Hi guys,

Does anyone have experience in selecting fans for cooling electronic systems?

I have read several online guides from the various manufacturers but there is something that remains unclear to me. They all have this formula where you can calculate the minimum required air flow created by the cooling fans. This calculation is based on the total heating power of the system and a parameter called "temperature rise".

Well I don't actually understand what this "temperature rise" parameter means or how could I set a limit for it. It seems to be the allowed temperature rise inside the enclosure in comparison to the ambient temperature. But this is somewhat confusing to me. I mean, I have the system design specification that my electronics should work at a max ambient temperature of 70 degC. So what about the temperature rise?

Could anyone help in this?

Thanks,
Nikos

 

Can you give a link of one of the online guides you are referring to?
Do you want to cool a PCB with electronic components? Or something like a heat sink with power semiconductors covering the base plate?

 

Basically, there is

dT [K] = Pv [W] * Rth [K/W]

dT ... temperature rise from ambient to maximum acceptable surface temperature of component
"ambient" is the temerature of the air which enters the cabinet or or with the electronics inside
Internally, the components will be hotter since it is about surface temperature

Pv ... thermal losses of the electronics components

Rth ... thermal resistance
In case you blow air, there is convective cooling and we can write
Rth [K/W] = 1/(Asurf [m2] * h [W/Km2])

Asurf ... surface of the components to be cooled
h ... heat transfer coefficient: typ. 5-8 in natural convection, 10-20 and higher for forced convection (that means a fan is blowing), much higher if heat sink or water cooling is applied

 

Can you give a link of one of the online guides you are referring to?
Do you want to cool a PCB with electronic components? Or something like a heat sink with power semiconductors covering the base plate?

So for example:
http://www.sunon.com/uFiles/file/03_products/07-Technology/004.pdf and
http://www.etrinet.com/tech/pdf/selection.pdf

Other manufacturers also have similar guides.

However I don't understand your second question! I need to cool a pcb that contains electronic components (like processors and FPGAs), with and without heat sink, and also PoL DC-DC converters.

Basically, there is

dT [K] = Pv [W] * Rth [K/W]

dT ... temperature rise from ambient to maximum acceptable surface temperature of component
"ambient" is the temerature of the air which enters the cabinet or or with the electronics inside
Internally, the components will be hotter since it is about surface temperature

Pv ... thermal losses of the electronics components

Rth ... thermal resistance
In case you blow air, there is convective cooling and we can write
Rth [K/W] = 1/(Asurf [m2] * h [W/Km2])

Asurf ... surface of the components to be cooled
h ... heat transfer coefficient: typ. 5-8 in natural convection, 10-20 and higher for forced convection (that means a fan is blowing), much higher if heat sink or water cooling is applied

So you actually mean that the temperature rise is meant between the components' (heat sources) surfaces and the ambient air outside the enclosure (not heated by the heat sources of the system)?

 

Ok, now it is clear what you want to cool.
Yes, temperature rise is related to the component surface and the temperature of the incoming air (which is typically outside of the enclosure).

 

Typically, ambient in industrial environments is 40°C (or 50°C), and maximum semiconductor junction temperatures should be below 110°C to get some lifetime. Some components (like e.g. elkos) like a little bit lower temperatures. So 70° as average component surface temperature should be sufficient. Then, in this little example, temperature rise is dT=70-40=30K

 

Start with three basic pieces of information:
1. The maximum ambient air temperature around the device. This is your source of cooling air. Colder is better, but the worst case is its warmest temperature. For example, if your device is going in a car that could be sitting out in the Arizona sun, then the max ambient could be +50C or +122F. If it never leaves an office environment, then the max ambient might be only +25C or +77F.

2. The maximum power being dissipated by the device. If the cooling fans are intake fans blowing on the electronics, remember to include the fan motor power in the total because the motor adds heat to the incoming air. If the fans are exhaust fans sucking air through the system, like the power supply fans in most desktop pc's, then the fan motor heat does not enter the system and can be ignored. Fan power does come from the system power supply, and raises the power supply temperature a little bit.

3. The maximum allowable temperature of the internal electronics. This is the hard part, because there is no universal standard for semiconductor data sheets. Usually the larger and more complex parts are the most delicate. Also, some parts come with several possible temperature ratings, often coded into the last few letters or numbers in the part number. You might see a memory chip rated for 70C next to an I/O driver rated for +125C. You usually can ignore resistors and most capacitors. Large aluminum electrolytics should be checked, as they have all kinds of temperature ratings.

The difference between #1 and #3 above is the max allowable temperature rise, or delta-T. The formula is this:

minimum air flow in cubic feet per minute (CFM) = 3.16 x power in watts / delta-T (degreesF)
or 1.755 x W / delta-T (degreesC)

This gives the bare minimum amount of airflow needed to cool the system. The problem with this number is that it is based on the specific heat of air, and assumes that 100% of the air molecules come in contact with 100% of the electronic components. In the real world this is not true, so the result has to be adjusted upward. If you know the pressure drop through the system there is a more detailed equation for a better first result, but that number still needs adjusting. If there are no big restrictions to the air flow path, then you probably can start with 2x or 3x the base number. Also, the formula is based on low altitude. At 10,000 ft in the Rockies, you'll need more air.

Example:
Office environment, but the device might be sitting near a window (sunlight heating) - max ambient = +30C
Device has an intake fan, 12 VDC, 0.8 A (starting guess at fan power) = 10 W
Device has a 200 W power supply making about 90 W into the electronics (about 70% efficient) = 90 W (electronics) + 10 W (fan) + 40 W (power supply heat) = 150 W
Max internal temperature = +50C (delicate A/D converter)
CFM = (1.76 x 150) / 20 = 13.2 CFM minimum airflow
13.2 x 3 = approx 40 CFM, a reasonable fan size for this system, and available with a 5 W motor. Smaller fans turn faster and make more acoustic noise. Larger fans are more quiet and last longer, but cost more.

ak