MOSFET Power Loss Calculator
Generated infographic and interface snapshot for MOSFET Power Loss Calculator
Stop guessing why your switching FET is burning up.
Free MOSFET Power Loss Calculator Online — Big Das
Every switched-mode power stage lives or dies by MOSFET losses. The Big Das MOSFET Power Loss Calculator separates conduction, switching, gate-drive, and dead-time body-diode losses, then estimates junction temperature from a package θJA — live, in your browser.
What Are MOSFET Losses?
A power MOSFET is not a perfect switch. It dissipates energy in four distinct ways:
- Conduction loss — while fully on, the channel behaves like a small resistor (Rds(on)), so I²R heating scales with duty cycle.
- Switching loss — during rise and fall transitions, voltage and current overlap, burning power each edge, every cycle.
- Gate-drive loss — charging and dumping the gate charge (Qg) costs energy that the driver ultimately dissipates.
- Dead-time body-diode loss — in half-bridges, the current freewheels through the MOSFET's intrinsic diode (Vf ≈ 0.7 V) during dead time, before the channel turns on.
Miss any of these and your thermal estimate will be wildly optimistic.
How to Use the Calculator
- Enter Vds and the drain current (use the RMS current through the device).
- Enter Rds(on) in mΩ from the datasheet — ideally at your actual gate voltage and Tj.
- Set the PWM duty cycle; conduction loss scales with it.
- Add tr, tf, and fsw — switching loss is proportional to all three.
- Add Qg, Vgs, dead time, and diode Vf for the remaining terms.
- Pick the package θJA preset (or Custom) and ambient temperature to see the junction temperature estimate.
The Formulas Used
Conduction: Pcond = I_rms² × Rds(on) × D
Switching: Psw = ½ × Vds × Id × (tr + tf) × fsw
Gate drive: Pgate = Qg × Vgs × fsw
Dead time: Pdt = Vf × Id × fsw × (2 × tdead)
Total: Ptot = Pcond + Psw + Pgate + Pdt
Junction: Tj = Ta + Ptot × θJA
Rise/fall times come from the datasheet switching specs; θJA from the package thermal table. Junction temperatures above ~125 °C risk reliability — most silicon is rated 150 °C or 175 °C absolute max.
Worked Example
A synchronous buck low-side FET: Vds = 24 V, Id = 10 A, Rds(on) = 8 mΩ, duty = 50 %, tr = 20 ns, tf = 15 ns, fsw = 100 kHz, Qg = 60 nC driven at 10 V, dead time 200 ns, diode Vf = 0.7 V, Ta = 25 °C, TO-220 with heatsink (θJA = 30 °C/W).
Conduction: 10² × 0.008 × 0.5 = 0.400 W
Switching: ½ × 24 × 10 × 35e-9 × 1e5 = 0.420 W
Gate: 60e-9 × 10 × 1e5 = 0.060 W
Dead time: 0.7 × 10 × 1e5 × 2 × 200e-9 = 0.280 W
Total ≈ 1.16 W → Tj ≈ 25 + 1.16 × 30 ≈ 59.8 °C — comfortably cool.
Common Use Cases
Sizing MOSFETs for buck, boost, and motor-drive bridges before ordering parts.
Comparing two candidate FETs: lower Rds(on) vs. lower Qg/tr/tf trade-offs.
Checking whether a bare TO-220 needs a heatsink at your switching frequency.
Estimating driver dissipation and supply current from the Qg × fsw term.
Frequently Asked Questions
Should I use peak or RMS current?
Use RMS for the conduction term — I²R heating responds to the mean-square current, and ripple makes RMS larger than the average. The calculator assumes the RMS current you enter flows during the on-time.
Why does switching loss grow with frequency?
Each switching event dissipates a fixed ½·V·I·(tr+tf) of energy. Multiply by events per second (fsw) and the power scales linearly — which is why high-frequency designs favor fast, low-Qg FETs or GaN.
Is θJA from the datasheet accurate?
It's a standardized JEDEC-board figure and is often pessimistic vs. a well-cooled board (or optimistic for a cramped one). Treat the estimate to ±20 % and verify with a thermocouple or IR camera.
What happens above 125 °C?
Rds(on) rises with temperature (positive temperature coefficient), increasing conduction loss further — a possible thermal runaway loop. Most silicon MOSFETs are rated 150–175 °C absolute max, but sustained operation above ~125 °C degrades lifetime.
Can I use this for a linear (non-switching) MOSFET?
For pure linear operation, conduction loss becomes Vds × Id instead of I²R — that mode is not what this tool models. It's built for PWM switching stages.