what-is-in-the-wattage

By Jonny Gerow, on 1 de septiembre de 2012

When people talk about computer power supplies, there are a lot of terms that get thrown around that we here at Corsair realize that not everyone understands. Unlike a CPU or GPU, a power supply doesn't have clock speeds. A PSU doesn't have 4MB vs. 8MB of L3 cache. A power supply doesn't have 2GB vs. 4GB of GDDR memory. So what number does everyone know when they look at a power supply box? The wattage. But, truth be told, the actual capability of one power supply can't be explained by wattage alone. One 600W power supply may not be the same as another 600W.

First, let's talk about what this "wattage" number on the box and power supply label is. The wattage of a power supply is an indication of how much DC power the power supply can put out. But a power supply only puts out as much DC power as demanded of it by the computer. And depending on what you're doing with that computer, that number can vary a great deal. Even though you have a 600W power supply, the power supply may only be putting out 300W, or even as little as 1 watt if it's just sitting in standby.

Here's the rub: that maximum wattage number comes with a number of conditions. A number of conditions that aren't always laid out on the retail box somewhere. And, as a consumer, you don't know what these conditions are unless a power supply is properly reviewed and exposed as the fraud, for the lack of a better word, that it may be.

 


Corsair fully discloses that the GS600 is rated at 600W, continuous, at 40°C.

 

Continuous vs. Peak

All Corsair power supplies state their output wattage as a "continuous" number, while some others use a "peak" number. What's the difference? Well, if a power supply is rated at 600W continuous, like a Corsair GS600 for example (pictured above), that means that the power supply can output as much as 600W and it can do so relatively forever.

 


This chart shows a 600W load on a power supply rated at 600W continuous.

 

If a power supply is rated at 600W peak, that means it can hit 600W DC output if pushed, but it can't continue to put out that much power for any extended period of time. The actual continuous output of this power supply may only be 550W, or 500W. In some cases, the continuous output may only be 450W or less, but as long as that power supply can "peak" at 600W, they're allowed to put 600W on the box!

 


This is a power supply that is rated at 600W peak. It can do 450W continuously, but can only do 600W for a fraction of a second.

 

How long can a peak power supply peak power?

If a power supply is rated at peak power, that means it can only put out its advertised power for a short period of time. But how short? Unfortunately, nobody's obligated to put out that information. Unless you have actual documents from the engineers, you would never really know. The way this is tested in the lab, the power supply in question is hit with a load that is greater than what the continuous capability of the PSU actually is. With modern day test equipment, you can set the duration of this load from a few milliseconds to a full minute. If the engineers hit a 450W power supply with a 600W load for 10 milliseconds (that's 1/100th of a second, by the way) and it doesn't blow up like a frog in the microwave, they go ahead and say it's a 600W power supply.

 


When we try to crank the load up to 600W on a power supply rated at 600W peak and maintain that load, the power supply eventually fails.

 

With a Corsair power supply, a full load can be put on the power supply and left on that power supply, and the unit will continue to run.

 

Does temperature play a part in the power supply's output capability?

A lot of power supplies on the market are only rated to put out power at room temperature, which in the industry is 25°C. As the temperature of the chassis increases during use, the power supply's capability to output its maximum continuous output decreases. This decline in power output capability is called a "derating curve".

 


A power supply rated to output 600W at 25°C has a decreasing output capability as temperatures increase. Eventually, OTP (over temperature protection) kicks in and the PSU shuts off.

 

It is very unlikely that a computer power supply will be taking in air that is only of room temperature. The inside of a typical PC easily reached 40 to 45°C. High end gaming machines, under full load, can easily hit 50°C. Even if you have your power supply configured to intake air from the outside (for example, a PSU mounted in the bottom of a chassis with the fan pointing down), the hot air exhausted from the rest of the PC will get sucked back in by the power supply. While these temperatures aren't as high as 40°C+, they're still higher than 25°C.

