Thermal Resistance (Rth, °C/W)
The quantity showing how many degrees of temperature rise a given power loss turns into; its unit is °C/W. On a path with a thermal resistance of 2 °C/W, a 5 W loss creates a 10 °C temperature difference. The path from the semiconductor to the ambient is calculated like a chain of series resistances: junction-to-case, case-to-sink and sink-to-ambient.
Thermal resistance maps one to one onto an electrical circuit: power loss corresponds to current, temperature difference to voltage and thermal resistance to resistance. The temperature rise is simply the product of the lost power and the total thermal resistance.
- The links of the chain: From junction to case (the semiconductor's own structure), from case to sink (the contact surface and the interface material), from sink to ambient (surface area and air flow).
- The weakest link: Since the chain is in series, the largest term determines the total resistance. A dry and uneven contact surface can easily wipe out the effect of the best heatsink.
- Example: With 4 °C/W total and 8 W loss, in a 40 °C ambient the junction rises to 40 + 32 = 72 °C.
This calculation is the basic tool that determines what current a driver can carry continuously at a given ambient temperature. The temperature inside a cabinet is typically 10–20 °C higher than the outside ambient; you must put the in-cabinet value into the calculation.
The context in which this term is used: DC Motor Driver Selection Guide