Taming power in PXI
Richard A. Quinnell, Contributing Technical Editor -- Test & Measurement World, 5/1/2007
Today's communications systems are placing an increasing demand on test-system performance, requiring more extensive use of digital signal processing and faster logic-clock speeds. At the same time, a need for portability for wireless system test is increasing interest in battery-powered PXI. While one trend is pushing power demands higher and the other trend requires lowering power, they both spell an increased need for careful power management by PXI system designers.
Whether the goal is to maximize the power available or to minimize it, you should start by developing a power budget. The PXI specification states the minimum currents that a chassis must provide to a board location (table), and recommends a maximum total power of 60 W for a 6U board and 30 W for a 3U board. Within those ranges, developers are relatively free to allocate power as they wish.
One limitation does arise in the specification for a low-power PXI chassis. Developed to provide a lower-cost alternative that addresses smaller installations with limited power needs, the low-power PXI chassis establishes a maximum total current that the system must provide.
While a low-power chassis must still be able to supply at least 2 A to each slot at 5 V, for instance, it only needs to provide a total of 10 A to the system. A low-power chassis must bear special marking to identify it so developers do not try to over-draw power.
Even with the full-power chassis, there are limitations that must be considered. According to David Manor, VP of engineering at Geotest—Marvin Test Systems, the power pins used in the board design need consideration. The specification allows a maximum of 1 A per pin, so power to a board can be limited by the number of pins that are connected to the rails. Manor pointed out that the 5-V rail has 11 pins available, for a total of 11 A to a board that needs high power. He also noted that both the per-slot and total chassis power budgets need to be evaluated together.
The supply must guarantee enough current to meet the needs of all boards in the system. As a conservative estimate, use 125% of maximum total board demand when determining the power supply requirements to allow for possible upgrades to the system and for degradation in the power supply over time.
Don't forget power-supply de-ratingJust as aging can affect the availability of current from a supply, so can temperature. Spencer Stock, PXI product manager at National Instruments, said that developers should look at how the chassis power supply is to be de-rated with temperature and compare that to their overall system needs. A power supply that meets requirements at 25°C may not provide adequate current at 50°C. Stock also cautioned that developers should look at both the de-rating and the maximum temperature at which power levels are guaranteed when evaluating the power supply for their chassis.
Another point for evaluation, said Stock, is access to the supply. Many early PXI chassis designs had the power supply directly wired to the backplane. This complicates removal and replacement of the supply in the event of failure. Now, Stock noted, chassis vendors are creating more modular designs, with a power supply that plugs into a docking station, making removal and replacement a simple operation.
When dealing with an alternative power application, such as battery operation of a PXI system, the need for power budgeting becomes critical. Some DC-powered chassis are commercially available, such as the NI PXI-1000B, but often developers will choose to use a conventional chassis and an inverter to supply AC power from a battery. Either way, the power budget will need to be expanded to include chassis elements such as fans and indicator lights along with the modules. The power draw of an empty chassis becomes a useful specification.
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Be sure to factor in the efficiency of the inverter and the power supply along with the chassis and board demands when determining battery needs for a portable PXI system. Source: National Instruments. |
Dividing the inverter power demand by the inverter efficiency will give the power demand that the battery must meet. The system's required operating time between charges then determines the battery capacity required. Choosing a battery so that the demand is 80% of its specified capacity will provide margins for error and for battery degradation over time.
Avoid overheating in PXI chassisAlong with supplying the needed power for their systems, PXI developers need to address removal of the heat that components generate. In nearly every case, heat removal is accomplished by using fans to provide air cooling. While this may seem like a straightforward task, there are still some considerations to be addressed.
One of the first considerations is to ensure that the airflow in a PXI system is properly channeled. A commercial chassis design will typically provide the same cooling-air volume to each slot location, but only if the chassis is filled. To maintain uniform airflow, users should insert blank cards into their unused slots. Without the blank cards to help channel the airflow, most of the cooling air may end up flowing through the unused volume rather than the occupied slots.
Developers should also be on the lookout for airflow restrictions, such as cabling. Cables that cross the top or bottom of the card cage can cause hot spots within the cage. Similarly, cables at the back of the system have the potential of blocking the air exhaust from the cage, reducing cooling efficiency.
The use of cooling fans carries with it the generation of acoustic noise. That noise is becoming a problem in some system designs, according to Geotest's Manor, especially in desktop installations. To address this problem, PXI system developers and chassis vendors are using variable-speed fans rather than always running the fans at full speed. Typically, a temperature sensor at the air intake for a chassis automatically controls the fan speed. The warmer the intake air, the faster the fans blow. This helps ensure efficient cooling under a variety of ambient conditions while minimizing the fan noise.
By addressing the challenges of cooling and powering PXI systems, developers will be able to create the systems they need even if they exceed specifications in some areas. Manor points to a customer that had a system populated with 16 high-performance digital boards in a chassis, each of which drew more than the 60-W slot limit.
By sizing the power supply appropriately and incorporating an additional back panel with variable-speed fans to provide adequate airflow, the customer was able to adequately power and cool the design. As Manor noted, power requires attention from developers but should not be considered a barrier. “Don't let power concerns or cooling limit your design,” he said. “Pretty much any problem can be solved.”
| Voltage | Min. Per Slot Current | Max. Total for All Slots (Low-power systems) |
| +5 V | 2 A | 10 A |
| + 3.3 V (I/O Voltage) | 2 A | 10 A |
| +12 V | 0.5 A | 1.5 A |
| –12 V | 0.25 A | 0.75 A |
| Reference |
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