Power electronics plays an increasingly important role in the automotive industry. Hybrid electric drive technology is now entering the mainstream market with hydrogen powered fuel cell vehicles still in research and development. One of the clearest indications that the future of vehicle design is changing can be found in the steady increase in the number of new hybrid models being offered in North America and Europe. This trend is likely to increase the application of power modules in motor drives.
John Mookken, Project Manager, SEMIKRON Inc., Hudson NH, USA
Two facts make the importance of power electronics in future cars a certainty. The first well-known fact to consider is that DC motors are unreliable and less efficient when compared to AC types. An AC induction motor has fewer parts, such as brushes and commutators, which greatly improve its reliability. AC motors are generally smaller, less expensive and more efficient compared to similarly rated DC motors. It is also known that in the past, the cost and complexity of AC drives have been barriers to their acceptance in a number of applications.
The second fact to consider is that most hybrid electric vehicles (HEVs) and most leading drive concepts for future cars rely on the electric power being stored in the DC form for a certain length of time. This means, barring any major technological breakthroughs in DC motor design, power must be converted from one form to another several times on future cars, and that virtually guarantees a bright future for power electronics in the automotive industry. In current HEVs, electrical energy is generated by the internal combustion (IC) engine and stored in a battery pack. This remains true whether the powertrain design is a parallel, series or the more common split hybrid design. In future designs the fuel cell/battery pack will likely replace the IC engine. In either case, power must be converted during charging, braking or coasting (regeneration), and motoring. Power converters and electric motors remain the common denominator when comparing different powertrain concepts illustrated in Figure 1.
Figure 1. Split hybrid or full hybrid (above) is the most common power train concept in current hybrid cars. In hydrogen powered fuel cell vehicle concepts (below) there are no internal combustion engines
In today’s industrial applications dominated $40 billion power electronics market, modules are not produced specifically for use in the automotive industry. Industrial drives components often cannot meet the reliability requirements in the harsh automobile operating environments and military grade components are too costly for use in cars. The market is now changing. With more electrification in future cars and the transition of the electric motors and drives from R&D to production vehicles, power module manufacturers are recognizing the need to develop power modules specifically designed to support the near term and long term needs of the auto industry. The race has begun to produce an affordable module that can meet the demanding performance, reliability, cost and manufacturability requirements set forth by the automobile manufacturers.
In general, some of the key characteristics for any new power module in development or in production for automotive traction applications can be summarized as follows:
Integration – one can expect to see a high degree of integration between the power devices, bus work, heatsink, bus capacitor, and gate drive/controller to optimize overall performance. Performance is measured in terms of output power, size, weight, cost and manufacturability.
High temperature operation – the ready source of circulating liquids found in cars, like radiator coolant and transmission fluids, make liquid cooling of the power module on automobiles an attractive option to improve module performance. The challenge faced by power module manufacturers is to design power modules capable of being cooled by engine coolant or lubricant with inlet temperatures of 105ºC. For power modules this translates to a wider operating temperature range (-40 to 125ºC). Power devices will require higher operating junction temperatures (Tj=175ºC), and higher temperature ratings (150ºC) for packaging and electronic components.
Flexibility – the term ‘flexibility’ in this case encompasses the capability for the module to be scalable, modular and customizable. The present and near-term continuous load for automotive traction applications is between 10kW and 120kW. Just like IC engines in the past, this figure is expected to rise over time. Scalability allows the module output power to be easily scaled up or down without changing the module footprint. Modularity allows the design to be easily upgraded when new technology becomes available and customizability makes it easier to integrate customer furnished materials into the existing design.
Robustness – high reliability in the harsh automotive environment is a requirement for any automotive component. However, traditionally power modules have not had to meet the under-the-hood temperature and vibration requirements. Power modules used in cars must be designed for the life of a car, which is usually 100,000 miles (160,000km) or 15 years. The new power module designs will have to pass tougher electrical, mechanical, EMI and environmental tests.
