VPX power systems are a defense industry mainstay. New standards help boost efficiency by eliminating the need to over-specify supplies when handling outlier situations.

GERALD HOVDESTAD
BEHLMAN ELECTRONICS

POWER Supplies have always been the problem child in electronic system design. Often, high-end systems are configured for specialized applications. As a result, power requirements tend to be quite different from one configuration to another. VITA (formerly the VMEbus International Trade Association) attempted to standardize some aspects of VME system design, which also put constraints on power. Mechanical standards were agreed upon for circuit cards, and there was an effort to organize voltages and such qualities as regulation, noise, ripple, and rise time. There was really no successful attempt to control the size and shape of VME power supplies.

After the introduction of the VPX standard VITA 46, the standardization process continued to advance and define many more features of system cards. Chassis, backplane, and card profiles are defined in VITA 65, although this standard appears to be a monster with unlimited growth potential.

In addition, the VITA 62 specification was written in an effort to standardize power supplies. This specification defines mechanical configurations, connectors, voltages and general interfaces, so system integrators can design platforms with standard backplanes and can plug in power supply cards from a variety of suppliers.

The expectation was that cards from these different manufacturers would function properly and identically. This expectation may be true with some less complex systems, but there are still many variables, such as user-defined pins, which allow designers to make use of sophisticated, “undefined” features. These features are specification enhancements offered by some power supply manufacturers but not by others, even if they are VITA 62 compliant. A further complication is the programmability often offered with these power supplies, which is loosely defined in the VITA 46.11 system management specification.

VITA 62 does define a starting point that system designers can use for basic power requirements. They must add actual power levels, as well as decide how to incorporate other undefined system power needs. EMI filtering and hold-up requirements, as well as transient and lightning protection, are just some of the possible considerations. If multiple cards are used, issues that must be resolved include shared circuit performance, compatibility, sensing, and synchronization.

The VITA 62 specification, as currently released, allows for considerable flexibility in requirements. Output currents for each allowable voltage are the parameters that are most variable. Seldom does a power supply company see two requirements with the same output power levels. So it is important that products can be tailored to meet these needs. System integrators should not be afraid to ask for what is important, and power supply manufacturers must be able to discuss these needs and offer products that will meet them. Sometimes it might be important to stick with a fully compliant power supply, while other programs may allow some configuration changes in the interest of space savings or cost.

An example is a system with a 5-V, 120-A requirement but which needs less power on the 12 and 3.3-V outputs. This type of situation is fairly common because some processor assemblies, widely used in VPX systems, need a significant amount of 5-V power. Standard 3U VPX cards come with outputs up to 5 V at 40 A because the pins designated for the 5-V output are rated at 40 A.

One system designer may decide to meet the requirement using three cards and stay fully compliant with the VITA 62 specification. Another may opt for some tailoring of the card to reduce size and cost. The latter system designer may settle on using one standard card and a second card with two 5-V outputs by replacing the 12 V on that card with a second 5-V model. This approach would save a card slot, and it would cut the cost of the power supply roughly 33% because it used fewer cards.

Each program designer must decide what degree of specification compliance or deviation they’ll accept. The designer should not be reluctant to discuss the requirement with the supplier; this interaction is usually the best way to optimize the design. In fact, it is important to consult with the power supply manufacturer early in the design stage because a reconfiguration of the power supply could affect other parts of the system design. The result could be added cost for reworking and more development time.

Besides hardware requirements, VPX power supply software capability is also important for customizing and optimizing systems. Modern power supplies can be quite flexible. While IPMI-based VITA 46.11 can be used to optimize system performance, it doesn’t do much for defining programmable functions. Some manufacturers continue to use a PMBus-based I2C structure because the functions available are well defined.

The process of tailoring a system can bring many desirable features. Some of those features include special over-voltage, over-current, temperature warnings and limits. It is quite common to see wide variations in these requirements from system to system. And it would be quite difficult and expensive to have each variation fixed in hardware. The ability to program these settings lets standard hardware handle such modifications.
Temperature warnings and limits are other factors that are often system dependent. The ability to read output and input currents can also help in monitoring supply operation and in diagnosing system problems.

VITA 62 is an important step forward in the effort to have an open VPX standard power supply. But there is still significant work necessary to meet new and existing system needs. Established standards don’t cover several hardware areas well, and software definitions need to be addressed.

In hardware, there is no real definition of an energy storage card that would let system designers and manufacturers have a standard configuration. While VITA 62 does mention these devices, there is no connector pin assignment. As a minimum, such a card needs definitions for input and output pins. It would also be desirable to have basic logic and control functions defined, such as “card ready” or “fault.”

Another need is a specification for three-phase inputs for 3U cards. The current 3U connector lacks enough input pins for this function. The original VITA 62 committee did not foresee a need for these inputs, but many systems now have such a requirement. There is, however, an effort underway to tie up some of these loose ends.

Industry is also paying more attention to the VPX arena. The DOD “Host” for hardware and “Fast” for software are just two of the industry thrusts to reinforce the Open VPX Standard. The most recent release of VITA 62 adds a number of different pitches (heights). This change is especially important when incorporating hold-up, as the required components might need additional space.