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A switching MOSFET dissipates power in three ways. Conduction loss is I²R heating while it is fully on. Switching loss happens during the brief moments it turns on and off, when voltage and current overlap. Gate-drive loss is the energy to charge and discharge the gate each cycle (dissipated mostly in the driver).
| Loss | Formula | Grows with |
|---|---|---|
| Conduction | Iload² × RDS(on) × D | current², RDS(on), duty |
| Switching | ½ VDS Iload (tr+tf) fsw | voltage, current, frequency |
| Gate drive | Qg × VGS × fsw | gate charge, frequency |
| Junction temp | TA + Ptotal × θJA | total loss, thermal resistance |
At low frequency conduction loss dominates (choose a low RDS(on) part); at high frequency and high voltage, switching loss dominates (choose a fast part with low gate charge).
Three: conduction loss (I²R while fully on), switching loss (during the turn-on/turn-off transitions when voltage and current overlap), and gate-drive loss (energy to charge the gate each cycle).
The I²×RDS(on) heating while the MOSFET is fully on, scaled by the duty cycle for a switch. It grows with the square of current, so it dominates in high-current, low-frequency circuits.
The power dissipated during the short turn-on and turn-off transitions, when the device has both voltage across it and current through it. It is proportional to voltage, current, transition time, and switching frequency.
The energy Qg×VGS needed to charge and discharge the gate each cycle, times the frequency. Most of it is dissipated in the gate driver, not the MOSFET, but it still costs system efficiency.
Lower RDS(on) directly reduces conduction loss. But very low-RDS(on) parts have larger gate charge, which raises switching and gate losses — so the best choice depends on your frequency and current.
Each on/off transition dissipates a fixed packet of energy. The more times per second you switch, the more of those packets you pay for, so switching loss rises linearly with frequency.
They are how long the drain voltage/current take to transition during switching, from the datasheet. Longer transitions mean more voltage-current overlap and higher switching loss.
Lower RDS(on) for conduction loss; pick a faster, lower-gate-charge device and drive the gate harder for switching loss; reduce frequency; or use soft-switching (ZVS/ZCS) and wide-bandgap (SiC/GaN) devices for high-voltage designs.
The total charge needed to fully turn the gate on, in nanocoulombs. It sets the gate-drive loss and how fast you can switch for a given driver current.
Yes — it rises significantly as the junction heats up (often 1.5–2× at 125 °C). For accurate conduction loss, use the RDS(on) value at your expected operating temperature.
Conduction dominates at high current and low frequency; switching dominates at high voltage and high frequency. Balancing the two is the heart of converter MOSFET selection.
θJA is the junction-to-ambient thermal resistance in °C/W. Multiply it by the total loss to get the temperature rise above ambient. A heat sink lowers the effective value — see our Heat Sink Calculator.
Compute the junction temperature here. If it exceeds ~110–125 °C (leave margin below the datasheet max), add a heat sink or improve airflow, or pick a lower-loss device.
A MOSFET has resistive conduction loss (I²R), so loss falls at low current; an IGBT has a roughly fixed voltage drop, so it is better at high current/high voltage but has more turn-off (tail) loss.
It uses the standard first-order model and is excellent for device selection and thermal sizing. For final designs, also account for reverse-recovery, output-capacitance (Coss) loss, and temperature-dependent RDS(on).
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