MOSFET Power Dissipation Calculator

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Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

RDS(on): The drain-to-source resistance of the MOSFET when it is fully turned on. Lower RDS(on) usually means less heat at the same current.

On time: In PWM mode, the percent of each switching cycle when the MOSFET is on.

Rise plus fall time: The total transition time used for the simple switching loss estimate. Longer transitions usually make more heat.

Coss: Output capacitance from the MOSFET drain-to-source region. Charging and discharging it can add switching loss.

Qg: Total gate charge. It estimates gate-drive power, which is shown separately from MOSFET drain heat.

Thermal resistance junction-to-air: The temperature rise from the MOSFET chip to nearby air per watt of heat, in deg C/W.

Temperature margin: Max junction temperature minus Estimated junction temperature. A negative value means the estimate is over the entered limit.

Mistake: Entering the full load current for Load current while on when only part of the current flows through this MOSFET.
Fix: Use the current through this one MOSFET while it is on, not the total system current.

Mistake: Using a room-temperature On-resistance RDS(on) without adjusting for heating.
Fix: Enter the datasheet hot value, or use Hot on-resistance multiplier to raise the resistance.

Mistake: Mixing up On time with off time in PWM switch mode.
Fix: Enter the percent of each cycle that this MOSFET is on in On time.

Mistake: Counting Gate-drive power as part of Estimated MOSFET heat.
Fix: Keep Gate-drive power separate unless your specific layout makes that driver loss heat the MOSFET package.

Mistake: Entering Thermal resistance junction-to-air from a datasheet test board and expecting the same board temperature.
Fix: Use a value that matches your copper area, airflow, and package, or treat Thermal check as a first estimate.

• This is a first-order estimate for a hard-switched MOSFET. It does not replace a datasheet safe-operating-area check, lab temperature test, or circuit simulation.
• DC mode ignores On time, Voltage during switching, Switching rate, Rise plus fall time, Coss, and Qg. Hidden PWM fields do not affect the DC result.
• Turn-on and turn-off loss uses a triangle overlap estimate. Real waveforms, gate resistance, driver strength, Miller plateau behavior, and layout parasitics can change the result.
• Output-capacitance loss uses the entered Coss as a fixed value. Real Coss changes with voltage, so the estimate can be high or low.
• Gate-drive power is shown separately and is not added to Estimated MOSFET heat. In some packages or layouts, part of that energy may still heat nearby parts.
• Thermal resistance junction-to-air depends strongly on copper area, airflow, package mounting, and nearby hot parts. A pass result still needs a real temperature check.
• The calculator does not include avalanche energy, reverse recovery, body-diode dead-time loss, pulsed transient thermal impedance, current sharing, or switching-node ringing.

Calculation steps

The calculator first converts the entered On-resistance RDS(on) from milliohms to ohms and applies the Hot on-resistance multiplier.

R_hot_ohm = (rds_on_mohm / 1000) * rds_temp_factor

It then sets the on-time fraction. DC mode uses a full on-time fraction of 1. PWM mode uses the entered On time percent.

D = 1 for dc mode; D = duty_percent / 100 for pwm mode

On-resistance loss is current squared times hot resistance times the on-time fraction.

P_cond = load_current_a^2 * R_hot_ohm * D

In PWM mode, turn-on and turn-off loss uses a simple voltage-current overlap estimate with a 0.5 factor for the triangular transition shape [1].

P_sw = 0.5 * switch_voltage_v * load_current_a * (rise_fall_ns * 1e-9) * (switch_freq_khz * 1000)

If Output capacitance Coss is entered, the calculator adds output-capacitance switching loss using energy equals one-half capacitance times voltage squared, repeated at the switching rate [3].

P_coss = 0.5 * (coss_pf * 1e-12) * switch_voltage_v^2 * (switch_freq_khz * 1000)

If Total gate charge Qg is entered, the calculator estimates gate-drive power, but it does not add that value to MOSFET heat.

P_gate = (gate_charge_nc * 1e-9) * gate_drive_v * (switch_freq_khz * 1000)

The main heat result is the sum of drain-path heat terms only.

P_heat = P_cond + P_sw + P_coss

When Thermal resistance junction-to-air is entered, estimated junction temperature is nearby air temperature plus heat times thermal resistance. Temperature margin is the entered maximum junction temperature minus that estimate.

T_j = ambient_temp_c + P_heat * theta_ja_c_per_w

margin = max_junction_temp_c - T_j

Mini-example

For a PWM switch with 8 A load current, 15 milliohms RDS(on), 1.2 hot multiplier, 40% on time, 48 V switching voltage, 100 kHz switching rate, 60 ns rise plus fall time, 500 pF Coss, and 30 deg C/W thermal resistance at 40 deg C air, the calculator gets 0.4608 W on-resistance loss, 1.152 W turn-on and turn-off loss, 0.0576 W output-capacitance loss, and 1.6704 W Estimated MOSFET heat. The estimated junction temperature is 90.112 deg C, so the margin to a 150 deg C limit is 59.888 deg C.

Important calculation choices

All numeric parsing uses the units shown in the labels: milliohms, kHz, ns, nC, pF, volts, amps, watts, and deg C. Optional fields are only included when entered. Gate-drive power is separated because it is usually handled by the driver path, while Thermal check uses Estimated MOSFET heat.