AUTOMATION & CONTROLS In hybrid and electric vehicle power trains, power inverters are exposed to considerable temperature swings. The resultant non-homogenous temperature distribution limits the output power and reduces the service life of the power converter. Power modules for hybrid and electric vehicles As a result of these temperature swings, by Dr. Volker Demuth, Semikron steps must be taken to ensure that power modules used in hybrid and electric vehicles are able to meet typical application requirements, such grow by an impressive 20% per year. The packaging solutions today are soldered as being able to withstand more than application possibilities are vast, and modules with and without base plate, three million active thermal cycles. As inverters for hybrid and electric drives and, most recently, modules with no regards the future development of hybrid can already be found in lorries, buses base plate in sinter technology. These and electric vehicles, semiconductor and agricultural vehicles, as well as in packaging technologies have different technology reliability is an important automotive and racing car applications. advantages and disadvantages, which quality factor. As different as the requirements may be is why the service life design calls for At the moment, 4% of all power in the different areas of application, the an evaluation of these technologies modules in use today are found in main focus in all cases is to develop with regard to the requirements for automotive applications. Over the next reliable packaging technology for the hybrid and electric vehicle applications. few years, this market is expected to power modules. The most prevalent Changing ambient temperatures, e.g. in the cooling water cycle, are responsible for passive thermal cycles. In addition, the power loss that occurs in the power semiconductors produces brief (5 – 20 s) temperature lifts of DT = 40°C to 60°C. Here, the power semiconductors are heated from e.g. the cooling water temperature of 70°C to between 110°C and 130°C, after which they drop back to the cooling water temperature. Owing to the different coefficients of thermal expansion of the materials used, every temperature change that occurs results in mechanical stress. This causes material fatigue in the solder and bond connections and, ultimately, component failure. Avoiding solder connections In modules with no base plate featuring pressure contact technology, several paths are pursued to boost module reliability. By consistently avoiding solder connections, solder fatigue – a key failure mechanism in power modules – Fig. 1: The SKiM 63 power module. can be completely eliminated. Here, the October 2010 - Vector - Page 43 by a factor of 15. A further advantage of the removal of solder inter-connects and base plate is that, in modules with base plate the soldered DBC areas should be reduced to a minimum in order to reduce material fatigue in the solder joints; here, the high thermal conductivity of the base plate ensures the necessary thermal spreading. When designing a module with no base plate, in contrast, the DBC area can be larger. Optimum heat distribution This paper looks at the positioning of IGBT and free-wheeling diode in a Fig. 2: Cross-section of a SKiM module with base plate (left) and a solder-free module without base plate (right). The removal of solder joints eliminates solder fatigue, a common 3-phase 400 A, 600 V inverter module. failure mechanism in power modules. The removal of the base plate also eliminates a large In the case of the modules with a proportion of the thermal stresses. base plate, two 200 A IGBTs and two 200 A free-wheeling diodes are used per semiconductor switch. A complete phase therefore consists of 4 IGBTs and 4 free-wheeling diodes. The optimum arrangement for modules with no base plate is four 100 A IGBTs and two 200 A free-wheeling diodes per switch (eight IGBTs and four free-wheeling diodes per phase). This means that the base area of a 3-phase module with no base plate will be around 10% larger than that of a module with base plate. When the inverter is in operation, conduction and switching losses occur, meaning that the power semiconductors act as local heat sources. With the help of 3D Finite Element calculations, thermal spread in an inverter module and heat sink for any given operating state can be calculated. For example, when a hybrid or electric vehicle is accelerated, the majority of power losses are produced in the IGBTs, while the free-wheeling diodes are subjected to a lower load. Fig. 3: Chip layout for a module with base plate with 4 x 200 A IGBTs and 2 x 200 A free This is why in the thermal image, the wheeling diodes. By comparison, the layout of a SKiM module without base plate with IGBT positions are seen as strong heat 8 x 100 A IGBTs and 2 x 200 A free wheeling diodes utilises the larger DBC area for sources. In the case of modules with optimised heat distribution and for heat