Engineering Reference
MOSFET Package and Thermal Parameters Reference
Lookup MOSFET package thermal paths, RθJA, RθJC, RθJB, ψ terms, transient Zθ(t), mounting conditions and junction-temperature boundaries.
Package Thermal Paths
| Package | Role | Primary heat path | Boundary |
|---|---|---|---|
| SOT-23 / DFN | Low-to-moderate power SMD | Primarily PCB copper and thermal vias | Quoted performance depends strongly on land pattern and board construction |
| SO-8 / PowerPAK / LFPAK | Compact power switching | Source clip, exposed pad and PCB copper | Pin compatibility does not imply equal thermal performance |
| DPAK / D2PAK | Surface-mount power | Large drain tab to PCB copper | Drain-connected tab and solder quality must be considered |
| TO-220 / TO-247 | Heatsink-mounted power | Junction-case-interface-heatsink-ambient path | Tab isolation, torque, TIM and airflow determine results |
| TOLL / TLF / top-side cooled | High-current compact power | Specified bottom- or top-side thermal path | Use the exact package thermal model and assembly guidance |
Thermal Parameter Lookup
| Term | Meaning | Use | Boundary |
|---|---|---|---|
| RθJA | Junction-to-ambient thermal resistance | Steady-state approximation for the stated board and environment | Not an intrinsic package constant |
| RθJC | Junction-to-case thermal resistance | Useful when heat is deliberately removed through the defined case surface | Case location and method are package-specific |
| RθJB | Junction-to-board thermal resistance | Characterizes heat flow to a defined board reference | Requires the specified board and measurement standard |
| ψJT / ψJB | Thermal characterization parameter | Estimates junction temperature from measured top/board temperature | Not generally interchangeable with thermal resistance |
| Zθ(t) | Transient thermal impedance | Time-dependent temperature response to pulsed dissipation | Use normalized curves and pulse-train superposition correctly |
| TJ(max) | Maximum rated junction temperature | Absolute upper limit | Design margin and reliability target should remain below it |
Steady State
TJ ≈ TA + P × RθJA
Valid only when RθJA represents the actual board and environment and the dissipation is effectively steady.
Pulsed Operation
ΔTJ(t) = Ppulse × Zθ(t)
Use the correct transient curve, duty cycle and pulse-train method; do not replace Zθ(t) with steady-state Rθ for short pulses.
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FAQ
Is RθJA a fixed MOSFET property?
No. It depends strongly on the specified PCB, copper area, orientation, airflow and measurement standard.
When should I use RθJC?
Use it when the modeled heat path is from junction to the datasheet-defined case surface and the remaining interface/heatsink path is known.
Is ψJT the same as RθJC?
No. ψJT is a characterization parameter for estimating junction temperature from measured package-top temperature.
What is transient thermal impedance?
Zθ(t) describes time-dependent junction temperature response and is used for finite pulses rather than steady-state heating.
Can I compare package thermal numbers from different datasheets directly?
Only when test standards, board, copper, mounting and definitions are comparable.
Does the MOSFET tab need electrical isolation?
Often the tab is connected to drain, but the exact package must be checked before mounting.
Does lower RDS(on) guarantee a cooler MOSFET?
No. Switching loss, gate drive, diode loss, current waveform, thermal path and temperature dependence also matter.
Can TJ(max) be used as the design target?
It is an absolute limit, not a recommended continuous target. Apply margin for tolerance, transients and reliability.