Thermal Resistance in Electronics

Thermal resistance is the ratio between temperature difference and power dissipated. In electronics, Thermal resistance is a great interest for engineer. It is due to every electronic equipment produce heat and need to be cooled. If they cannot be cooled properly, it will be harmed because of overheat problem.

Thermal resistance can be analogized by electrical resistance. Current represent heat flow, voltages represent temperature differences, and Resistor represent thermal resistance. It can be simplified: R_th=∆T/Q is analogized by R=V/I.

There are several kind of thermal resistance, such as conductive thermal resistance, convective thermal resistance, and spreading thermal resistance. Thermal resistance is basically needed to calculate heat transfer from one point/surface to another point or surface.

In electronic, Thermal resistance is the parameter that informs how effective heat dissipated can be transferred to the ambient. Moreover, it can be used to determine heat source temperature, usually chip junction temperature. In other words, heat source temperature can be known if thermal resistance is already known.

Let’s consider a simple electronic device with heat sink. Heat transfer is started from junction to casing and finished at ambient. This construction results several thermal resistances, those are junction to casing thermal resistance, casing to ambient thermal resistance through heat sink. Total thermal resistance of this system is R_total= (T_j-T_amb)/Q= R_JC+R_CA

Power dissipated from electronic device is assumed that all electricity is converted to heat. Therefore heat dissipation is : Q=VxI, where Q represent heat dissipation, V represent Voltage, and I represent Current. For LED heat dissipation, sometimes optical power from LED is considered, so heat dissipation becomes, Q= P_electrical-P_optical = (V*I)-P_optical.