What is the Best Heat Sink?

A heat sink is not just a reservoir for thermal losses
Most modern power electronics devices require controlled temperature environments, in order for them to function correctly. If the device overheats, the performance can be impacted, or in worst cases it fails and can become a safety hazard. The heat sink provides the pathway for heat to be efficiently transferred away from the device to the surroundings. This means that heat needs to be dissipated at least as fast as it is generated by the source. The heat sink is not the final destination for the losses of the device.

How to choose a heat sink

The heat flow or dissipation through a given heat sink is determined largely by the thermal resistance, thermal conductivity, cooling method and effective surface area. However there is a need to optimise any system not just for performance, but also in terms of cost, mass, size and complexity. Copper for example has around twice the thermal conductivity of Aluminium, but is much denser and more costly. Aluminium can also be extruded into complex shapes, which can only be achieved with Copper by machining and mechanical assembly. The best heat sink solutions combine the thermal management requirement with a practical and cost effective mechanical design.

Obstacles to overcome
Heat losses from a power electronics device need to bridge the various interfaces between materials. For example heat from a power semiconductor must move from the die to the casing of the component, then from the casing to the surface of the heat sink. Finally the heat sink will conduct the heat to the surroundings. Each transfer has its own thermal resistance, which when combined determines that of the system as a whole. Minimising the system thermal resistance (or maximising the heat flow) is key, and the best results are achieved by optimising the machining and finishing of the surfaces, and correct selection of interface materials.

Heat sink cooling options
Natural convection, forced air convection, liquid cooling – it really is determined by how much and how quickly heat is to be dissipated. For low level dissipation, heat sinks may be relatively simple Aluminium extrusions relying upon natural air convection. The addition of fans will improve the heat flow rate from the system. Beyond this, there are several more advanced technologies:

Welding & controlled atmosphere brazing for producing complex assemblies, with much higher effective surface areas, that absolutely rely upon forced air or liquid cooling to work efficiently.

Liquid cooled plates that can be anything from simple extrusions or drilled plates, to more complex assemblies inlaid with copper tube for the coolant to be carried to specific areas.

What makes the best Heat Sink?
Aluminium alloy with high thermal conductivity is the starting point. Selecting the right type of heat sink is crucial.

Once you know the ambient operating temperature, heat dissipation characteristics of the devices, and maximum operational limits for key components (such as junction temperatures), the dissipation of the overall system can be calculated. The optimum solution will be defined by available space, location, accessibility and cost targets.

For the more complex applications, simulations modelling the heat flow through the heat sink could prove a valuable tool in determining the best of alternative or competing technologies.

The finish of the surfaces, both those dissipating the heat, and those in contact with the heat source is a critical factor in minimising thermal resistance.

Some applications are such that high efficiency forced air complex fin solutions are the only practical way to meet the requirements. These include certain traction and electric vehicle designs, with high power and high dissipation needs.