MOSFET Power Dissipation Calculator
Calculate MOSFET conduction and switching power loss from drain current, R_DS(on), switching frequency, and gate charge.
MOSFET Power Loss Analysis
MOSFETs are used as switches in power electronics, motor drivers, and DC-DC converters. Understanding their power dissipation is essential for thermal design and preventing device failure.
Two Types of Power Loss
1. Conduction Loss — heat generated while the MOSFET is fully on:
P_cond = I_D² × R_DS(on)
Where:
- I_D = drain current in amperes
- R_DS(on) = on-state drain-source resistance (from datasheet)
This loss is proportional to the square of current — doubling the current quadruples the conduction loss.
2. Switching Loss — heat generated during the transition between on and off states:
P_sw = 0.5 × V_DS × I_D × (t_rise + t_fall) × f_sw
Or using gate charge method:
P_gate = Q_g × V_GS × f_sw
Where:
- V_DS = drain-source voltage (supply voltage)
- f_sw = switching frequency in Hz
- Q_g = total gate charge from datasheet
Total Power Dissipation
P_total = P_cond + P_sw
R_DS(on) Temperature Coefficient
R_DS(on) increases with temperature — typically doubling between 25°C and 150°C junction temperature. For accurate high-temperature estimates, multiply the room-temperature R_DS(on) value by 1.5–2.0.
Thermal Resistance and Heat Sink
Once you know P_total, check the junction temperature:
T_junction = T_ambient + P_total × R_θJA
Where R_θJA is the thermal resistance junction-to-ambient (from the datasheet). If T_junction exceeds the maximum rated temperature (usually 150–175°C), a heat sink is required.
Design Guidelines
| Parameter | Guideline |
|---|---|
| Conduction loss | Minimize R_DS(on) — choose a lower-R_on device or parallel multiple MOSFETs |
| Switching loss | Minimize gate charge Q_g — choose a faster device, or reduce f_sw |
| Safe operating area | Always stay within the SOA curve in the datasheet |
| Derating | Use MOSFETs at ≤ 80% of max voltage and current ratings |