Power Loss Budget
Calculating and adding up every item in a driver installation that turns into heat: switch conduction loss, switching loss, freewheeling diode loss, shunt loss and cable-terminal losses. The cooling decision comes out of this total; an estimate based on a single item usually fails to explain the real temperature rise.
The way to build the budget is to calculate each item with its own formula:
- Conduction: Current squared times switch resistance. For 15 A and 10 mΩ, roughly 2.25 W per switch; in an H-bridge two switches conduct at the same time.
- Diode: A 0.4–0.7 V drop times current during freewheeling. At 15 A this item alone can reach 6–10 W; that is why synchronous rectification matters.
- Cable: Current squared times the total out-and-back resistance. Five metres of 4 mm² copper is about 44 mΩ for both directions; at 20 A that makes 17 W, and this heat is generated on the cable itself.
- Switching: Directly proportional to frequency; at 20 kHz its share of the total cannot be ignored.
When you multiply the total loss by the thermal resistance you get the expected temperature rise. That number is the only solid basis for deciding whether a heatsink is needed and at what current you can operate.
The context in which this term is used: DC Motor Driver Selection Guide