How to read datasheets
A silicon power MOSFET's datasheet gives RDS(on) = 10 mΩ (Tj = 25 °C, VGS = 10 V). Which is the correct approach when using it at around Tj = 125 °C?
- ASemiconductor resistance falls as temperature rises, so estimating with the 25 °C value errs on the safe side
- BRDS(on) increases at higher temperature, so correct it to the high-temperature value using the temperature characteristic graph and estimate the loss
- CRDS(on) hardly depends on temperature, so the table value can be used as is to calculate conduction loss
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Answer: B. RDS(on) increases at higher temperature, so correct it to the high-temperature value using the temperature characteristic graph and estimate the loss
The RDS(on) of a silicon power MOSFET increases with temperature because carrier mobility decreases; at 125 °C it is typically about 1.5 to 2 times the 25 °C value. Datasheets include a graph of normalized RDS(on) versus temperature, with the 25 °C value as 1. Calculating conduction loss I²R with the 25 °C value underestimates the loss and overlooks the vicious circle in which heating raises the resistance further. This positive temperature coefficient also has the advantage of tending to equalize current among devices connected in parallel.