Economical – one of the key characteristics of the new power module that is also in opposition to the other characteristics listed above is affordability. A reasonable price target for power modules in automotive quantities is estimated to be less than $6/kW for the design to successfully penetrate the automotive market.
SKAI ADVANCED INTEGRATION MODULES
Incorporating most of the desirable characteristics mentioned in the previous section into a new first generation power module for the automotive industry is SEMIKRON’s SKAI (SEMIKRON Advanced Integration) module. It integrates all the necessary hardware for a drive into a single package and gives us a glimpse into the future of power modules.
The module integrates the power stage with the liquid cooled heatsink, gate drivers, controller, and protection logic to provide a ready solution to the automotive tier 1 suppliers. Figure 2 shows the key components that make up the SKAI module.
Figure 2. The major sub-components that make up a 600 or 1200V IGBT based SKAI Power Module (components shown in red can be substituted with customer furnished parts)
A high level of integration can be seen between the AC and DC bus structure on each of the baseplate less half-bridge circuits (see Figure 4).
Figure 4. SKiiP technology in SKAI
Each Direct Bonded Copper (DBC) AlN ceramic substrate contains twelve 600V NPT or 1200V trench IGBTs, six matching diodes and a positive coefficient thermistor (PTC). The bus structures (AC and DC) and the populated DBCs are all held together under more than 153kg/cm2 of pressure using SEMIKRON’s SKiiP technology, an unique and proprietary pressure contact system. Tight integration of the DC and AC bus bars on the DBC keeps parasitics to a minimum. Bus filtering is included with a 1mF metalised polypropylene capacitor which is also closely integrated with the bus bars. Two magneto resistive current sensors are integrated with two of the three AC bus bars which are attached to the control board via flexible cables. The control board interfaces with the DBCs via spring pins and includes the power supply, gate drives, DSP, communications hardware (supports CAN/IEE485) and protection circuitry.
The SKAI module is designed to operate in the -40 to 85ºC temperature range. The module is available on either liquid cooled or air cooled heat sinks. Coolant inlet temperatures are limited to 70ºC with higher temperature operation possible with power derating.
The modular nature of the SKAI design allows the units to be easily customized. Only minimal restrictions are imposed in cases where the customer wants to furnish the heatsink which could also be a part of the customer’s system enclosure or supply custom control boards with special connectors or processor.
The SKiiP technology used in the SKAI module was first introduced by SEMIKRON in early 1992 in power modules. In addition to simplifying module assembly, the pressure system eliminates the large solder interfaces typically seen between the DBC and the module baseplate which greatly improves reliability. The pressure system is shown to be superior to traditional power modules by a factor of 10 in power cycling tests. The use of AlN substrate also improves reliability; studies have shown that the module lifetime can improved by a factor of 2 using AlN rather than the less expensive Alumina (Al2O3). The pressure system also reduces the number of wire bonds in the power module by using spring pins to contact the gate and sensor connections from the DBCs to the driver/control board. The use of Ag plated spring pins have been shown to be a highly reliable method for low current control and sensor contacts for use in power modules.
In addition to automotive applications in hydrogen powered fuel cell vehicles, SKAI modules are currently undergoing field tests in applications as varied as driving radiator fans and providing auxiliary power on railroad locomotives and agriculture vehicles, traction and hydraulic pump motor drive on mining vehicles and as traction drives on small electric vehicles.
The second generation of the SKAI module, which is under development, will feature a wider operating and storage temperature range. A more advanced heatsink design has the potential to significantly improve power density of the existing SKAI module. Higher junction temperature Si trench IGBTs, high temperature conductive plastics or light weight metal enclosures, new current sensor technologies and high temperature capacitors are being developed for the next generation. The new SKAI will include a more capable controller such as the TI 28xx series DSPs and triple sealed automotive style connectors with more I/O lines. The lure of high volumes in future cars will continue to drive the power module manufacturers to push the envelope of performance and affordability of future power modules.
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