Unfortunately, not everyone puts the temperature at which their power supply is continuously rated at on the box or in the documentation. It is safe to say that if there is no temperature rating either on the website, box or documentation, odds are that power supply is only rated to output at 25°C.

 


The Corsair TX Series power supplies use industrial grade components that allow them to run at maximum output, continuously, even at 50°C temperatures.

 

Corsair's TX, HX and AX power supplies are rated to output continuous power at 50°C. What this means is that those power supplies can put out whatever they're rated at (650W, 750W, etc.), continuously, even if the temperatures in the PC chassis reach 50°C.

 


A power supply rated to output 600W at 50 °C. Eventually, OTP (over temperature protection) kicks in and the PSU shuts off.

 

I'm good, I found the operating temperature range on the box!

Sorry, but no. It's not unusual to see something like "operating temperature 0°C to 50°C" on the box or in the manual for a power supply, but those numbers only tell you that the power supply can operate within those temperature parameters. It doesn't tell you whether or not a 600W power supply can actually put out 600W at 50°C or only 100W. The derating curve still plays a part while the PSU is running within the advertised operating temperature range. The only message that's being conveyed is that the power supply may not operate at all if you are outside of this temperature range.

 

 

"Operating temperature range" only means that the PSU will operate within a certain temperature range. The de-rating curve still applies. The image above is an example of a power supply with an operating temperature range of 0°C to 50°C, but is only rated to put out 600W continuously at 25°C.

 

Can I expect that the power supply will provide good power to my components, even if it's pushed to its peak power capability?

Absolutely NOT! As some power supplies reach the upper limits of their capability, such as the output wattage range between continuous and peak, the ripple can get quite high and the output voltages can drop considerably. Why is this a bad thing? Ripple that is not filtered by the power supply has to be filtered by the components the power supply is powering. The filtering capability of most computer components is not as complete as that of a quality power supply, so prolonged exposure to this excessive ripple can shorten the life of your components. Likewise, if voltages drop too low, voltage regulators on your components that regulate your power supply's output voltages into other voltages have to work harder to maintain the voltages that your components require. Over working the components that regulate these voltages can, in time, cause them to fail.

 

 

The above image shows a "cheap" power supply, running within the wattage rating advertized on the box, but with ripple that is so bad, that we could actually measure it in volts, rather than millivolts. This shows 2V of ripple on the +12V rail. ATX specification states that ripple cannot exceed 120mV. That means the ripple on this power supply's +12V rail is more than 16 times ATX specification.

Corsair power supply all maintain very tight voltage regulation and suppresses most ripple, even when pushed to the upper limits of the unit's capability.

 

 

The above image shows the ripple from a Corsair power supply's +12V rail under full load. Note that the settings on the oscilloscope are the same as they were when we tested the previous power supply (lower right hand corner). The ripple is actually less than 20mV. 1/6th the ATX specification.

 

Ecova (80 PLUS) tests power supplies at 20%, 50% and 100% loads, so is it safe to assume that any power supply on 80 PLUS's website is perfectly capable of performing admirably at any load?

80 PLUS is a great resource for finding out if your power supply is efficient or not, but a user can't find out much more about a power supply beyond that. Ecova tests all of their units at room temperature (25°C) and they do not measure ripple during testing. And while Ecova does measure the output voltages of the rails they are loading during testing, they only measure voltages to 1/10th of a volt. That means voltage can drop as much as .04V and not be recorded as a drop in voltage, so you can't really use their test results to determine how good the voltage regulation is on a particular unit.

As we can see, we might know what wattage power supply should be able to power our computer, but do we really know if we're getting our money's worth? Two power supplies may be the same price and advertise the same wattage, but one might actually be considerably less capable when taking into consideration all of the variables we've discussed today. For the sake of making sure you're getting enough power for your computer, or to make sure you don't inflict long term damage to your computer components, it's worth it to do a little research before jumping on that "good deal".


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