Every switching MOSFET turns part of its power into heat, and that heat decides whether the part lives for years or fails in weeks. This guide breaks the loss into conduction, switching and gate parts and shows how to reach a safe junction temperature.
A MOSFET loses power while it conducts and every time it switches. We calculate each loss with datasheet values, correct Rds(on) for temperature and check the junction against its limit.
What Is MOSFET Power Loss Calculation?
MOSFET power loss calculation is the process of adding the conduction loss, switching loss and other small losses of a MOSFET to find the total heat it produces in a circuit. That total is then multiplied by the thermal resistance to predict the junction temperature and decide on a heat sink.
P_cond = I² × Rds(on) × D
P_sw = ½ × V × I × (tr + tf) × fsw
P_gate = Qg × Vgs × fsw
Tj = Ta + P_device × Rθ
The device itself works as a voltage controlled switch, as explained in MOSFET working principle. In a perfect world it would have zero resistance when on and switch in zero time, but real parts do neither.
DMC lists conduction, switching, reverse recovery, dead time, gate charge and output capacitance losses in its online tool, which is a good map of every loss term. In most low voltage designs the first two terms dominate, so we start with them.
Conduction Loss: I² × Rds(on)
When the MOSFET is fully on it behaves like a small resistor, so the loss is current squared times Rds(on). DMC notes that this loss rises with temperature, because Rds(on) increases as the device heats up.
In a switching circuit the part conducts only for part of each cycle, so multiply by the duty cycle D. ROHM writes the high side conduction loss of a synchronous buck as I² × R_ON × Vout ÷ Vin, which is the same idea with D = Vout ÷ Vin, as explained in buck converter working principle.
Rds(on) at 25 °C = 10 mΩ
Hot factor at working Tj = 1.5
P_cond = 10² × 0.010 × 1.5 × 0.5 = 0.75 W
In any mosfet power loss calculation, never use the 25 °C datasheet value alone, because the junction is always hotter in operation. Read the normalised Rds(on) versus junction temperature curve in the datasheet and pick the factor at your expected Tj.
Rds(on) of a silicon MOSFET has a positive temperature coefficient, so a hotter part conducts less current. This helps several MOSFETs in parallel share current, unlike bipolar transistors that tend to run away thermally.
Check which gate voltage the datasheet uses for Rds(on). A part rated at 10 V gate drive can show a much higher resistance at 4.5 V, and that change alone can double the conduction loss.
How Do You Calculate MOSFET Switching Loss?
During each turn on and turn off the MOSFET passes through its linear region, where voltage and current are both high at the same moment. All About Circuits points out that the exact calculation is not straightforward because channel resistance changes continuously during the transition, so engineers use the ROHM estimate below.
P_sw = ½ × V × I × (tr + tf) × fsw
V = 24 V, I = 10 A
tr = tf = 20 ns, fsw = 100 kHz
P_sw = 0.5 × 24 × 10 × 40 ns × 100000
P_sw = 0.48 W
Switching loss scales directly with frequency, so the same circuit at 400 kHz would lose 1.92 W in switching alone. This is the main reason a designer cannot raise the frequency freely to shrink inductors, a trade off also seen in linear vs switching regulators.
Rise and fall times depend strongly on the gate driver strength and gate resistor, not only on the MOSFET. A stronger driver shortens the transitions, which is why MOSFET gate driver ICs matter so much at high frequency.
Gate Charge, Output Capacitance and Dead Time Losses
ROHM gives gate charge loss as Qg × Vgs × fsw, which for 30 nC at 10 V and 100 kHz is only 0.03 W. Most of this power is dissipated in the driver and gate resistor rather than inside the MOSFET die, so it is often listed separately.
| Loss term | Formula | Example value | Where the heat goes |
|---|---|---|---|
| Conduction | I² × Rds(on) × D | 0.75 W | MOSFET die |
| Switching | ½ V I (tr + tf) fsw | 0.48 W | MOSFET die |
| Gate charge | Qg × Vgs × fsw | 0.03 W | Driver and gate resistor |
| Output capacitance | ½ Coss V² fsw | 0.009 W at 300 pF | MOSFET die |
| Dead time | Vsd × I × t_dead × fsw | Depends on dead time | Body diode |
| Reverse recovery | Qrr × V × fsw | Depends on diode | MOSFET and diode |
DMC adds output capacitance loss as ½ Coss V² fsw and reverse recovery loss as Qrr × V × fsw. At 24 V these are small, but in a 400 V PFC boost circuit the Coss term grows with V² and can become a major loss.
Dead time loss comes from the body diode conducting while both switches are off. A Schottky in parallel or synchronous rectification reduces it, as covered in synchronous rectification and Schottky diode advantages.
How to Calculate MOSFET Junction Temperature
Once the mosfet power loss calculation gives the device loss, the junction temperature is ambient temperature plus loss times thermal resistance. DMC suggests a typical Tj limit of 125 to 150 °C and notes that reliability drops sharply as temperature rises.
Tj = Ta + P_device × Rθ
Without heat sink: Rθ = RθJA
With heat sink: Rθ = RθJC + RθCS + RθSA
Then recheck Rds(on) at the new Tj
The iteration matters in mosfet power loss calculation because higher Tj raises Rds(on), which raises loss, which raises Tj again. Two or three passes are usually enough for the numbers to settle within a few degrees.
