Power electronics – thermal design of SCR/thyristor A thyristor has a maximum allowable junction temperature of 110°C. The ambient (cooling air) temperature is 40°C. If the device's maximum internal power dissipation is 50 W, compute the required overall thermal resistance from junction to ambient (°C/W) so that the junction temperature limit is not exceeded.

Introduction / Context:
In power electronics thermal design, the allowable thermal resistance from junction to ambient determines whether a thyristor (SCR) can safely dissipate a specified power without exceeding its junction temperature limit. This governs the choice of heatsink and any interface materials.

Given Data / Assumptions:

• Maximum junction temperature Tj,max = 110°C.
• Ambient temperature Ta = 40°C.
• Maximum internal power dissipation Pmax = 50 W.
• We assume steady state and that the single overall thermal resistance Rth,ja lumps junction–case, case–sink, and sink–ambient as required.

Concept / Approach:

The thermal analogy mirrors Ohm’s law: temperature rise equals power multiplied by thermal resistance. Hence, Rth,ja = (Tj,max − Ta) / Pmax. Keeping this value at or below the computed number ensures the junction does not exceed 110°C under 50 W dissipation.

Step-by-Step Solution:

Verification / Alternative check:

If you used a heatsink such that the measured temperature rise at 50 W is near 70°C, the device junction will be near its limit. Selecting a heatsink to achieve Rth,ja less than 1.4 °C/W adds safety margin.

Why Other Options Are Wrong:

• 2.0 or 2.2 °C/W would allow a rise of about 100–110°C at 50 W, pushing Tj toward 140–150°C, which exceeds the limit.
• 0.7 °C/W is safe but not the minimum required; it implies extra margin beyond the calculation.

Common Pitfalls:

Ignoring interface resistances (thermal pad/grease) or using sink ratings without accounting for airflow can underpredict Rth,ja. Always include junction–case and case–sink contributions.

Final Answer:

1.4° C/W