MOSFET Power Dissipation Calculator

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) × D

Where:

• I_D = drain current in amperes
• R_DS(on) = on-state drain-source resistance (from datasheet)
• D = duty cycle, the fraction of each switching cycle the device spends on

This loss is proportional to the square of current, so doubling the current quadruples the conduction loss.

The duty cycle term gets left out of a surprising number of calculators, and it matters. A MOSFET in a buck converter running 50% duty spends half its life off, dissipating nothing, so its conduction loss is half what the bare I²R figure suggests. Leave duty at 100% only when the device is genuinely held on, as in a load switch or an ideal-diode controller.

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

Where:

• V_DS = drain-source voltage (supply voltage)
• f_sw = switching frequency in Hz

3. Gate drive loss, which is a real cost but not the MOSFET’s problem:

P_gate = Q_g × V_GS × f_sw

Q_g is the total gate charge from the datasheet, in nanocoulombs. Every switching cycle you push that charge into the gate at V_GS and then dump it to ground, and all of it ends up as heat.

Here is the part that gets lumped in wrongly almost everywhere: that heat comes out in the gate driver IC and the gate resistor, not in the MOSFET channel. It loads your supply and it can cook a small driver, but it does not raise the MOSFET junction temperature, so it does not belong in the P_total you feed into the thermal calculation. The calculator prints it on its own line for exactly that reason.

Total Power Dissipation

P_total = P_cond + P_sw

R_DS(on) Temperature Coefficient

R_DS(on) climbs with temperature, and it roughly doubles between 25°C and 150°C junction temperature.
For a realistic hot-running estimate, multiply the room-temperature R_DS(on) figure by 1.5 to 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 to 175°C), a heat sink is required.

Design Guidelines