PXI Power Limitations for Switch Modules

    Information on the power issues on this page apply to PXI modules when fitted into any vendors PXI chassis or Pickering Interfaces LXI Modular Chassis (60-102/3/4).

    The PXI Standard requires a chassis to be able support 25W of power dissipation per slot through the use of forced air cooling, the air being required to enter the module from the bottom and then to be extracted from the top. The PXI specification also requires a PXI chassis to be able to support 6A to any one peripheral slot on the +5V supply - the most commonly used power supply for switch operation.

    For high current and some high density applications this power limitation can affect the maximum number of relays that can be closed or the maximum permitted power loss in the module.

    The thermal limit of 25W per slot is an average, it can be exceeded in time varying conditions but it should not be exceeded for times greater than the thermal time constant of the module (usually measured to be between 2 minutes and 30 minutes depending on construction). The power supply limit can be an average, but there is a much higher risk that the power supply will shutdown if an excessive number of slots are taking high currents. The PXI standard requires that the chassis is capable of supplying greater than 2A average to the peripheral slots on the +5V supply.

    The PXI specification requires the vendor to supply information on the power supply current for each of the PXI supplies, Pickering Interfaces provides this information for all its PXI modules on the data sheet. The chassis data sheet will also provide information on the capability of the power supply.

    Pickering Interfaces also provides guidelines for the limitation on modules that might exceed the thermal limits imposed by the standard under exceptional circumstances in the operating manuals, for example for 40-139. In most cases users do not use switching systems in ways that thermal limitations apply.

    This page is provided to help give some guidelines on these thermal management and power supply issues.

    Power Consumption of PXI Switching Module

    The power dissipation in a PXI switching module has three principle thermal sources:

    • Static power load. This load is made up of the general infrastructure of the module. It includes the PCI interface and the drivers that operate the relays. The PCI interface load is generally low, but for large switching systems (such as those deployed in BRIC's and other modules) the relay drivers can consume significant amounts of current. The PCI interface and the relay drivers can draw current either off the 5V supply, the 3.3V supply or both.

    • Relay coil current. Each relay that is energized requires coil current to operate it, as the number relays closed increases the power they dissipate and the current load in the module increases. Reed relays tend to be low power, typically around 10mA per coil, while EMR's vary greatly. Often it is EMR based modules that cause the greatest difficulty on power loss and power supply draw.

    • Switch path loss. This is often neglected by users, but can be the most significant source of heat in a module. For example, an array of SPST switches rated at 2A and having a path resistance of 0.15 ohms dissipates 600mW in each path when fully loaded. The heating will then tend to increase the copper resistance in the module (by 0.039% per C), further increasing the power dissipation. A 160 pin connector can support 80 off SPST switches in a single module, so having all these paths carrying 2A is not an advisable operating condition. Fortunately it is also a very unlikely operating condition.

    In most cases the static load of the module can be considered to be constant and relatively low to simplify any calculation of the likely thermal loss - though this is not always the case in systems like large matrices where there are many more open relays than closed relays.
    The relay coil load can be calculated from the manufacturers data on coil power.

    Signal power loss can be calculated from the typical path resistance and the current flowing in each path. As this can vary across different paths some simplification may be needed, in general only paths carrying more than about 33% of the rated current contribute significantly (the path loss is dependent on the square of the current so a path carrying 33% of the current is generating about one tenth of the power it does at full load).

    A simplified spread sheet can be found here - PXI Power Calculation

    Provides a method of assessing the power dissipation of a module. By entering the static power, the coil power and the path characteristics a a calculation can be performed that plots the thermal load in a table and on a graph. A green box indicates power dissipation of less than 25W.

    12V Relays

    Some products use relays which use 12V coils, typically those using automotive relays or microwave relays. The power supply capacity of of a chassis on the +12V supply is more limited than the +5V supply so 12V relays can be an issue on power supply current. Microwave relays are generally large and occupy multiple slots so although their current draw may be high users are unlikely to encounter power supply limitations. The same situation exists for Automotive relays, their density is not high enough to cause excessive power supply loading.

    Variable Speed Fans

    Some chassis are fitted with variable speed fans, the fans can be set to be automatically controlled. This is particularly attractive in applications where fan noise may be of concern, for example in an office environment.

    For applications when chassis thermal load is always low the use of the automatic mode results in the fans running at slow speed. However the automatic mode can cause some problems when a chassis has a highly variable thermal load or if the load varies considerably between modules. Here are two cases where the automatic setting can cause issues:

    • When the chassis is being used one or two modules with high thermal loads might not create enough temperature rise to force the fans to a high speed setting, so those high thermal load modules will only be being cooled by a low air flow and may get hotter than their design intended.

    • If a chassis has a time variable thermal load that can be very large then as the thermal load suddenly increases the modules may heat up much quicker than the chassis temperature, in some cases overheating before the chassis responds by increasing the fan speed.

    For chassis which are expected to support modules with high thermal loads it is sometimes advisable to set the fan speed manually rather than rely on the automatic mode.

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