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I've been wondering recently about why PSUs have fans. Some PSUs (like the one here: http://www.newegg.com/Product/Product.aspx?Item=N82E16817151122) don't, and they're only marginally more efficient than the ones that do. At 50% of the maximum power draw, a platinum PSU is only 7% more efficient than a bronze PSU. In the case of the unit I linked to, that's equivalent to 18.2 W. What this really means is that a bronze, fanned PSU must use forced convection to dissipate 18.2 W more than the platinum, fanless PSU dissipates with only natural convection.

I find it difficult to believe that a bronze unit couldn't operate at the slightly (only slightly) higher temperature resulting from the additional 18.2 W is must dissipate due to increased inefficiency (relative to the platinum unit). Either the platinum PSU is intended to operate at a higher temperature, and is built with higher quality components that can tolerate elevated temperatures for extended periods of time, or fanned, bronze (and higher) units don't really need fans at all. I've noted that the fanless units usually are better vented, which allows for more natural convection; if this is the only difference, though, then it would be pretty easy to use better venting on less efficient units too.

One other note: this is all approximate, because in a case, there would be a fair amount of forced convection due to other fans in the case, so really no PSU needs to dissipate heat using only natural convection.

I suppose I'm looking for comments you might have. I don't think there's anything wrong with the analysis, but if there is, please let me know.

PSUs have fans because there was once a time when the only fans moving air out of the case WERE the PSU fans.

I run my power supply with "Hybrid Silent Fan Control" enabled. It turns off the fan when it is lightly loaded and only spins it up when the components inside the PSU need cooling.
http://www.seasonicusa.com/Platinum_Series_XP2.htm

Captain Ned wrote:

PSUs have fans because there was once a time when the only fans moving air out of the case WERE the PSU fans.

If that's the case, then is there any good reason for modern PSUs to have fans? Maybe very high-power (~1000 W) ones need them, but it seems that for more reasonable PSUs, the fans are vestigial.

A PSU fan is required by the ATX specification for cooling of components in the case, as Captain Ned described.

JustAnEngineer wrote:

A PSU fan is required by the ATX specification for cooling of components in the case, as Captain Ned described.

I think that reason goes out the door the last few years with all the newer ATX cases adding in a intake grill in the bottom of the case for the PSU fan. If you mount the PSU with the fan intake down, using the case grill, its not venting anything from the case. Then again my 600T has plenty of exhaust in the rear and top of the case. If that isn't enough I can use the side panel grill that came with the case and add in four more fans as either intake or exhaust.

But where that reason does still make sense is with the trend of smaller and smaller cases that just do not have room for a lot of fans. Then yes, using the PSU as a case exhaust port makes sense again.

From what I can tell, Fanless PSUs (often):

1. Have higher quality components rated to higher temperatures.
2. Have (marginally) better efficiency than regular PSUs
3. Have bigger heatsinks than regular PSUs
4. Have more open airflow than regular PSUs.
5. Sometimes, are based on more advanced technology like DC-DC units

Granted, many PSU makers seem to be either overly conservative about fanspeeds (resulting in more noise) or will nickel and dime the internal heatsinks down to the last erg of material (resulting in more airflow needed to carry away waste heat). Part of the conservcatism with fanspeed may itself be a cost-cutting feature - cut back on monitoring, feedback and control systems, design for greater variances in component performance (VRMs, heatsinks, fan RPMs) etc.

Voldenuit wrote:

From what I can tell, Fanless PSUs (often):

1. Have higher quality components rated to higher temperatures.
2. Have (marginally) better efficiency than regular PSUs
3. Have bigger heatsinks than regular PSUs
4. Have more open airflow than regular PSUs.
5. Sometimes, are based on more advanced technology like DC-DC units

This ^

i think cooling is very required to control the humidity of psu......

think cooling is very required to control the humidity of psu......

ahhhh.... no.

Stepping 120V(or 240v in other parts of the world) down to 12v is always going to generate some waste heat.

While PSU have gone from 70% to 92% efficiency in the last 15 or so years, the components inside are still heat sensitive.

Excessive thermal build up will destroy, and the higher the temperature, the faster the destruction in a non-linear acceleration.

Even the highest quality capacitors are only rated for 50C use.

So, the answer to your question is that cooling is required, and a fan is still the simplest and cheapest method.

PS...you ever wonder why Apple products are sealed and cannot have the battery replaced?
When you make things insanely thin and without fans, they are going to fail.
Making everything fail at about the same time makes sense to keep the consumer addicted.
After2, 3 years, your warranty is gone, your battery is dead, and the thermal load most likely has worn everything out so that attempting to replace the battery is pointless anyway...

Time to buy the latest and greatest Right?
Thank goodness they planned this out to give you the justification to upgrade without guilt.

PSS

Just FYI, my calculator, HP15, form 1982 still works.