Select the Right Power Supplies for Your Servers

Most system builders don't know how to do this, and it's a shame. They're leaving money on the table, wasting time, and possibly building servers that are less reliable than they could be.

Sure, you can simply call a systems engineer at your distributor and ask them to make the calculations. But why depend on them when you can do it yourself? Instead of leaving a message and waiting for someone to call back, spend your time more wisely, and be more productive in the process.

In this recipe, I will attempt to demystify the method of calculating power requirements. You will learn the ability to discern which power supply is the best one for your server. By going through this process, you will be sure you are spending your customers' money where it counts. You'll also learn how to create a more-reliable server. The benefits should include increased customer satisfaction at a variety of levels, and fewer support calls for power problems—which are among the most difficult to troubleshoot.

For system builders seeking to make their mark in the white-box server market, there are tools that can help. One of the best tools out there is Intel's Power Budget Analysis Tool, which is available as a free download from Intel's Web site. This tool is basically a Microsoft Excel spreadsheet, customized with built-in calculations. I'll explain how to understand and use this tool as part of this recipe.

Getting Started

When selecting a power supply for your server system, be sure to choose a unit with enough capacity to keep your utilization rate low. In other words, you need output above your actual system requirements. As the load on the power supply is decreased, the system's longevity is increased.

Calculating the system's power requirements is fairly straightforward. First, look at the manufacturer's datasheet for each component installed, and find the power consumption specs. This will be written as a number of amperes (amps) at a particular voltage.

Sometimes, one device can use current at multiple voltage levels. If this is the case, just add these figures together to arrive at your total power consumption. Add up the power consumption for all devices in the system, and add some overhead to account for your utilization rate. You want to keep utilization down to between 60 percent and 80 percent. The lower, the better.

Diving A Little Deeper

There is one mathematical equation you should familiarize yourself with: Watts (W) = Volts (V) x Amps (A).

Of course, some unit conversion might be necessary, since current requirements for "standby power" (often shown as +5VSB on the power supply) are sometimes written in milliamps instead of amps. A milliamp is one thousandth of an amp; in other words, 200 mA equals 0.2 A.

A standard ATX-compliant power supply has six lines or "rails." These are: +3.3V, +5V, +12V, +5VSB, -3.3V, and -12V. Each rail has wiring to supply the indicated voltage. They also have a variety of connectors to plug into devices inside the server.

For every device in the system that consumes electricity from the power supply, you will need to multiply the current at each voltage level to arrive at the total wattage required by that device. If that seems like it's a lot to keep track of, you're not alone. On the next page, I give you a starting point...