Heat Sinks

Mission and purpose of a heat sink

The electric power loss (Ploss) is located at the junction of semiconductor devices and resistors to heat (Q) and causes a temperature increase. The temperature of the barrier layer (θJ) must not exceed a maximum value to ensure a stable operation, and to avoid the destruction of the semiconductor. This maximum allowable junction temperature can be taken from the data sheets of semiconductor manufacturers. Can not be sufficiently aware of the semiconductor body to the surrounding medium, usually air, the heat dissipated, so the component must be mounted on a heatsink. The effective surface area for heat dissipation housing is thus increased. This leads to higher reliability and lifetime of the semiconductor or the entire circuit. A heat sink is made of highly thermally conductive materials, generally of an aluminum alloy with an adjusted and appropriate for the application geometrical structure and surface characteristics.

Materials used are:

• AlMgSiO, 5 F22 for aluminum extrusions
• AlSi8Cu3 for aluminum die castings
• Al99, 9 hh aluminum strip material

Operation of a heat sink, heat and Konvektionsarten

The heat transfer from the heat source (eg junction of the semiconductor) over the heat sink to the surrounding medium is composed of:

• the heat transfer from the heat source to the heat sink
• the heat conduction within the cooling body to the heat sink surface
• the transfer of heat from the surface by free or forced convection to the surrounding medium
• the heat radiation depending on surface characteristics

The thermal resistance and the thermal equivalent circuit

The heat resistance is defined as the ratio of temperature rise at a power supplied and used as a measure for the heat-dispensing capacity of the heat sinks and comparability. The lower the thermal resistance, the lower the expected temperature increase and the more "better" is a heatsink. The thermal resistance is expressed in K / W (Kelvin / Watt). Heat sink and semiconductor form a functional unit that can be prepared analogously to the Ohm's law in electrical engineering as a thermal equivalent circuit:

Structure in the following areas:

• Supply of the power loss (Ploss) is reacted in the heat flow (Q)
• Heat conduction from the barrier layer on the mounting surface of the component
• Heat output of the heat sink to the surrounding medium

Calculation of the required thermal resistance for a given power dissipation and the allowable temperature gradient

RthK = Heatsink thermal resistance in K / W θjmax = Maximum junction (junction) temperature of the semiconductor in ° C (from data sheet) θU = Ambient temperature in ° C Pv = the heat source supplied power dissipation in W RthjG = Thermal resistance, junction to case of the semiconductor in K / W RthGK = Heat transfer resistance at the mounting surfaces in K / W (can be reduced by means of thermal compound to a minimum value). With isolated installation, the spec. Thermal resistance of insulation materials considered.