Thermal (heat) dissipation: Power dissipation performance must be well understood to ensure that any given device is operated within its defined temperature limits. When a device is running, it consumes electrical energy that is transformed into heat. Most of the heat is typically generated by switching devices like MOSFETs, ICs, etc. As power consumption increases, components like linear voltage regulators can heat up during normal operation. Some heat is okay, however when things get too hot, the performance of the linear regulator suffers.
The ambient air temperature TA for cooling the devices depends on the operating environment in which the component is expected to be used. Typically, it ranges from 35°C to 45°C, if the external airflow through a fan is used and from 50°C to 60°C, if the component is enclosed. The interface resistance θCS depends mainly on the interface material and its thickness and also on the surface finish, flatness, applied mounting pressure, and contact area. Reliable data can be obtained directly from material manufacturers. With all the parameters defined, θSA becomes the required maximum thermal resistance of a heatsink for the application. In other words, the thermal resistance value of a chosen heatsink for the application has to be equal to or less than the previous θSA value for the junction temperature to be maintained at or below that specified .
The following are the various important parameters in selecting a heatsink.
1. Thermal resistance θSA
2. Airflow
3. Volumetric resistance
4. Fin density
5. Fin spacing
6. Width
7. Length
The thermal resistance is one parameter that changes dynamically depending on the airflow available. Airflow is typically measured in linear feet per minute (LFM) or CFM (cubic feet per minute). LFM is a measure of velocity, whereas CFM is a measure of volume. Typically, fan manufacturers use CFM because fans are rated according to the quantity of air it can move. Velocity (speed) is more meaningful for heat removal at the board level, which is why the derating curves provided by most power converter manufacturers use this. Typically, airflow is either classified as natural or forced convection. Natural convection is a condition with no external induced flow and heat transfer depends on the air surrounding the heatsink. The effect of radiation heat transfer is very important in natural convection, as it can be responsible for approximately 25% of the total heat dissipation. Unless the component is facing a hotter surface nearby, it is imperative to have the heatsink surfaces painted to enhance radiation. Forced convection occurs when the flow of air is induced by mechanical means, usually a fan or blower.