Jon Gerow · 06-26-2006 · Category: Guides

The Derating Curve

One could say that the fundamental limiting factor of a power supply is it's operating temperature. As components within a power supply heat up, either the efficiency is going to drop below a desired level, voltages are going to drop below a usable level... or something's going to just plain burn up (hopefully thermal overload protection kicks in before that happens.)

Essentially, being able to squeeze more juice out of a power supply is really the same train of thought used when overclocking a CPU. You can't overclock a CPU that can't be kept nice and cool and you can't get good, clean power out of a power supply if it's not kept nice and cool.

The effect of temperature on a power supply's performance is called a "derating curve." The warmer a power supply gets, the less capable the power supply is of supplying wattage. Deratng curves are typically measured as Watts per degree. For example: "2W per 2°C" would be a "derating curve." What this means is if a power supply was rated at 500W maximum sustained output at 20C, then it's maximum sustained wattage capability is reduced by 2W for every 2°C it's temperature is increased. So at 50C, the maximum sustained output of this power supply may only be 470W. Another thing to remember is this derating curve only applies to a unit operating within it's recommended operating temperature. Once you go beyond the maximum operating temperature, say 70°C, then the derating curve starts to increase exponentially.

Unfortunately, a power supply's derating curve is not information typically found on the power supply box or in the manual. Probably the best known example of a derating curve is in PC Power and Cooling's advertisement for their Turbo-Cool 510. And although the derating curve exhibited in their example (24W per 2°C) has to be either grossly exaggerated or the worse 500W power supply in the world, an excellent point is made. As the temperature at which full load ratings are derived change, so does the effective output of the power supply. So, as implied by the advertisement, a power supply that has it's full load output rated at 50°C is not an apples to apples comparison to one that has been rated at 25°C.

So now that your head is swimming, I'll break it to you as to what that REALLY means to you.

The facts are, you may not know at what temperature your power supply was rated at and you may not know what your power supply's derating curve is, but you CAN make sure that your power supply is kept nice and cool so it gives you long, dependable service as a provider of good, clean DC power.

Some PSU's use 80MM fans mounted on the ends because they cool better, but 120MM fans are typically quieter.

To be perfectly honest, I could write a whole other article on chassis thermodynamics. But for now, here are some tips....

• Remember to consider your power supply as part of a PC's entire cooling solution. If an equal number of intake fans are used along with an equal number of exhaust fans, you may not be moving enough air through your power supply!
  
  
• Keep high CFM fans away from the power supply. I've seen examples of fans mounted just below the power supply's 120MM fan that were of a higher CFM then the power supply's fan. The resulting venturi effect actually created a vacuum in the power supply housing!
  
  
• Remember that some "low noise" power supplies can sometimes mean "high temps." Certainly a power supply that is more efficient or has a better heatsink design isn't going to require as much CFM from a cooling fan than another, but not every power supply is designed with the consideration that it's going to be implemented as part of the entire cooling solution of a PC. Don't get me wrong. I appreciate a low noise power supply just as much as the next guy. But if you're going to implement a low noise solution, consider positive air pressure in the case or perhaps an upside down case that puts the power supply at the bottom of the chassis instead of at the top. Otherwise, you might run into voltage fluctuations, mysterious shut downs, reboots or premature failure of the power supply.

Contents

1. Introduction
2. The PC power supply:
3. The PC power supply label:
4. Defining the connectors of an ATX power supply
5. ATX power supplies DO NOT turn on at the flip of a switch
6. Testing your power supply's voltage: Software vs. Multimeter
7. Power Supply Efficiency
8. The Derating Curve
9. Power Factor Correction
10. The resistance of modular connectors, adapters and splitters
11. Conclusion

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