There is no doubt that the operating environment has a major effect on the life and reliability of electronic assemblies. Power electronics in particular are perhaps most susceptible to life and reliability degradation, as there are usually several components within the system whose losses all conspire to raise the operating temperature. Losses include switching and conduction losses in semiconductor devices as well as losses in passive components such as magnetics, filter damping resistors, and bus bars.
Understanding the maximum ambient temperature your product will be required to operate in, and properly analyzing the losses in the design will directly affect its reliability and life. Operating temperature usually has the highest impact on electronic component life and reliability – the rule of thumb in electronic product design is that every 10 degrees Centigrade rise in temperature decreases the average life by 50 percent*.
It stands to reason that if you can manage temperature and lower it by 10 degrees, you’ll effectively double the life of your power electronics. Put another way, if you’re evaluating the mean time between failures (MTBF), you can double your MTBF, on average, by lowering the temperature by 10 degrees. Often time’s inadequate thermal analysis results in improperly designed thermal management solutions. This in turn causes products to operate much hotter than intended, ultimately leading to premature product failure.
A more subtle consideration is cyclical temperature change. The temperature excursion of key components needs to be taken into account on products with cyclical loadings, such as motor drives. Each time a part heats or cools, mechanical stresses are induced due to mismatches in thermal coefficients of expansion, differences in material thermal conductivity, and the particular characteristics of the heat flow path, which incrementally reduce its life. And as one might expect, the larger the temperature swing, the larger the stress, and the greater the impact on life.
This is a major consideration for semiconductors, which utilize a number of different materials, and operate with very high power density. The cooling system design, therefore, needs to take into consideration not only average temperature rise, but also temperature change with expected cyclical loading.
What is the best method of controlling heat in power conversion systems? The rate of heat transfer between bodies depends primarily on the environmental contact medium – solids, liquids, and gases all have rates of transfer or thermal resistance. Air cooling, either by convection or forced convection, as well as liquid cooling alternatives, are all options. Radiant cooling, which does not require a medium for heat transfer, is generally ineffective for power electronics given the high power levels that must be handled and the moderate temperature differentials between heat-producing and heat-absorbing components.
Power density, available space, noise, system reliability, and maintenance are all factors in selecting a cooling method. Many engineers with minimal high power experience tend to shy away from liquid cooling, but it can be enormously beneficial in many applications, particularly for those with high power and/or high ambient temperature requirements.
Liquid cooling has multiple benefits over air cooling. In general, liquids have much higher thermal conductivities than air, and thus much higher heat transfer coefficients associated with them. As such, liquid cooling is a far more effective, higher performance method when compared to air cooling, which often allows the use of smaller, less expensive semiconductors.
A second benefit is that it that less space is required than air cooling. Liquid-cooled systems can be designed with little or no airflow. This allows electronics to be packed in more tightly, providing a number of functional benefits, including lower component costs, better electrical characteristics, and less audible noise. Liquid cooling also allows the use of sealed enclosures/cabinets, which can be very beneficial for systems intended for rugged environments.
There are a number of factors involved in effective thermal management and developing the most efficient, effective, and reliable power conversion solution for your application. The thermal analysis and design of your cooling system are just as important to your product’s success as the controls and hardware implementation. Consider partnering with an experienced power conversion system design and manufacturing firm such as Oztek to ensure optimal performance and reliability while saving time and money.
* Based on the Arrhenius equation, which says that time to failure is a function of e-Ea/kT where Ea = activation energy of the failure mechanism being accelerated, k = Boltzmann’s constant, and T = absolute temperature.
