Solid state power electronics systems that do not exceed recommended operational temperature limits will regulate the flow of electricity in an efficient and reliable manner.   Insulated Gate Bipolar Transistor modules (IGBT modules) are an integral part of modern shipboard power systems. For high voltage applications, 1700V IGBTs are ganged into a Power Electronics Module (PEM); and the PEMs are integrated into a Line Replaceable Unit (LRU). The LRU is a self-contained system containing drivers, control circuits, monitor circuits, and a coldplate typically made from copper or aluminum, which serves as the heat sink for the IGBT system.  The coldplate is ground extremely smooth and flat to ensure complete contact between the IGBT modules and heat sink. A thermal grease or Thermal Interface Material (TIM) is placed between the heat sink and the IGBT module to promote heat transfer through conduction. Cooling the heat sink can be accomplished by many means including recirculation of a fluid.  In a recirculation system, the fluid passes through channels inside the heat sink and heat is transferred into the fluid.  The heat is removed from the fluid using an external heat exchanger that transfers the heat to cold sea water as showin in  Figure 1-3.

One of the biggest challenges for cooling PEMs, lies in the way an IGBT is designed. The IGBT channels current from the back of the silicon device through its face. As a result, the IGBT must be electrically isolated from the heat sink. This adds an additional thermal barrier between the IGBT and the base plate of the IGBT module. Studies indicate that 75% of the thermal bottleneck in PEMs comes from the IGBT module package itself.  If a method could be devised to immerse the IGBT into a cooling liquid, the largest thermal barrier would be removed, resulting in an efficient system that would reduce the overall number of IGBTs required onboard a ship.  This has the twofold effect of reducing weight and space on a ship.

In 2002, the 3M™ Corporation introduced a new hydrofluoroether (HFE) heat transfer fluid that is environmentally friendlier that perfluorocarbons (PFCs) and 3M™ can tailor this material for specific applications. Moreover, the fluid has a very high electrical resistivity and dielectric strength, making it suitable for use in immersion cooling of high voltage IGBT based systems.  Figure 1-2 shows selected material properties of Novec™ 7500 HFE liquid.

There are some considerations that must be taken into account when developing immersion cooling systems. Hydrocarbons are soluble in HFEs. If an immersion cooling system uses hydrocarbon based plastics or plastics with hydrocarbon plasticizers, the HFEs can slowly leach away the plasticizer or decompose the plastic. These contaminants can redeposit onto other surfaces, clogging pumps or generating static charges. However, by carefully selecting the materials used in the immersion cooling system, this issue can be mitigated.

Recently, 3M™ began development of a proprietary coating technique for immersion boiling systems that has the potential to increase the heat removal to 250 W/cm2.  Developmental research using the coating system in immersion cooling systems is ongoing. Reports can be found on the 3M™ website which provide in-depth analysis of immersion cooling techniques using HFEs. If 3M™ achieves its goal of reaching 250 W/cm2 for heat removal, the door will be opened for very efficient IGBT-based PEMs for all electric busses, trains and automobiles.

Immersion cooling system design is dramatically different from the transfer molded IGBT module shown in Figure 1-1. In an immersion cooling set up, the bare IGBT module is immersed into a liquid as shown in the Figure 1-3 illustration.

Conclusion
Immersion cooling has a history of successful use. Raytheon used passive two-phase immersion cooling on transformers manufactured in the late 1950’s. Siemens used passive two-phase cooling on locomotive train traction converters designed in the 1990s.  The Cray Corporation’s X-1 supercomputer released to the market in 2002 used spray-impingement cooling. What these methods had in common was that they all used either a  PFC or an HFE based fluid.

The migration of conventionally packaged high voltage IGBT devices from the technology base used in Figure 1-1 to an immersion cooled system will occur when there is an application for the immersion cooled systems to provide an advantage over the current methods. Hybrid vehicles may prove to be the area where a higher efficiency electrical conversion system will provide a market differentiator that attracts automotive designers to adopt and utilize advanced cooling techniques.