Junction temperature calculation (SCR) A thyristor dissipates 40 W and operates in 20°C ambient. If the overall junction-to-ambient thermal resistance is 1.6°C/W, estimate the junction temperature (°C).

Introduction / Context:
Accurate junction temperature estimates are essential to ensure device reliability. The thermal resistance model provides a straightforward way to compute Tj from ambient temperature, power, and Rth,ja.

Given Data / Assumptions:

• Ambient temperature Ta = 20°C.
• Power dissipation P = 40 W.
• Overall thermal resistance Rth,ja = 1.6°C/W.
• Steady-state operation; linear thermal model applies.

Concept / Approach:

Temperature rise above ambient equals power times thermal resistance. Then junction temperature Tj = Ta + P * Rth,ja.

Step-by-Step Solution:

Verification / Alternative check:

Confirm that Tj is below device maximum (commonly 110–125°C for many SCRs). At 84°C, the device is operating within typical safe limits.

Why Other Options Are Wrong:

• 94°C or 114°C would imply larger Rth or power than given.
• 164°C exceeds typical device limits and contradicts the calculation.

Common Pitfalls:

Omitting interface thermal resistances in practice can raise Tj. Measurement should be cross-checked with thermocouples or IR methods during prototyping.

Final Answer:

84° C