A MOSFET in a TO 220 package without a heat sink often has a junction to ambient thermal resistance of around 60 °C/W. At that value, just 1.5 W of loss raises the junction about 90 °C above ambient.
MOSFET Power Loss Calculation Calculator
Enter the load current, datasheet values and operating conditions to get each loss and the junction temperature. The defaults match the solved example of a 24 V, 10 A, 100 kHz switch in a D2PAK soldered to a copper pour with Rθ of 40 °C/W.
P_cond = 10² × 0.010 × 1.5 × 0.5 = 0.75 W
P_sw = 0.5 × 24 × 10 × 40 ns × 100 kHz = 0.48 W
P_gate = 30 nC × 10 V × 100 kHz = 0.03 W
P_device = 0.75 + 0.48 = 1.23 W
Tj = 25 + 1.23 × 40 = 74.2 °C
At 74 °C the real hot factor is probably lower than the 1.5 assumed, so this answer is slightly pessimistic and safe. If the same design ran at 400 kHz, the device loss would be 2.67 W and Tj would reach about 132 °C, too close to the limit without a heat sink.
To bring that 132 °C case down, use our heat sink calculation guide to pick the sink thermal resistance. On surface mount parts the board itself is the heat sink, so add thermal vias under the drain pad.
Ways to Reduce MOSFET Losses
- Choose a lower Rds(on) part
- Drive the gate fully at rated Vgs
- Parallel two MOSFETs to halve the current in each
- Keep Tj low so Rds(on) stays low
- Reduce switching frequency where possible
- Use a stronger gate driver for faster edges
- Pick a part with lower Qg and Coss
- Use soft switching topologies at high voltage
There is always a trade off, because a large die with low Rds(on) usually has higher gate charge and capacitance. At very high voltage and current many designers move to an IGBT, which has different loss behaviour.
- Use hot Rds(on) at the expected junction temperature
- Include duty cycle in conduction loss
- Use real rise and fall times from the circuit, not only the datasheet
- Add Coss and reverse recovery terms at high voltage
- Iterate Tj until it settles
- Keep at least 20 to 25 °C margin below the Tj limit
Measure the case temperature with a thermocouple after 30 minutes at full load. Then work back to the junction with Tj = Tcase + P × RθJC to confirm your mosfet power loss calculation in the real enclosure.
Airflow changes the case to ambient term a great deal, so fans are often the cheapest fix, as discussed in electronic cooling fans. For DC to AC stages the same method applies to every switch in the bridge of a power inverter.
The ROHM note and the video both split the loss into the same terms we used here. Work through them alongside this mosfet power loss calculation guide to build confidence with real datasheets.
MOSFET Power Loss Calculation FAQ
For a mosfet power loss calculation, add the conduction loss I² × Rds(on) × D to the switching loss ½ × V × I × (tr + tf) × fsw, then add small terms such as Coss and dead time loss. Use the hot Rds(on) value.
Multiply the total by the thermal resistance and add the ambient temperature. This gives the junction temperature you must keep below the datasheet limit.
Conduction loss is the heat produced while the MOSFET is fully on and carrying current through its channel resistance. It equals current squared times Rds(on), multiplied by the fraction of time the part conducts.
Because Rds(on) rises with temperature, conduction loss also rises as the part heats up. Using the hot value avoids underestimating the loss.
The common estimate is ½ × V × I × (tr + tf) × fsw, where tr and tf are the rise and fall times of the switching edges. ROHM and many other makers use this form in their application notes.
The loss grows in direct proportion to frequency, voltage and current. Faster gate drive and lower frequency are the main ways to reduce it.
Use Tj = Ta + P × Rθ, where P is the device loss and Rθ is the total thermal resistance from junction to ambient. With a heat sink, add the junction to case, case to sink and sink to ambient values.
Then check Rds(on) at the new junction temperature and repeat the loss calculation. Two or three passes are normally enough for the result to settle.
Only a small part of it does, because most gate drive power is dissipated in the driver and the external gate resistor. The total gate loss is Qg × Vgs × fsw for each switching cycle.
At high frequency this loss can heat the driver noticeably. That is why driver package power and gate resistor wattage should also be checked.
Carrier mobility in silicon falls as temperature rises, so the channel and drift region resistance go up. Datasheets show this with a normalised Rds(on) versus junction temperature curve.
This positive coefficient raises conduction loss in a hot part. It also helps parallel MOSFETs share current, because the hotter device naturally takes a smaller share of it.
Related Articles
- What Is a MOSFET Working Principle
- MOSFET Gate Driver ICs
- Heat Sink Calculation
- Synchronous Rectification Buck Converter
- IGBT Working Principle and Applications
External References
- ROHM: Calculation of Power Loss (Synchronous) Application Note (PDF)
- DMC: MOSFET Power Loss Calculator
- Wikipedia: Power MOSFET
What We Learn Today
- MOSFET power loss calculation adds conduction loss I squared Rds(on) times duty cycle to switching loss half V I times edge time and frequency.
- Rds(on) rises with junction temperature, so use the hot value from the datasheet curve and iterate until the junction temperature settles.
- Junction temperature equals ambient plus device loss times thermal resistance, and it should stay well below the typical 125 to 150 °C limit.
