Conduction is the transfer of heat through contact with another solid material. Metals are generally good conductors of heat. When heated, electrons in the structure of the metal separate from their atoms leaving the ions to vibrate with a kinetic energy that is greater the hotter the metal is. This kinetic energy is transferred from hotter to cooler parts of the metal by the flow of electrons that collide with ions as they move, until they are recaptured once they lose enough energy. The rate of heat flow depends on the temperature differential, or gradient, and the thermal conductivity of the material. Within a power supply, heat is readily conducted from a component such as a resistor through the printed circuit board (PCB), with its high copper content, to the supply’s metal enclosure. Further conduction through the equipment’s main PCB board or system chassis may also remove heat from a power supply, although traditionally this has been considered less effective than convection cooling, which will be examined in the following section. Calculating heat transfer for conduction cooling is usually expressed in terms of thermal resistance rather than thermal conductivity, which is its reciprocal. Thermal resistance, measured in °C/W, is defined as: θ= ΔT/Q, where ΔT is the temperature difference in °C, and Q is the heat flow in watts. Different nomenclatures may be used to denote the thermal resistance between different points in a system. For a power transistor the resistance between the semiconductor junction, where the heat is generated, and the device’s case might be referred to as θJC while the thermal resistance from the case to the ambient air would be θCA. The combined thermal resistance from the conduction and convection heat flow paths simply sums to give θJA = θJC + θCA. The inclusion of a heatsink can also be modeled, allowing for the thermal resistance of any heat transfer pad (or thermal compound), by substituting the thermal resistance from heatsink to ambient for the case to ambient figure.