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• Fundamentals of Thermal Resistance and Heat Dissipation: About Thermal Resistance

2021.03.10 Thermal Design

# Fundamentals of Thermal Resistance and Heat Dissipation: About Thermal Resistance

Thermal Design of Semiconductor Components in Electronic Equipment

From here, we begin technical discussions relating to thermal design. The knowledge required for thermal design spans a broad range of fields. We begin by explaining the basics of thermal resistance and heat dissipation, an understanding of which is the bare minimum background knowledge needed for this subject.

Thermal resistance is a numerical representation of the difficulty of propagation of heat. It is the temperature difference between two arbitrary points, divided by the amount of heat flowing between the two points (the amount of heat that flows in a unit of time). The higher the thermal resistance, the harder it is for heat to propagate; if the thermal resistance is low, heat propagates easily. As symbols, Rth and θ (theta) are used. Rth derives from the term "thermal resistance".

Units are ℃/W （K/W）.

A Thermal Version of Ohm's Law

Thermal resistance can be regarded as more or less similar to electrical resistance, and the basic equations for thermal calculations can be handled similarly to Ohm's law.

 Electricity Heat Current I（A） Voltage difference ⊿V（V） Electrical resistance R（Ω） Amount of heat flow P（W） Temperature difference⊿T（℃） Thermal resistance Rth（℃/W）

Hence, just as a voltage difference ⊿V is calculated as R×I, so a temperature difference ⊿T can be calculated as Rth×P.

#### Key Points:

・Thermal resistance is a numerical expression of the difficulty of propagation of heat.

・As symbols, Rth and θ (theta) are used; units are ℃/W （K/W）.

・Thermal resistance can be considered to be roughly analogous to electrical resistance.