When is a fan needed to cool a FET?
Hello,
I beg to know what is the maximum dissipation (Watts) of a TO220 or TO247 Mosfet attached to a heatsink of any size , where a fan is NOT needed?
In other words, what is the dissipation level in a mosfet, where any size increase of heatsink provides no further cooling improvement , and a fan becomes necessary?
I am speaking of a mosfet in a well ventilated enclosure with 30 degrees C ambient.
Comments
Thanks Spock but your reply shocks me........
Please see the five heatsinks on pgae 4 of RDR203......
http://www.powerint.com/sites/default/files/PDFFiles/rdr203.pdf
....surely a TO220 stuck to that kind of heatsink could dissipate 40W+ without a fan being needed.
I am speaking of a general domestic product such as a guitar amplifier in 30degC ambient.
Well, be prepared to be shocked then... Don't say I did'nt warn ya!
If you think about it, its actually a very simple calculation. The datasheet for those heatsinks in RD203 describe the thermal resistance to be around 13 deg C/Watt with no forced air cooling.
Given that this device is processing about 54 Watts (which is where the thermal tests were performed)and assuming an efficiency of around 76%.... It leaves us with around 4.5 W to be dissipated in the device. With 13 deg C/W and 4.5 W dissipation we get 13 * 4.5 = 59 deg C temp rise which is approximately what was observed on that board.
This is very difficult to answer because its a very subjective topic. It depends on your thermal environment, air flow path, acceptable temperature rise etc.
In general you can use flat customised aluminium sheets you can probably getaway with 2 or 2.5 Watts max.
If you use external Al extruded heatsinks and get away without forced air cooling for 4 W maybe 5 Watts dissipation within the device. With forced air cooling you can now afford to have much more dissipation within the MOSFET (even 13-15 Watts possible)