Is load the only thing that affects the amount of heat a PSU generates?

Title says it all really. Basically I'd like to know if power load on a power supply is the only thing that affects the amount of heat it generates. For example, will a 500W PSU generate the same heat as a 200W PSU if they are both connected to exactly the same equipment?

Perhaps a 500W is more efficient and so doesn't generate as much heat at a given load? My main reason for asking is just out of interest but it's worth knowing these sort of things when I'm choosing components for a micro PC I'm building.

Many thanks!
Will,
http://wjdashwood.co.uk/


P.S. I don't suppose anyone knows where I can get hold of the JRex range of tiny single board computers?
wjdashwoodAsked:
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Adrian DobrotaNetworking EngineerCommented:
Mainly it should be based on power demand not max_output_power.
So, a 500W PSU will generate more heat than a 200W one only if the demand on the PSU is higher than 200W (where the small one will collapse :-)
If you don't change the equipment, the heat should be the same .... with small differences besause of the PSU construction....but insignificant.

Hope this helps

Kronos
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wjdashwoodAuthor Commented:
Thanks, yep that helps. It's what I assumed but no harm in asking for some reassurance. I'll leave the question open for this afternoon and if no one else has some useful info the points are yours.

Cheers,
Will
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jhanceCommented:
In theory kronostm is correct but in practice what he says is not the case.  A 500W PS has bigger "iron" and will have greater hysteresis loss under all load conditions vs. the 200W supply.

The different will not be great but is easily measurable.  I'd estimate that you'll see power dissipation of about 10% greater for the 500W supply than the 200W one under identical loads.
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wjdashwoodAuthor Commented:
What does greater hysteresis loss mean? I thought hysteresis was the behaviour of say a thermostat when it turns on a one temperature but turns off at another so that it doesn't fluctuate when around the threshold temperature.
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jhanceCommented:
Hysteresis is a behavioral property of many systems not limited to thermostats but you are on the right track...

Hysteresis occurs in MAGNETIC materials (like iron or ferrite) where it takes a magnetization above a certain threshold to get the magnetic field in the material moving in a certain direction.  The same with getting it going the other way.  The amount of additional "push" it needs is related to the properties of the material but also the AMOUNT of the material.

In a power supply of the switching variety (lke your computer power supply) there is an output inductor with a ferrite core.  Such an inductor is sized based on the maximum power output of the supply.  Bigger output bigger inductor.  Bigger inductor, more ferrite.  More ferrite more hysteresis.

The energy used to overcome the hysteresis is lost to the power supply and comes out of the system as heat generated in the ferrite of the inductor.
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wjdashwoodAuthor Commented:
An excellent explanation! Truly raised the intellect level there for a brief moment ;) and well deserving of some points. In fact, an increase is due me thinks.

Thanks for all the help.
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Adrian DobrotaNetworking EngineerCommented:
thank you jhance for completing this Q.
I tried to explain in the easy way why the amount of energy loss should be equal. I stated that there actually IS a difference , and considered it insignificant.
In the given conditions 10% seems to me in an acceptable range , however I personally doubt it's 10%, but there's no need to contradict since a calculation on this loss can be made only with detailed technical specifications, so , that'ss just an appraisal ...
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jhanceCommented:
kronos,

Agreed that 10% is just a "WAG".  It's likely in that range on an "order of magnitude" level.  I'm sure that 1% is way too low, 100% way too high.
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Adrian DobrotaNetworking EngineerCommented:
:-)  
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