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... here's a simple table that shows some common devices and the voltage levels they may use. This should be a good starting point for system builders new to the world of power supplies:

Here's an example of how to use this information. Let's say your server has four Seagate ST373455LC hard drives installed. From the table above, you can see that these devices use power at +5V and +12V. Next, let's calculate the power requirements for these hard drives. Here's how to do it:

• Looking at the datasheet from Seagate's Web site, we discover that the current in amps is 0.41A @ +12V and a maximum of 0.84A @ +5V.
• Next, we multiply 0.41 by 12 to get 4.92 Watts on the +12V rail.
• Then we multiply 0.84 by 5 to get 4.2 Watts on the +5V rail.
• Add these two products together. That gives us 9.12 Watts for each drive.
• Multiply this per-drive sum by the number of drives. In this case, we have four drives. 9.12 x 4 gives us 36.48 W total consumption for the hard drives in the system.
• Repeat this process for all of the different devices in the server: processors, memory, add-in cards, etc. Then add it all together. The sum is the absolute minimum wattage needed for your power supply at 100 percent utilization.

Next, let's move on to adding overhead to our capacity calculations. Overhead is necessary to handle the transient loads, such as initial system start-up, and various devices activating and de-activating. Overhead is also needed to keep your power supply from working too hard and overheating. You should not keep your power supply at 100 percent utilization; you must give it room to "breathe."

The utilization rate expresses the percentage of the power supply's capability that is being tapped at peak output. So if you limit your power supply to 80 percent utilization, then you need 25 percent more power capacity than you are actually using. You calculate this by taking the inverse of 0.80, which is 1.25.

For a system that runs 24 x 7 (like an Internet-ready server), a lower utilization rate—such as 60 percent or 70 percent—will help the computer run with greater durability. For a computer that is used only intermittently, such as a home PC, the power supply can tolerate a higher utilization rate.

Some of the newer power supplies out there, like the ATX12V v2.2 compliant models, have two (sometimes three or four) +12V rails, often shown as +12V1 and +12V2 on the power supply. This helps to increase the system's stability by letting you isolate some of the electrically "noisy" loads. In this scenario, you would put the disk drives on one rail, the circuit boards and CPU chips on another.

To minimize the amount of energy lost to heat in the system, you'll want to use a power supply with the highest possible efficiency rating that still meets all your other requirements. Usually, this number will range from 60 percent to 70 percent, though some of the newer ATX12V v2.2 units demonstrate as much as 85 percent efficiency. Less heat in the system translates directly into greater longevity. It also reduces your cooling requirements.

When using redundant power supplies, be sure the units have load-sharing capability. This means the load will be distributed among all power supplies in the group until there is a failure. At that point, the entire load will fall on the remaining unit(s). This will lower your utilization rate. But make sure the remaining power supplies in the group can handle the load (at least for a short time) if one of the units fails. This is another important reason for calculating your power requirements. The Devil In The Details

Now, let's use what you've learned to select the best power supply for your next server. At one extreme, you might find a $9 generic power supply; at the other end of the scale, a unit with a list price of $549. Which is the right one for your server? After close examination of several manufacturers' data sheets and scrutiny of their specifications, I've weeded out a lot of possibilities. In my opinion, here are the product offerings you should be evaluating for most of your servers:

• Turbo-Cool 510 XE, by PC Power and Cooling. Manufacturer's suggested retail price (MSRP): $199.
• Neo HE 550, by Antec. MSRP: $149.95.
• EG851AX-VH, by Enermax. MSRP: $199.99.

If these units aren't right for your server, check alternate products from the same companies. These manufacturers have offerings that will handle the most stringent requirements. But remember, marketing hype can carry a product only so far. Look at the data sheets, and check the stats. Judge for yourself which is cheaper or more efficient, or provides the amount of current you'll need at +12 Volts, etc. Then use what you know about your server (and what you learn here about power requirements) to evaluate the options yourself.

Working With The Intel Power Budget Analysis Tool

Intel's Power Budget Analysis Tool is a handy Microsoft Excel spreadsheet that can be customized to include components not already in Intel's pre-configured format.

While this tool was originally intended for system builders working with Intel's server platforms, the mathematical principles behind it apply equally to any computer system " whether desktop, laptop, or otherwise. If you customize the tool by adding the power requirements for your most commonly used devices as the need arises, then over time, the tool will grow to cover a wide variety of configurations. Best of all, you won't have to do the math yourself.

All you need to start is a PC with Microsoft Excel 2003. First, download the Intel Power Budget Analysis Tool. Once you've got it installed, open the spreadsheet with Excel. Enable macros. Then begin using the tool.

The Intel Power Budget Tool is divided into three parts: Configure System, Configure Server Board, and Summary/Results. These three don't need to be completed in any particular order, and you can switch back and forth as you input your numbers. This feature comes in handy. It lets you to try several scenarios before arriving at your final results. Use the drop-down lists and counters to fill in the types of devices in your server and the number of each type. You'll see the spreadsheet perform its calculations and show its results as you go.

In addition, the Intel tool also has buttons to the left that let you create your own peripherals for inclusion into your system. Do this by clicking on the appropriate Define button, entering the power consumption in amps at each different voltage level (as discussed above), and then saving the changes to your spreadsheet. Once you've saved the changes, the new devices you've created show up in your drop-down lists, and they are available to you each time you use the tool. You can configure any kind of peripheral device. Even a different chassis can be added to the tool.

Anywhere that your system's power capacity is exceeded, the Intel Power Budget Tool highlights and flags the problem area. For instance, if you have overburdened the +5V rail, this is brought to your attention, and you can try alternate configurations to resolve the problem.

By spending time with the Intel Power Budget Tool, and customizing it to suit your needs as a system builder, you can put more reliable systems into the field and prevent potential problems before they become a reality. The effort you've spent on these calculations to arrive at your system's power consumption can be thought of as the first step in your detailed system design that will focus on determining the best components for your server.

By taking this approach to building a server, you can stop guessing at which parts to use, and instead proceed with certainty that you're delivering a reliable and powerful system that will meet your customer's needs and prevent downtime (and service calls) concerning your customer's site. You can also use your results here as a springboard into calculating your system's heat dissipation and cooling requirements. Stay cool!

DAVID GILBERT is the owner of Appalachian Computer Systems, a system builder based in West Virginia that specializes in multiprocessor SCSI RAID servers.