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
Switched-mode power stages depend on accurate MOSFET loss estimation. The calculator separates conduction, switching, gate-drive, and dead-time body-diode losses, then estimates junction temperature from a package θJA.
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 to 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.