Jon Gerow · 06-26-2006 · Category:
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
- 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
- The PC power supply:
- The PC power supply label:
- Defining the connectors of an ATX power supply
- ATX power supplies DO NOT turn on at the flip of a switch
- Testing your power supply's voltage: Software vs. Multimeter
- Power Supply Efficiency
- The Derating Curve
- Power Factor Correction
- The resistance of modular connectors, adapters and splitters