PXI Chassis Power Supply Considerations

Like all modular standards the PXI Standard specifies the use of a centralized powers supply. The specification requires a power supply with +12V, +5V, +3.3V and -12V supplies (PXIe slots do not have -12V and +5V supplies, they have 12V and 3.3V supplies). The standard does not guarantee a PXI module can be supported without restriction in a particular chassis, as we shall see in this article.

The standard does not specifically define noise or ripple performance, so it is largely up to module vendors to check for compatibility of their modules with typical PXI backplane power supplies. The dynamic behavior of the power supply under changing load conditions is also not controlled, so module vendors need to make sure that modules with high dynamic loads do not cause unwanted behavior in a typical chassis. In practice this lack of a definitive specification has not caused significant problems in module development, and a lightweight approach to this eases the testing requirement (and therefore the PXI design costs) for module vendors compared to standards such as VXI which are more closely controlled.

A more significant problem is sometimes encountered concerning power supply ratings. Each version of the PXI standard specifies a minimum power supply capacity, early versions of the standard (which vendors can still construct to) have generally lower requirements for minimum power supply capacity. The current version of the hardware standard for PXI (which is different to PXIe) is as follows:



System Slot
Peripheral Slot

System Slot
Peripheral Slot

All slots

All slots
Power supply

+5V
+5V

+3.3V
+3.3V

+12V

-12V
Current

6A
2A

6A
2A

0.5A

0.25A

So at 5V and 3.3V the PXI power supply should be capable of supplying 6A to the system slot and 2A to EVERY peripheral slot at the same time, plus 0.5A to every slot (including system slot) on +12V and 0.25A to every slot on -12V. For a chassis with a given number of slots a minimum specification for the power supply to be compliant to the standard can be calculated.

The peripheral modules are permitted to take 6A on the +5V and the +3.3V supplies and 1A on the +12V and -12V supplies. If a module with high current demand on particular rail was used in many locations in the chassis it can clearly overload the chassis if it only meets minimum requirements. The module current in each case is limited by the backplane connector pin current rating, which is 1A. In the case of the +12V and -12V supplies just one pin provides power to the module. In the case of the +5V and +3.3V supplies multiple pins provide power to the module, allowing a higher current draw.

The PXI standard does NOT try to specify that a chassis should support applications where all the slots are filled with modules taking the maximum permitted current on a supply - such a situation is an unnecessary and expensive burden that would not be in the interest of users.

Applying to switching systems

In most switching applications (but not all) relays are powered from the +5V supply, so often this has the biggest load on the backplane supplies. The more relays that are closed the higher the load on the backplane.

Some types of switches are operated from the +12V supply, typically in cases where automotive or microwave relays are used.
In general if the user is concerned about power supply loading the 5V and +12V supplies are therefore usually the ones to be the most concerned about.

How to check

Pickering Interfaces clearly publishes the power supply loading numbers for every PXI module on its data sheet under the highest loading conditions (maximum number of closures permitted by the module). We use the most efficient relays we can to minimize the loading and make more of the supply available to other modules.

For our PXI chassis we make sure they have power supplies capable of more than the minimum requirements and again publish this information in the data sheet posted on our web site.

This information makes it easy for users to check if they are likely to run into power supply loading issues, not all vendors provide this information in a clear and simple way.

In most circumstances PXI systems do not run into power supply loading issues because:

  • the modules are not used in their maximum load conditions

  • there is usually a variety of different modules in the chassis some of which load the supplies more heavily than others

  • there may be empty slots

  • some modules load different supplies more heavily

  • the chassis often supports more than the minimum power supply requirements.

Excessive loading of a chassis is much more likely in systems where power is drawn from the chassis, for example to provide power supplies to the test system.

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