Power Electronics Design Tools

MOSFET Loss & Thermal Calculator

Optimize DC-DC converter efficiency and estimate junction temperature accurately.

Accurately estimating power dissipation in switching regulators is critical for thermal management. This calculator breaks down total loss into two distinct phases: Conduction Loss (I²R static loss) and Switching Loss (V-I overlap dynamic loss).

Select Direct Input to calculate based on datasheet parameters, or use the Topology Wizard to derive RMS currents for standard Buck (Step-down) or Boost (Step-up) converters.

MOSFET Datasheet

Specs
mΩ
ns
ns
°C/W
°C

Switching Conditions

Circuit
V
kHz
A
A
A

Loss Breakdown

Analysis
Total Power Loss
— W
Cond: — Switching: —
Estimated Junction Temp
— °C
Enter parameters
Switching Overlap
Vds Id Turn On Turn Off

Formulas Explained

Conduction Loss

Pcond = Irms2 × RDS(on)

Note: The calculator applies a 1.5x multiplier to RDS if “Simulate Hot” is checked.

Switching Loss

Psw = 0.5 × VDS × fsw × (Iontr + Iofftf)

Energy lost during the rise/fall transition overlap.

Buck RMS Current

Irms ≈ Iout × √D

Approximation used in the Wizard for High-Side switches.

Boost RMS Current

Irms ≈ (Iout / (1-D)) × √D

Approximation used in the Wizard for Low-Side switches.

How to Use

Two ways to use this tool:

A

Option 1: Direct Input

If you already have your circuit simulation values, simply enter RDS(on), Irms, and switching times. This works for any topology.

B

Option 2: Topology Wizard

Click “Topology Wizard”. Select Buck or Boost, enter your voltage/load, and click Apply. We will calculate the RMS currents for you.

3

Check Temperature

Review the Junction Temp. If it’s red (>125°C), you need a better MOSFET, a heatsink, or a lower switching frequency.

Common Questions regarding MOSFET Losses

Why is my MOSFET overheating?

Overheating occurs when the total power dissipation (Ptotal) multiplied by the thermal resistance (RθJA) raises the junction temperature above the silicon limit (usually 150°C or 175°C). Common causes are high RDS(on) (Conduction loss) or slow gate drive causing prolonged rise/fall times (Switching loss).

Does Switching Frequency affect efficiency?

Yes. Switching losses are directly proportional to frequency (fsw). Doubling the frequency will double the switching loss, although it allows for smaller inductors. Conduction losses are generally independent of frequency.

What is the “Hot” simulation setting?

MOSFET on-resistance (RDS(on)) increases as the device heats up. A common rule of thumb in power electronics is that RDS(on) at 100°C is approximately 1.5 times the value at 25°C. Checking this box provides a more realistic worst-case scenario.