[Design Application]
When It Comes To CompactPCI Supplies, Standards Are Helping
Despite Evolving Standards, The Wide Variety Of Supply Topologies Leaves It Up To The User To Match The Product To The Application.
Although an increasing number of embedded-systems integrators are using the CompactPCI architecture, the power-supply requirements for these systems have not yet been standardized. This task belongs to the PCI Industrial Manufacturers Group (PICMG), which is charged with generating both mechanical and electrical specifications for PCI-based systems and boards for industrial computer applications. The group currently is working on establishing the necessary standards for the power supplies used in these systems. These standards will ensure compatibility among systems as power requirements rocket skywards.
Unfortunately, the term standard can be misleading, as manufacturers have a number of power-supply methods for achieving that electrical compliance. Thanks to a wide range of available topologies, different power supplies can provide varying levels of performance on a number of key parameters. These parameters can include output-power/current capability, power-factor correction, filtering, current sharing, and live insertion. The topology each manufacturer uses is a key differentiator when it comes to choosing the supply most suited to the application.
CompactPCI (cPCI) itself is a leading bus architecture for embedded systems in the industrial-control, computer-telephony, and telecommunications markets, to name a few. By combining the PCI desktop architecture and the rugged Eurocard form factor, cPCI increases the functionality and/or flexibility of embedded systems.
The existing cPCI specification (Rev. 2.1) includes portions detailing general power-supply requirements. While these basic requirements are expected to remain, they do not leave room for expansion as systems evolve. In fact, most front-end, pluggable power supplies used in cPCI systems prior to this year were less than 150 W. As power requirements increase, integrators will need power supplies that provide more power, are scalable, and are cost effective. The cPCI power-interface specification will ensure the commonality of such power supplies, both mechanically and electrically.
The mechanical definition of cPCI power supplies generally will follow the guidelines set in various IEEE 1101 specifications. The power supplies will be limited to the size of a particular system being designed, typically 3U or 6U high, 160-mm deep, and 8HP wide. The existing cPCI specification details the use of a DIN "M"-type connector for use on power supplies. The new specification also will include other connector types to enable flexibility as power requirements increase and different features are needed.
With regard to electrical requirements there is, to date, no single document that would specify all cPCI power-supply parameters. There are, however, a number of preliminary document drafts. These drafts,which list only a few basic requirements, include the Power Interface specification PICMG 2.11 D0.3, CompactPIC specification PICMG 2.0 R2.1, and Hot Swap specification PICMG 2.1 R1.0.
For example, the document PICMG 2.0 (Sept. 2, 1997) sets nominal voltages (5 V, 3.3 V, ±12 V), tolerances (±5%), and maximum ripple (50 mV), measured with external 22-µF and 0.1-µF capacitors at the measurement point. The PICMG 2.11 D0.3 draft (Oct. 30, 1998) sets tighter tolerances of +4% and -2% for 5- and 3.3-V outputs. This draft also lists some basic power-supply design rules, such as the use of internal OR-ing diodes, enable and inhibit signals, and a new connector pinout. It's expected that the next revision of PICMG 2.11 D0.4 already will include the above mentioned parameters from PICMG 2.0. Most of the important power-supply features, like power and current capabilities, power-factor correction, method of current sharing, and EMI requirements, are not specified by any cPCI document. Instead, they will be determined by the general system requirements.
Power-Supply Design Features The predominant dc bus voltages required for cPCI systems are 5-V dc, 3.3-V dc, and ± 12-V dc. Other voltages may be required for unique features in some systems. The 5-V dc and 3.3-V dc outputs will provide the bulk of the power, and therefore have the higher current ratings. The input voltages for the power supply will be wide-range ac for worldwide use, and 48-V dc for telecom applications.
One of the challenges for cPCI power supplies is to achieve ever-higher power density. Power supplies use slots in a cPCI system that otherwise would be filled with other feature-filled boards. The smaller the power supply, the more features can be offered by the system integrators. For example, a typical quad-output, full-featured, 150-W model supply and its dc twin can deliver about 4.6 W/in.3, and have a 3U by 8HP cPCI form factor (Fig. 1).
Thanks to the lack of standards governing electrical performance, power supplies designed for compliance with the power-interface standard theoretically could use many different topology designs, and provide varying levels of performance on key parameters. Depending upon the type of systems being designed by the system integrator, some particular performance parameters will be more important than others. The following sections relate to different performance parameters, and how different designs lend themselves to better or worse performance.
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