demand or shut down the system before thermal overload kicks in.
Sequencing and Power Control
You may have entire sections of your equipment that do not need to be powered unless in use. Consider an IoT application such as a vending machine that sends inventory data. Does the cellular radio need to always be on for that? Probably not, maybe it only wakes up every so often to send data. This type of functionality would be useful in both low power battery applications where you try and squeeze as much run time out as possible and in high power systems where shutting down power to a motor bank could create dramatic savings in electrical power. You can also easily control startup and shutdown sequencing as well as slew rate control.
Want to know more?
I certainly didn’t cover all the potential benefits but what I've discussed should give you a taste of what is possible. Check out these videos that I created highlighting the functionality of the PMBus on two of Avnet’s products, the Xilinx Kintex UltraScale development board and the UltraZed SOM (system on module). These should give you a sense of how you can interact with the bus and the type of information and control you gain. PMBus compliance doesn’t require every device to support every command, but standardizes the commands and the hardware interface.
