TECH TALK

A journal of bus architecture tips & techniques

Choosing CompactPCI Power Supplies: Review of ATX

You have more choice than you think in specifying a CompactPCI power supply for your enclosure. Various supply configurations serve different purposes: inexpensive for cost sensitive projects; dual redundant and hot swappable required by high availability systems; or, mammoth outputs for high power applications.

DC voltages required for CompactPCI systems are 5V, 3.3V, and ± 12V. That mandates a quad output power supply. Since the 5V and 3.3V outputs have high current ratings, they provide the bulk of the power to the system. CompactPCI power supplies support a range of AC input voltages for worldwide use: 100 - 127 VAC and 200 - 240 VAC. The correct input range is set manually (switch-selectable) or automatically by the power supply (auto-ranging). DC input versions are also available: -48V DC for telecom and 24V DC for industrial applications.

In general, you may specify one of four styles of power supplies for your CompactPCI system. Designed with specific applications in mind, each has advantages and disadvantages. Choices are:

• ATX
  
• plug-in
  
• open frame
  
• modular

This article will discuss the ATX power supply, popular in CompactPCI systems. Future articles will detail the three other styles.

ATX Power Supply
Since it became standard in desktop PCs, ATX supplies are manufactured in high volume and are omnipresent. Cost savings drove designers of CompactPCI to choose the ATX power supply. ATX supplies follow the ATX12V Power Supply Design Guide, Version 2.01. The guide specifies standard dimensions (150 mm x 146 mm x 86 mm) and a standard power connector (20 pin Molex 39-29-9202). An enclosure manufacturer can easily integrate supplies from multiple ATX vendors.

Power can be connected to the backplane in a couple ways. Since ATX supplies are built with a standard power connector, the mating connector can be mounted onto the backplane. The ATX wire harness plugs directly into the backplane. A second method uses an additional wire harness. One end mates to the ATX power connector and the other to the backplane. The backplane side of the harness accommodates power bugs or lugs, whichever the backplane uses. ATX supplies have an AC input receptacle; you plug input power directly into the supply. Chassis manufacturers can mount the power-on switch in the rear or in the front on the enclosure.

ATX supplies range from approximately 90 to 600 watts and can easily power an eight slot CompactPCI system. Even though they are the least expensive of the four solutions, one must be aware of ATX supplies' limitations. The 3.3V and 5V outputs are often shared. When devices such as disk drives draw too much power from the 5V line, they will reduce the current available on 3.3V. A Pentium single board computer depends on a minimum amount of power on the 3.3V line. Also, some supplies built for economical reasons sacrifice ruggedness and reliability. A quality chassis manufacturer avoids the lowest cost models. ATX supplies cannot be swapped out quickly in case of failure. However, they are available in redundant, hot swappable configurations. Data sheets for several ATX supplies are found here.

Stay tuned. I will review the three other types of power supplies commonly specified in CompactPCI systems: plug-in, open frame, and modular.

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