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Question 19.3: A transistor has an absolute maximum junction temperature ra...

A transistor has an absolute maximum junction temperature rating of 150° and a thermal resistance from junction to case of 1.0°C/W. If the device is fitted with a washer and mounting kit having a thermal resistance of 1.25°C/W and a heatsink of 2.75°C/W, determine whether the maximum ratings are exceeded when the total power dissipation reaches a maximum of 25 W at a maximum ambient temperature of 40°C.

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Applying equation (6) gives:
T_{Jmax} = (25 × (1.0 + 1.25 + 2.75) + 40 = 165°C
The absolute maximum junction temperature rating (150°C) is exceeded and the designer should either reduce the power dissipation to a safe value or reduce the thermal resistance present (or both!).

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