Thermal Derating
Reducing the current a component can safely carry as the ambient temperature rises. Catalogue values are usually given for a 25 °C reference; inside a panel at 50–60 °C the same current does not provide the same safety margin. In addition, because the resistance of copper and semiconductors increases with temperature, the loss grows at the same current.
The physics of derating is simple: the junction temperature is the ambient temperature plus the loss multiplied by the thermal resistance. When the ambient rises, to stay within the same junction limit you have to reduce the loss, and therefore the current.
- Double effect: In a hot environment the starting point is higher and, because the MOSFET channel resistance increases, more loss is produced at the same current. The two effects feed each other.
- Conductor side: The resistance of copper increases by roughly 27–33% at around 90–105 °C compared with room temperature; the voltage drop and the heating on the cable grow by the same proportion.
- Application: Selecting right at the limit using 25 °C figures comes back as thermal shutdown or shortened life in a closed panel during the summer months.
The data for AXI products does not include an operating temperature range or a derating curve. For that reason, when choosing current it is a sound approach to leave a clear safety margin above the continuous current you have calculated.
Where this term is used: DC Motor Driver Selection Guide