Over the past five years, CompactPCI has become the architecture of choice for equipment makers developing high-availability embedded applications. Its powerful, standards-based, cost-effective computing solutions, as well as hot-swap and high-availability capabilities, make it ideal for communication and military applications.

Packet-based advanced managed platforms take the CompactPCI architecture further by employing comprehensive management and a suite of highly compatible and feature-complete components to deliver high-availability, "application-ready" platforms. But as companies consider developing their applications with advanced managed platforms, there are several power architecture issues to consider. One of the most challenging is calculating the power budget.

Typically, 12U advanced managed platforms with dc input deliver power to the platform through two isolated -48-volt feeds. For high reliability, the two feeds are not ORed together with diodes. This is important for telco facilities that provide redundant power plants, because a diode failure could allow faults in one power plant to affect the other plant.

When dealing with high-performance applications in a CompactPCI platform, it is important to develop a power-budget analysis to ensure that all components operate correctly. High-performance applications will tax both the power and cooling architectures supported in the platform.

Poor power-budgeting skills look something like this: "Well, the platform delivers a total of 1,300 watts of redundant power, and I've got boards that draw a maximum of 50 W each, so I can install 26 boards in the chassis." Not only did we not account for all the infrastructure elements that draw power, such as fan trays, switches and shelf managers, but we did not account for the fact that there are four voltage rails in the platform, and each has limited power.

To properly budget power for 12U platforms, designers need to go through three steps. Let's look at each in more detail.

• Max current available/platform. CompactPCI defines four independent and limited voltage rails that deliver power to the platform: 5, 3.3, 12 and -12 V. It is important to analyze power budgeting at each voltage rail in addition to overall power of the chassis, because one specific configuration of boards in a chassis may not tax the 5-V rail but could significantly tax the 3.3-V rail.

Each voltage rail is independent and does not share current. After one rail is maxed out, you have reached the limit of boards you can integrate into the chassis that use that specific voltage. Step 1 is to calculate the maximum current for each of the four voltage rails from the total set of power supplies in the platform, discounting any redundant supplies.

• Peak power rating/component. In Step 2, designers list the maximum current rating for each component and board to be integrated into the platform. For each unique board, write down its maximum current draw in amps for each voltage rail. It is important to use the maximum power draw for each component to calculate the worst-case scenario.

Be sure the maximum draw on the V (I/O) pins is included with the 3.3- or 5-V rail depending on the V (I/O) configuration of the system. The maximum power rating should take high environment temperature conditions into consideration. Typically, 40 degrees C is used for each board and component.

• Power budget analysis. Now let's configure these boards into the platform and calculate the total power budget. Create a table that lists each board, the quantity of unique boards in the platform and the total current consumed per voltage rail.

Designers need to look at total current draw, total current available and the delta between the two. The total current draw row represents the sum of each voltage column, while total current available is the maximum current available from the platform as we calculated above. The delta is the difference between the currently available and the total current draw from all components and boards for each voltage rail.

By using these three steps, designers can make this complex task much easier to handle.

Download our white paper on "The Power Architecture of Advanced Managed Platforms" for more information on budgeting power.