Power electronic systems are used in day-to-day applications. The sophisticated technology behind the scenes, however, often goes unnoticed. Take, for instance, public transport vehicles such as the underground, trams or trolley buses: the only time the systems that run these vehicles are noticed is when they fail to work properly, for example, if people get trapped in a stalled metro train. The utmost care, therefore, has been taken to guarantee the reliability of the power electronics driving the vehicle. This article outlines the cooperation between two manufacturers that fulfil the demands for public transport.
Dr. Ladislav Sobotka, ŠKODA ELECTRIC, Pilsen, Czech Republic and Ralf Herrmann, SEMIKRON, Nuremberg, Germany
The environmental requirements for traction applications are very demanding in terms of ambient temperature, power and temperature cycling, as well as size. Ambient temperatures under operation normally exceed 100°C, but could also drop at night during standstill to below zero centigrade. This passive temperature swing shows the need for extreme temperature cycling capability. Equally important is the need for a compact overall package, a high power density and a robust design in terms of vibration and shock. These somewhat conflicting requirements have to be taken into consideration when designing a new IGBT traction converter. The need for high power density at high coolant temperatures often goes at the expense of the power cycling capability.
IPM for traction
Many conventional power modules with copper base plate are available for traction applications. These modules then still have to be matched with driver boards and heatsinks from several suppliers. The user is responsible for matching the components and ensuring the proper functioning of the individual components in interplay. A SKiiP intelligent power module comprises three matched components: heatsink, IGBT half-bridges and driver with integrated protective functions. High power density, load and temperature cycling capability are ensured, thanks to the patented SKiiP pressure contact technology.
ŠKODA's more than ten years of experience with challenging applications such as trolley buses, trams and metro cars is a confirmation of the high reliability of the intelligent power module SkiiP. When developing unique technologies for public transport vehicles, there are applications where the use of intelligent IGBT modules is the only possible option, e.g. the 100% low-floor tram ŠKODA ForCity with rotary bogies.
Figure 2. SKiiP pressure contact technology without baseplate. The thermal cycling capability is five times higher than that of a standard module with baseplate
Figure 3. Power module on heat sink
The modules (Figures 2/3) are used for asynchronous traction drives for voltages of up to 1000VDC on a direct current intermediate link. Almost twenty years of experience with pressure contact technology has gone into this power module.
The principle behind the technology is a mechanical pressure system that presses the DBC (direct bonded copper) onto the heatsink without soldering. This results in homogenous pressure distribution with a thermal connection between the ceramic substrates carrying the semiconductor chips and the heatsink. A 40% improvement in thermal resistance (Rth(j-s)) compared to standard modules is achieved. SKiiP has no baseplate and fewer solder layers, resulting in lower thermal-mechanical stress inside the module. The thermal cycling capability is five times higher than that of a standard module with baseplate, is reached even under the harsh climatic conditions common to the electric traction industry, and brings a number of crucial advantages for this field of application. High load and temperature cycling capability is ensured thanks to the use of patented SKiiP pressure contact technology. The pressure contact technology has been optimised to provide low thermal resistance and high load cycle capability. Well-matched materials with careful consideration of the coefficients of thermal expansion (CTE), state-of-the-art packaging and bonding technologies make this module the fitting choice for this area of application.
For lower outputs, ŠKODA uses modules with air-cooled heatsinks. For traction drives in metro cars, especially in the electro-dynamic brake mode, liquidcooled modules are preferred, thereby utilising the maximum current load of the IPM. The low thermal resistance is used to provide high load capability. This means that the dynamic behaviour of the semiconductor structure is optimised to achieve maximum reliability and minimum switching losses. Thermal and over-current protection elements are already integrated in SKiiP. The corresponding sensors can be used for traction converter semiconductor diagnostics and for automatic intervention when admissible limits are exceeded. The current sensors are used for traction motor control.
