Power dissipation: Milliwatts to kilowatts
08/01/2000
By Daniel K. Ward
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In this column I will briefly review some of the common techniques used to cool electronic modules in vehicles and some specific requirements that currently limit their use. In my opinion, substantially improved heat transfer methods/materials are needed to cost effectively dissipate power from many emerging vehicle electronic systems.
Automotive electronic modules/systems can dissipate power levels from a few milliwatts to a kilowatt or more, depending on their function or application. A worst-case thermal condition is typically at vehicle start and run following a hot soak (hot engine off) condition. Ambient temperatures for this type of condition are 85°C for the passenger compartment, 85 to 100°C for body mounted units and 95 to 125°C or more for underhood applications. Operating junction temperatures for digital and analog integrated circuits (ICs) can reach 150°C for some applications, and power devices can have operating junction temperatures as high as 175°C. Also, in certain applications (e.g., ABS, generators, steering), the worst-case power condition may be transient in nature. These transient conditions can range from less than a millisecond (power device switching) to several minutes. Transient power levels can be ten times higher than typical steady state power levels.
Air Cooling
Because of cost pressures and robust design requirements, most automotive electronic modules are air-cooled using metal cases or convectors with natural convection. Most modules mounted outside the passenger compartment are sealed or have very restricted airflow to prevent intrusion of contaminants. These modules typically use conduction though packaging materials or air convection to transfer heat from components to the case surface. In these applications, low-cost, high-thermal-conductivity materials and thermal structures are needed to improve heat transfer from components to the case.
Forced air cooling through an open box is currently used for some passenger compartment applications in which dirt, mud, automotive fluids and other contaminants are not present to damage fan motors or electronic circuits. Forced air cooling using dedicated fans or the moving air from the vehicle heating/cooling system will likely be used more extensively as multimedia (e.g., communication, Internet, PC, entertainment) functions become more prevalent in vehicles.
Heat Pipes
To date, heat pipes have found limited utility in automotive applications. As heat pipe costs decrease and the long-term reliability increases, it is likely that heat pipes will be used in multimedia applications, and for high-current switching and motor control/drive applications. To become viable for automotive applications, heat pipes must be capable of efficient operation up to 150°C (for underhood applications) with heat-source-to-heat-sink temperature differentials of less than 5 to 10°C. They also must be able to retain heat exchange fluids for more than 20 years in the automotive environment.
Vapor Compression Cycle
It is unlikely that stand-alone refrigeration systems for electronics cooling will be viable for the automotive environment because of cost and complexity issues. It may be possible to use existing vehicle air conditioning to cool some electronic modules with the use of cooled airflow.
Internal Chilled Water
Some new very high current/power generation applications may use engine liquid cooling systems or stand-alone liquid cooling for thermal dissipation. It is possible that existing vehicle air conditioning systems could be used to cool the liquid in stand-alone systems below ambient temperature if lower cost solutions can not be found.
Transient Thermal Absorption
Most automotive thermal transients are currently absorbed by the module packaging/case. As power and current values increase and package size decreases, there will be a need for low-cost phase-change materials for transient absorption. Designs that use these materials will need to reliably contain the material for the life of the vehicle.
Thermal Electric Cooling
Current thermal electric junctions either are non-functional or have low efficiencies at automotive ambient temperatures. The development of low-cost junction materials that could efficiently operate in automotive ambient temperatures may have future applications for electronic cooling.
Looking Forward
Future automotive electronic products will undoubtedly have increased power dissipation requirements. It is anticipated that high-power 42-volt electrical systems and hybrid (gas and electric powered) vehicles will substantially increase the need for high-power thermal management systems. These applications will also drive the need for higher IC and power transistor operating junction temperatures and high-temperature passive devices.
The development of low-cost semiconductor devices and device packaging that can reliably operate at junction temperatures of 175°C for digital and analog devices and 200°C for power drivers has the potential to significantly reduce thermal design cost for many automotive applications. If available, high-temperature devices could be used today in some underhood applications. This is particularly true for high-value capacitors used for electrical noise and switched current ripple suppression.
In summary, many emerging products for automotive electronics require very low-cost, robust electronic thermal management systems with lives of 20+ years that are capable of dissipating up to one kilowatt or more in +100°C ambient environments. Passive thermal cooling systems are preferred, where possible, for minimum cost and maximum reliability reasons. Electronic components and system-level packaging materials for high-temperature operation are also needed to reduce dependence on high-cost and more complex active thermal cooling systems for high-power electronic modules.
AP
DANIEL K. WARD is manager of advanced electronic packaging for Delphi Delco Electronics Systems, One Corporate Center, P.O. Box 9005, Mail Station: D-16, Kokomo, IN 46904-9005; 765-451-3093; Fax: 765-451-3115; E-mail: [email protected].