Switching and conduction losses
Where the watts actually go inside a power module.
- Explain switching and conduction losses at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real drive system
Overview
Every semiconductor loss falls into one of two buckets: loss while it is fully on, and loss during the brief moments it is turning on or off. Getting the balance right across the whole speed range is the art of inverter design.
In depth
Conduction loss depends on the device's on-state characteristic and the RMS current flowing through it while conducting.
Switching loss scales with how often you switch and the energy lost per transition, which itself scales with the voltage and current at the moment of switching.
Power Loss Calculator
Copper, iron and switching losses in a drive unit.
P_loss = I²R + k_iron × f + switching lossesCopper loss scales with the square of current and dominates at high torque. Iron loss scales roughly with frequency and dominates at high speed. Inverter switching loss scales with switching frequency and device energy.
Example: 300 A through 12 mΩ gives 1,080 W of copper loss alone.
- Halving current quarters copper loss — one reason 800 V systems are efficient.
- Silicon carbide devices cut switching loss by roughly half versus IGBTs.
- Every watt of loss is a watt the cooling system must remove.
Key takeaways
- Conduction loss depends on current and the device's on-state characteristic.
- Switching loss scales with frequency, voltage and current at each transition.
- Choosing carrier frequency is always a system-level trade-off, not a device-level one.
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