Thermal conductance and resistance

Definition of Thermal resistance

Regarding thermal resistance or thermal resistance in general

Semiconductor components heat up due to the electrical power P. In case of low power (e.g. LED) we do not have to worry about inadmissibly high heating or use of heat sinks. In case of higher power unacceptably high heating must be taken into account already during the development of a circuit.

The heating of an electronic component happens especially at the PN junctions. The maximum junction temperature T_j (junction) is specified by the manufacturer in the data sheet. In most cases, the maximum junction temperature is 150°C. Exceeding the junction temperature destroys the semiconductor.

Thermal resistance indicates how well or poorly a component dissipates heat. The smaller this value, the better the temperature is dissipated and the lower the temperature rise of the component. The calculation formula of the thermal resistance is as follows:

The equation for calculating the thermal resistance is very similar to the Ohm's law. In fact, we can derive the analogy of electrical resistance to thermal resistance:

Comparison Analogie Ohm’s Law and Thermal Resistance

Note: The heat flux q is directly related to the power dissipation P. Since the calculation of the thermal resistance is mostly used in electronics, the power dissipation P was used in the above calculation formula.

Use of heat sinks

Heat sinks are used to dissipate the heat. For very low power, the case is usually sufficient, or the semiconductor is mounted directly on the device case. For higher power, heat sinks are used, usually in combination with a so-called thermal conductive paste.

Thermal Resistance Schematic Ohm’s Law and Thermal Resistance with heatsink

Example

The housing of a transistor heats up to a maximum temperature of 105 °C during operation. The junction temperature is 133 °C and the ambient temperature is 30 °C. The maximum thermal contact resistance RthJC is specified as 40 K/W.

1. Calculate the power loss at the transistor P_L1.
2. Calculate the maximum allowed ambient temperature. The maximum junction temperature is 150 °C. The power loss calculated under point 1 remains the same.
3. In order to improve the maximum power dissipation of the transistor, a heat sink is used.
   The thermal contact resistance RthSG is 0.5 K/W. The heat sink used including thermal paste has a thermal contact resistance of 5 K/W. The ambient temperature TU can again be specified as 30°C.