The next generation of automobiles — whether hybrid, fuel cell or combustion engine — promises better fuel efficiency and greater options for in-car entertainment systems, including connections for computers, video players, navigation systems and MP3 players. However, these additional electronics systems present additional challenges for auto designers and manufacturers.

The waste heat from the growing number of electronic devices — such as insulated gate bipolar transistor (IGBT)/metal-oxide semiconductor field-effect transistors (MOSFET) modules, power ICs or PICs, passive components, and batteries — strains the capabilities of conventional air/liquid-cooling systems as they are called on to handle multiple high-heat-flux components spread throughout the automobile (Figure 1).

However, innovative technology for cold plates developed at Amulaire can deliver the thermal efficiency and small size required, while also taking advantage of existing cooling loops.

Electronic modules and integrated circuits (ICs) continue to shrink and become more powerful, and all must be cooled in an efficient, reliable manner. These thermal solutions must be somewhat ingenious to meet automobile manufacturers' requirements for decreased size and weight without any sacrifice in performance. Obviously, it wouldn't work to strap on a 50-pound aluminum heat sink with a blower attached.

Automobile designers already have many years of experience working with liquid cooling, and they are not afraid of mixing electronics and liquid in the same application. In addition, they already have a liquid-cooling loop circulating through the target zone for cooling the engine. The challenge becomes finding a way to use existing pumps, hoses and radiators within the engine compartment, tapping off these components to also cool the new electronics. And the only way to do this is with a very efficient cold plate, because you also must consider added pressure drop caused by such heat sinks. (If it is too restrictive, you will need a separate thermal loop for your electronics — which is cost-prohibitive.)

Let's look at a typical 100 kW drive in a hybrid vehicle. The waste heat is typically 2%, or 2000 Watts of heat that must be removed. Generally, an aluminum cold plate will experience a 30-40 °C rise per 1000 Watts of heat (depending on flow and the design of the cold plate). With typical inlet fluid temperatures around 85-100 °C, coupled with a 30-40 °C rise at the cold plate, the electronics you are trying to cool might experience case and junction temperatures well in excess of 130-140 °C, which is above the IGBT manufacturers' specifications.

Engineers have considered using micro channels and spray cooling for thermal improvements, but this approach would greatly increase the pressure drop within the loop. The goal is to use the existing loop within the engine compartment, thus removing the additional expense of redundant hoses, pumps and radiators. Adding a better cold plate would solve many other issues while reducing the cost of a separate thermal loop just for the electronics (Figure 2).

The traditional aluminum-and-copper embedded tube technology is not efficient enough to satisfy the thermal requirements needed to remove 2000 W with at 85-105 °C inlet temperature. Because aluminum thermal conductivity is approximately 200 W/mK (cast aluminum is roughly 160 W/mK) and copper is 400 W/mK, copper is the better choice in higher-heat-flux applications. However, copper is more expensive and difficult to machine, and it weighs more than aluminum. The design of a cold plate must take these issues into consideration and use the copper only where it is needed.