Power MOSFET Thermal Calculator

A Power MOSFET generates heat when it switches current — too much heat and the device fails. The Power MOSFET Thermal Calculator takes your MOSFET's drain current (IDS), RDS(on) resistance, switching frequency, duty cycle, input voltage, and thermal parameters to calculate the junction temperature and total power loss. Secondary outputs include conduction loss, switching loss, gate charge loss, and the required heat sink thermal resistance to keep your device within safe operating limits.

A
mΩ
%
Hz
V
ns
ns
nC
V
°C
°C
°C/W
°C/W

Results

Junction Temperature

--

Total Power Loss

--

Conduction Loss

--

Switching Loss

--

Gate Charge Loss

--

Required Heat Sink Rth

--

More Electrical & Electronics Tools

Frequently Asked Questions

What is the difference between conduction and switching losses?

Conduction losses occur when current flows through the MOSFET in the 'on' state, calculated as I²×RDS(on). Switching losses occur during transitions between on/off states due to simultaneous voltage and current during switching.

How do I calculate the required heat sink thermal resistance?

The required heat sink thermal resistance is calculated as Rth_hs = (Tj_max - Ta)/P_total - Rth_jc - Rth_ca, where negative values indicate no heat sink is needed for the given conditions.

Why are gate charge losses typically much smaller than other losses?

Gate charge losses are proportional to switching frequency and gate charge, typically resulting in milliwatts. They become significant only at very high switching frequencies or with large gate charges.

What happens if my calculated junction temperature exceeds the maximum rating?

If junction temperature exceeds the maximum rating, the MOSFET may fail or have reduced reliability. You need to reduce power losses, improve cooling, or use multiple MOSFETs in parallel.

How does duty cycle affect MOSFET power dissipation?

Duty cycle directly affects conduction losses since the MOSFET only conducts when 'on'. A 50% duty cycle means the MOSFET conducts half the time, reducing average conduction losses.

What is junction-to-case thermal resistance and why is it important?

Junction-to-case thermal resistance (Rth j-c) represents the thermal barrier between the semiconductor junction and the package case. It's typically much lower than case-to-ambient resistance and determines heat transfer efficiency to external cooling.

How do rise and fall times affect switching losses?

Longer rise and fall times increase switching losses because the MOSFET spends more time in the linear region where both voltage and current are present. Faster switching reduces losses but may increase EMI.

Can I use this calculator for parallel MOSFETs?

For parallel MOSFETs, divide the total current by the number of devices and calculate for one MOSFET. Each device will have similar power dissipation if properly matched, so multiply the result by the number of devices for total system losses.