Junction Temperature

Heat flows from a transistor's junction to ambient through a chain of thermal resistances — junction-to-case, case-to-sink (interface), and sink-to-ambient. This tool adds the chain, multiplies by the dissipated power, and gives the junction temperature rise and absolute Tj, plus the margin to a maximum rating and the maximum power the chain can handle. Essential for reliable PA and high-power RF design.

Equations & Parameters ▸

\(\theta_{ja}=\theta_{jc}+\theta_{cs}+\theta_{sa} \qquad T_j=T_a+P_{diss}\,\theta_{ja}\)
\(P_{max}=\dfrac{T_{j,max}-T_a}{\theta_{ja}} \qquad \text{margin}=T_{j,max}-T_j\)

P_diss	Power dissipated in the device (W).
T_a	Ambient (or reference) temperature (°C).
θ_jc	Junction-to-case thermal resistance (°C/W).
θ_cs, θ_sa	Case-to-sink (interface) and sink-to-ambient resistances (°C/W). Set θ_cs, θ_sa = 0 to use a junction-to-ambient figure alone via θ_jc.
T_j,max	Maximum rated junction temperature (°C), optional — gives the margin and max power.

Reference: Standard steady-state thermal-resistance network; JEDEC JESD51 thermal-characterization terminology.

Inputs

Dissipated power, P_diss

Heat load

Ambient, T_a

°C

Reference

θ_jc

°C/W

Junction–case

θ_cs

°C/W

Interface

θ_sa

°C/W

Heatsink

T_j,max (opt)

°C

For margin

Results

Temperature

Total θ_ja—

Temperature rise—

Junction temperature T_j—

Margin

Margin to T_j,max—

Max dissipation—

Diagram

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