Ultra High Ratio Extrusion Heat Sinks Improve Cooling

May 1, 2010 12:00 PM
Christopher Soule Engineering Director, Thermshield LLC, Gilford, NH.

Minimizing cost and maximizing thermal performance are the obvious goals of cooling a power semiconductor. Thermal performance is related to the power semiconductor's junction temperature; the lower, the better and the higher the reliability. Overall cost depends primarily on the thermal management device that absorbs and dissipates heat from the power semiconductor either directly or by radiation. Currently, heat sinks are the lowest cost, most widely-used thermal management device for power semiconductors.

One of the newest heat sink improvements is ultra-thin aluminum fins spaced closer together than traditional heat sinks. Ultra-thin fin extrusions are just an iteration of conventional technology. Their cost penalty is only an increased tooling cost, which is about 150% of traditional tooling; however, it is only a one-time cost. The benefit of ultra-thin fins is lower assembly labor costs compared with other heat sink types. For example, it lowers cost by eliminating the fin-to-bond joint, of the frequently-used bonded fin type.

To understand how the newer heat sinks provide better performance at lower cost, we have to look at a typical heat sink employed with a power semiconductor. Fig. 1 shows the typical configuration of a heat sink cooling a power semiconductor. The junction temperature reduction occurs in three steps. First, heat moves from the junction to the case via θj-c, the thermal resistance from the junction-to-case. Then, the heat moves from the case to the heat sink via the thermal interface material whose thermal resistance is θc-s. In the final step, the heat moves from the heat sink to the air through thermal resistance θs-a. In most cases, we can assume a thermal interface material with a very low thermal resistance, so the heat sink is the major contributor to removing heat from the power semiconductor's die. The heat sink's ability to reduce the temperature of the power semiconductor's die, or junction, is a measure of its thermal performance.

Fig. 2 is an example of determining the thermal resistance required by a heat sink based on a power semiconductor's characteristics. This power semiconductor has a 10W power input, a maximum junction temperature of 100 °C, and an internal thermal resistance of 1.0 °C/W (θj-c). The thermal interface resistance (θc-s) is 1.5 °C/W. Therefore, this system requires a heat sink with θs-a = 3.5 °C/W to achieve a maximum ambient temperature of 40 °C. In this case, the maximum temperature rise in the heat sink will only be 10W × 3.5°C/W, or ≤ 35 °C.

Basically, the heat sink removes heat from the power semiconductor by providing a cooler temperature direction for the heat to move toward. The overall low resistance path provides a place for the waste energy to drain off. Therefore, the heat sink must be maintained at a lower temperature than the heat source. Ultimately, all heat removed from the power semiconductor will be exhausted into the air.

EXTRUDED HEAT SINKS

Today, many heat sinks are produced by an extrusion process. Thermal extrusions, as with most aluminum extrusions, are formed as two-dimensional parts along a length. Most extrusions are made in large hydraulic presses that use a specifically shaped tool to form parts. The size of the press is designated by the number of tons of force it can apply and the maximum circle size of the tooling. The circle size of the tool limits the overall size of the maximum extrusion profile that can be produced by a given press. This largest allowable size is the combination of width and height that will fit within the given circumscribed diameter. Press circle sizes range from 2.0 inches to as large as 30.0 inches. Generally, there are fewer presses available for the larger the tool sizes.

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