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Hybrid Electric Vehicles (HEVs) or Plug-in Electric Vehicles (PEVs) typically have motor drive power modules (e.g., IGBT modules) that generate considerable waste heat—typically, in the range of 1,000 to 2,000W . The only practical way to cool these automotive electronics power modules is by liquid cooling—not air cooling, because the thermal conductivity of water is 20 times that of air. Water also has a specific heat (ability to absorb energy) that is four times that of air.
Power modules in most vehicle applications pose such a thermal challenge that, in fact, there is a separate cooling loop dedicated to cooling them. The cold plate, the component that transfers the heat of the power module to the coolant, forms a vital part of that cooling loop.
So how does one choose the best liquid-cooled cold plate? The choice will be a trade-off among various considerations (e.g., thermal performance, weight, cost, reliability and manufacturability)—but in reality the choice becomes fairly straightforward.
Let's briefly examine how heat generated at the IGBT chips can best get carried away by the coolant.
Improve conduction
First, the heat must transfer from the chips through the DBC (Direct Bond Copper) substrate; then through the module base plate; and typically across a thermal grease and into the wall of the bolted-on cold plate (see below). This portion of the heat transfer occurs entirely by conduction.
To improve conductive heat transfer, one should either choose materials with the highest thermal conductivity (k) or reduce the thickness of the layers—or better yet eliminate layers—between the heat source and the coolant.
For example, an aluminum nitride ceramic (k = 160 W/mK) used as the power module DBC substrate is a better choice than alumina (k = 25 to 35 W/mK). Copper (k = 390 W/mK) for the module base plate and the cold plate walls is a better choice than aluminum (k = 220 W/mK) or aluminum silicon carbide (AlSiC) (k = 170 - 180 W/mK). Simply put, you can not get a higher conductivity material than copper without resorting to very expensive materials such as silver or diamond.
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