Hi John, I have assumed you are referring to electronic circuits on PCBs. and if this is not correct, you'd do well to stop here.
You will probably get much better responses to your query than this one, but for what it is worth, here it is. It is possible to do the calculations you seek, but is an exceedingly difficult and very involving task, usually incorporating a lot of prior knowledge of product behaviour. While it is generally possible to get a fairly accurate calculation of the temperature rise of a device based on its thermal impedances and power dissipation, t is actually a very difficult thing trying to determine by sheer calculation what the temperature rise will be inside an enclosure because of the manner in which the materials, ambient conditions and thermal properties all interact. The closest you can get to a reasonably accurate prediction is to do a simulation. Commercial packages are available for thermal simulations, examples of which include Flotherm, but even these will require quite a lot of data, some of which may not all be available at the design stage and educated guesses will have to be made. A Web search on 'thermal management' should yield quite a few results. This power is then translated to an actual temperature rise on the surface of the device, this itself being a function of the device packaging and mounting. Most data sheets will give you the thermal impedances of the devices and some - e.g. for microprocessors, will give you the calculations and/or equivalent circuits. Some will go as far as suggesting an ideal layout for the best heat transfer. Even this is just rough, because for example, where a ceramic power resistor is mounted so that it stands off the PCB, the device tends to trap air under it and can raise the temperature of this air quite considerably, so much so that nearby devices such as semiconductors, are forced to be operating at ambients in excess of rating. Simulation itself requires that you have a good idea how your PCB is laid out, number of layers, thicknesses, percentage Cu:FR4 etc because the copper and PCB material all play a part in the thermal behaviour of the product. At design stage, this information is usually non-existent and it is customary to target the most important part of the product (from a thermal viewpoint) and to determine the heatsinking required to keep this at an acceptable temperature as far as the rest of the circuitry is concerned. When all this is done, you then have to move to the 3D picture because the orientation of PCBs, stacking, component proximity, etc can have a profound effect on the thermal characteristics of the whole enclosure because they govern airflow. Air inside a sealed enclosure tends *not* to flow, so it is quite possible to develop air-pockets at some locations and these can be at higher temperatures than others! I have picked up air temperature gradients of about 10?C in the same enclosure! For this reason - and others - it sometimes becomes necessary to fill or partly fill the sealed enclosure with thermally conductive potting compound so that the innards of the product have some means of conducting ( as opposed to radiating and convecting) to the outside world. The enclosure itself has a part to play. The shape, colour, size and material all matter as I am sure you are well aware. Also, if you will be relying on radiating to the enclosure to effect your cooling, then the proximity of the components becomes critical. Also note that radiating to the enclosure usually involves radiating to the ambient air first and this may not be desirable. It sometimes becomes necessary to ensure that all devices dissipating a lot (relative terms) of power have some sort of direct contact with a cooling surface which itself is able to lose heat to its environment. This can often mean undesirable changes to preferred layouts when production, EMC, rework and similar are taken into consideration. Should you find that you need ventilation, then the location, size and orientation of the vents themselves becomes a fairly involving process requiring some pretty good educated guesses and quite often some experimental work, especially if no forced cooling (fans) is employed. Here again, simulation is probably best. Should you decide that only heat-sinking is required, then that is another matter, the complexity of which depends on the nature of the product and already captured in part in my explanation of the potting above. Some heat sinks require very specially designed and intricate forms, and the shape of these, fin size, thickness and orientation can usually be another simulation exercise! Regards - Chris Chileshe - Ultronics Ltd - Cheltenham, UK -----Original Message----- From: Fee John [SMTP:[email protected]] Sent: Tuesday, February 06, 2001 4:37 PM To: Newsgroup (E-mail) Subject: Ventilation calculations Hi all, I have been asked if there is a way of calculating at the design stage whether a product needs ventilation openings to keep the product cool. I am not aware if there is a method of calculating this using variables such as power dissipation, component ratings, volume etc. If anyone can point me to a source of information I would be grateful. Thanks John Fee National Electronics Technology Centre Enterprise Ireland Phone +353-1-8082214 Fax +353-1-8370705 email [email protected] ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected] ________________________________________________________________________ This message has been checked for all known viruses, by Star Internet, delivered through the MessageLabs Virus Control Centre. For further information visit: http://www.star.net.uk/stats.asp ________________________________________________________________________ This message has been checked for all known viruses, by Star Internet, delivered through the MessageLabs Virus Control Centre. For further information visit: http://www.star.net.uk/stats.asp ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected]
