Contents
How is thermal resistance calculated?
Thermal resistance is a heat property and a measurement of a temperature difference by which an object or material resists a heat flow. Thermal resistance is the reciprocal of thermal conductance. It is the thermal resistance of unit area of a material. In terms of insulation, it is measured by the R-value.
What is junction and ambient temperature?
The ambient temperature refers to the temperature of the surrounding environment (typically air) when the device is powered. The junction temperature refers to the temperature of the silicon die within the package of the device when the device is powered.
Is a higher thermal resistance better?
What is thermal resistance? Thermal resistance is calculated by dividing the thickness of the material by its thermal conductivity, giving an R-value specific to that thickness. As a rule of thumb, the higher the thermal resistance the better, because there is a greater resistance to heat transfer.
How is Junction to ambient thermal resistance determined?
The overall junction-to-ambient thermal resistance (θ JA) of the system is given by: θ CS determines how easily the heat can flow from the package surface to the base of the heat sink. Due to the unevenness and irregularities of the package and heatsink surfaces, an air gap can be formed between the two surfaces.
How is the junction temperature of a heat sink calculated?
The ambient temperature shall be 25 [deg.C]. Junction temperature is calculated by using the above thermal resistance. For a semiconductor device package used with a heat sink, in order to ensure more accurate calculations, thermal resistance of junction-to-case values should be used instead of thermal resistance of junction-to-ambient values.
How to calculate the thermal resistance of a heat sink?
The following equation can be used to calculate θ CS from the TIM properties: θ SA specifies the thermal resistance from the base of the heat sink to the ambient environment. This can be found in the heat sink datasheet. For example, the graph below shows that Aavid ML26AAG has a thermal resistance of 10°C/W at 200 feet per minute air velocity.
When to use thermal resistance instead of thermal resistance?
For a semiconductor device package used with a heat sink, in order to ensure more accurate calculations, thermal resistance of junction-to-case values should be used instead of thermal resistance of junction-to-ambient values. In this example, it is calculable by the same formula as above by using P C values @ T c = 25 [deg.C].