dissipation. base plate, the heat is concentrated in the centre of the 3-phase configuration. solder connections between chips and base plate bending. In modules with no Owing to the close positioning of the insulating DBC ceramic substrate are base plate, the main problem that has semiconductors and the short distance between the phases, the temperatures replaced by a highly stable sinter layer to be dealt with is how to compensate of the IGBTs are highest at this point. and conducting connections in contact for the surface roughness of the heat Although in this operating state the pressure technology. The removal of the sink and DBC surface, which is why a freewheeling diodes are subjected base plate has a number of benefits: first 20 – 30 µm thick thermal paste layer is to moderate loading only, the IGBTs of all, the thickness of the thermal paste sufficient. The removal of the base plate cause the diodes in the centre of the layer between module and heat sink can means the removal of one of the main module to heat up considerably. At be reduced. Thermal paste is one of the causes of thermal stress. the edges of the inverter module, the main factors contributing to the total Temperature-induced stress is effectively temperature of the diodes, by way of thermal resistance in the power module; reduced and reliability thus significantly contrast, is 15°C lower. Despite base this is why as thin a layer of thermal increased, as accelerated passive plate, the power semiconductors paste as possible should be used. In thermal shock tests at 40°C and 125°C in the edge regions of the inverter modules with base plate, a thermal show: in the case of sintered modules module become far less hot than in the paste layer of between 75 and 150 µm with no base plate, the number of module centre, which ultimately leads in thickness is needed to compensate for possible thermal shocks was increased to non-homogenous heat distribution October2010- Vector - Page 44 a b Fig. 4: Temperature distribution in a module with base plate (left) and a module without base plate (SKiM, right). Load conditions: battery voltage = 350 V, output current = 250 A, output voltage = 220 V, output frequency = 50 Hz, switching frequency = 12 kHz, phase angle cosΦ = 0,85, cooling agent temperature = 70°C. overall system design. In addition to this, temperature sensors on each insulating DBC ceramic substrate allow for separate evaluation of the individual phases, providing an additional control possibility for operating temperatures. Temperature and service life For actual thermal loading on an inverter in operation, time-dependent loads must be taken into consideration. During the actual running of a hybrid or electric vehicle, various load states occur: during vehicle acceleration the IGBTs are under a particularly high load, while during deceleration where energy recovery takes place and the battery of the electric motor is re-charged, it is the free-wheeling diodes that are under the greatest load. To describe the time-dependent heating of the inverter Fig. 5: Time-dependent thermal resistance between IGBT and cooling medium. module, the behaviour of the power module has to be investigated for load cycles in the 0,1 to 30 s region, too. The to the 3 phases: the mean thermal distribution is far more homogenous: time-dependent thermal resistance of load on the IGBTs in the centre here, too, the IGBT positions can be IGBTs increases for both configurations phase is almost 10°C higher than seen as the strongest heat sources. in line with the duration of the load the mean temperature of the IGBTs However, since the thermal losses are impulses (see Fig. 4). The heat begins of the external (outer, at the module distributed across several positions to flow, spreading from the power edges) phase. The difference between and the distance between the DBCs is the maximum and minimum IGBT greater, more space is available for heat semiconductors in the direction of the temperature is more than 20°C. The dissipation. The losses produced can be heat sink, causing the entire module to centre phase limits the useable electric effectively dissipated, reducing mutual heat up. If the load impulses last longer power in the entire inverter module. heating between IGBT and diode. The than 30 s, the module will fully heat This has two consequences: on the optimum heat dissipation also ensures up and the thermal resistance will stop one hand, the cooling conditions and homogenous load distribution across increasing. the load have to be selected such that the different phases: the temperatures The time-dependent thermal resistance the temperatures in the centre DBC of the IGBTs and diodes between the values can now be used to calculate the do not become too high; on the other three phases of the power inverter are thermal load acting on the semiconductor hand, temperature-induced damage homogenous; the mean temperature of switches and valves during operation.