MOSFET Junction Temperature
Compute MOSFET junction temperature from drain current, RDS(on), thermal resistance, ambient temperature and duty cycle. Flags marginal and danger conditions against the configured Tj-max.
Find out whether a MOSFET will survive its load before you commit it to a board. Enter the drain current, on-resistance, thermal resistance to ambient, ambient temperature and duty cycle, and the calculator returns the conduction power loss, the junction temperature, and how much headroom is left below the datasheet maximum — with a clear ok / marginal / danger verdict.
curl -X POST https://toolsamurai.com/api/v1/electronics/mosfet-thermal-junction \ -H "Authorization: Bearer sk_live_•••••••••••••••" \ -H "Content-Type: application/json" \ -d '{ "drain_current_a": 5, "rds_on_mohm": 20, "theta_ja_c_per_w": 62, "ambient_temp_c": 25, "duty_cycle": 1, "max_junction_temp_c": 150 }'
The method behind the numbers
The model covers conduction loss, the dominant heat source in a MOSFET that's fully on. Power dissipated is P = I² × RDS(on) × duty cycle. That power flows out through the junction-to-ambient thermal resistance, so the temperature rise is ΔT = P × θ-JA, and the junction temperature is simply ambient plus that rise.
Headroom is the datasheet maximum junction temperature minus the computed value. The verdict turns green when there's comfortable margin, amber as you close on the limit, and red once you'd exceed it. Note this is a steady-state conduction model: it deliberately ignores switching losses, body-diode conduction and gate-drive losses. For hard-switching converters, add the switching loss separately or fold it into a higher effective duty term.
See it in practice
A common load-switch case on a modestly heatsunk footprint.
- drain_current_a
- 5
- rds_on_mohm
- 20
- theta_ja_c_per_w
- 62
- ambient_temp_c
- 25
- duty_cycle
- 1
- max_junction_temp_c
- 150
Frequently asked questions
Does this include switching losses?
No — it models conduction loss only (I²·RDS(on)·duty). That's the right answer for a MOSFET used as a static or low-frequency switch. For high-frequency PWM, switching and gate losses can dominate and should be added on top.
Where do I get θ-JA?
From the datasheet, but read the conditions carefully — θ-JA depends heavily on copper pour area and airflow, and the quoted figure often assumes a specific pad. If you run the part on a heatsink, model θ-JC plus the heatsink path instead.
What junction temperature is safe?
Silicon MOSFETs are typically rated to 150 °C and SiC parts to 175 °C, but running near the limit shortens life and raises RDS(on). Design for a junction temperature well below the maximum so you keep healthy headroom.
Why does RDS(on) rise with temperature?
It's a positive feedback path: higher current and resistance raise the junction temperature, which raises RDS(on), which raises loss again. Use the RDS(on) at your expected junction temperature, not the cold value, for an honest result.
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