Keep a power semiconductor alive — trace heat from the junction through case, interface, and heat sink to ambient air, find the junction temperature or the heat sink you need, and see the margin against the device's limit on a live thermal ladder.
Given the heat sink, how hot does the junction get?
Heat generated in the device — not its output power. For a MOSFET: I²·R_dson + switching losses.
Silicon limit, often 150 °C. Derate to 80–110 °C for reliability & long life.
Air around the sink — inside an enclosure this can be far above room temperature.
The junction temp you want to stay under when sizing the sink (≤ Tj max, with margin).
From the device datasheet. Small for power packages (TO-247 ≈ 0.3–0.6).
The interface: thermal grease ≈ 0.2–0.5, pad ≈ 0.5–1.5, bare metal-metal ≈ 0.1.
The heat sink rating (lower = bigger/better). Falls with forced airflow.
Heat flows like current through resistors in series: each θ (°C/W) drops temperature by P·θ. The junction sits hottest on the left, ambient coolest on the right. The biggest resistance is where most of the temperature is spent — usually the heat sink.
The electrical analogy. Steady-state heat flow is exactly like a DC circuit: power P is the current, temperature is the voltage, and thermal resistance θ (°C/W) is the resistance. Resistances in the heat path add in series, and each one drops temperature by ΔT = P·θ.
Junction temperature. Tj = Ta + P·(θjc + θcs + θsa). If Tj exceeds the datasheet maximum, the device cooks — thermal runaway, then failure. The whole job is keeping Tj under the limit with margin.
Sizing a heat sink. Rearranged for the sink you need: θsa = (Tj_target − Ta)/P − θjc − θcs. If this comes out negative, no passive sink can do it — you need forced air, a cold plate, or less power. Buy a sink rated at or below this θsa at your airflow.
Maximum power. For a fixed thermal path: P_max = (Tj_max − Ta)/(θjc + θcs + θsa) — the derating line every power datasheet plots.
Interface matters more than people think. θcs (grease, pad, or bare joint) is often comparable to θjc and quietly dominates on well-heatsinked designs. A dry or uneven joint can add several °C/W and wreck an otherwise good design.
Steady-state, single heat source, one-dimensional path. Transient spikes (use thermal impedance Zθ), multiple sources sharing a sink, spreading resistance, and radiation are separate analyses.