High reliability and long service life
The IGBT converters have become standard for power supply in asynchronous traction motors and are subject to considerable price pressure. This is why the technologies used in intelligent IGBT power modules are compared to conventional IGBT transistors in every new project. The good price-performance ratio, minimum failure rate and long service life of IGBT traction converters are important for end applications. Every year, ŠKODA ELECTRIC manufactures more than 300 SKiiP-based converters for public transport vehicles and prefers the IPM, especially in light of its extensive positive experience in operating these vehicles. The robustness and resistance to thermal cycles are particularly crucial selection factors.
The tram car consists of four fully rotating traction bogies which bear 16 synchronous low-speed traction motors with permanent magnets. Each of the traction motors is supplied separately from a voltage converter comprising SKiiP modules. The roof container houses the entire traction equipment for one bogie, including an isolated brake resistor. Traction converters work with 5kHz pulse modulation, which is the optimum modulation for this type of vehicle. Given the limited space and the parameters requirements for synchronous motors with permanent magnets, this constitutes the only possible design solution, owing to the compactness and thermal properties of SKiiP modules.
All SKiiP IPMs used are 1700V modules. The current parameters of the modules in the traction converters are 500A, and the modules of the brake converters and input recuperation circuits are 1000A. The traction container can withstand harsh type tests. The design meets the challenging demands of the fast IGBT technology, in respect of the very low inductance between the DC link capacitance filter and the IGBT modules. Easy access to these modules for measurement or service reasons is a further important factor.
Contracts for ForCity trams constituting a production volume of more than 1000 traction containers have already been signed, i.e. 4000 traction inverters comprising the proven SKiiP modules.
Another application featuring the liquid-cooled SKiiP modules is currently under development: traction equipment for the underground system in St.Petersburg, Russia. Here, the modules must be able to work for a short time in the electro-dynamic brake mode at an output that reaches almost 1MW. A challenging requirement is the -40°C temperature level required by the customer. Traction drives for metro cars are among the most demanding applications for thermal cycles, which is why all operating states were simulated carefully and the traction converters subjected to worst case dynamic load tests. The SKiiP module passed these tests, thus guaranteeing the high reliability and long service life of these modules for the future. SKiiP has also been used in the first ever hydrogenfuelled bus; the official presentation and first road tests have already taken place. In addition, ŠKODA ELECTRIC employs these modules in converters for auxiliary drives in the rail industry; electric locomotives and electric multiple units (EMUs).
Fully tested power module
The power module is 100% tested to meet the requirements of traction applications. Once the IGBT half-bridges, driver and all other components have been tested, the overall system qualification is carried out. If modules are bought separately and used in combination with the customer’s own driver or drivers from other suppliers, the overall system test has to be done by the customer. SKiiP guarantees lower costs (development of driver electronics and test plus the corresponding test equipment) and no time wasted on inhouse development.
An optional burn-in test for SKiiP modules is available, in which the modules are operated for approximately two hours under worst case real inverter conditions at elevated temperature and elevated voltage. All root causes of early failures are identified and eliminated. SKiiP undergoes one of two burn-in cycles. The modules are tested with cooling water at 80°C and cycling at a constant chip temperature. The junction temperature of the silicon reaches temperatures up to 140° (IGBT3) to ensure high stress levels for the module. High power densities at coolant temperatures of 105°C can only be achieved at a maximum junction temperature above 150°C.
For comparable conditions and module sizes, the SKiiP4, which was introduced this year, provides 33% more power than the current version of this module (Figure 4). On the one hand, this allows for the development of more powerful or more compact frequency converters, thus reducing costs. This increase in power is due to the use of an innovative pressure contact system, an improved heatsink and IGBT4, CAL4 diode chip technology. In addition, six parallel half-bridges have been used for the first time at the upper power end instead of four, as was the case up till now (Figure 5).
Figure 4. Power module featuring six parallel half bridges
Like its predecessors, SKiiP 4 is based on well-matched components such as heat sink, power module, driver and protective sensors/functions. Here, the mounting and connecting technology, which is based on the pressure system, still plays a crucial role.
For a reliable and smooth switching behaviour, a new driver was developed with digital signal transmission, fully galvanic insulated switching and sensor signals, a diagnosis channel (based on CAN open protocol) and a multi-output stage.
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