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Posted on 02 September 2019

A Power Module Concept for the Low Voltage MW Class

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Various applications like renewable energy generation and high power drives in the MW range have an increasing demand for high reliable power module concepts addressing the operation in a harsh environment. Next generation designs e.g. of wind turbine inverters need to be more reliable, more compact and less expensive to bring down the costs of installation and operation.

By Thomas Grasshoff, Reinhard Helldörfer Semikron Elektronik GmbH

Actual low voltage designs have reached their limits for current capability, weight and size. Medium voltage solutions below 10MW are expensive and do not allow an easy migration of existing designs. SEMIKRON has introduced the SKiN® technology as next generation assembly technology of power electronics after a long advanced development phase. SKiiP® X is the first serial product based on this technology. It is a modular, highly compact low voltage IPM overcoming today restrictions of 2MW+ applications. A primary failure root cause in wind and other demanding applications are pollution and condensation. Today available power modules are restricted to climate class 3K3 which does not allow condensation. The SKiiP® X is the first product on the market especially designed for climate class 3K4 and explicitly withstanding condensation in the system by allowing pollution degree 3.

The Design Concept

The power ratings of MW drives in wind applications increase step by step. Today 3 MW is the standard in the wind market. By increasing the power to 6000amps the size and weight of the inverter has to be reduced by half. The need for compact systems, high reliability and low cost means that new technology approaches are demanded and the classical modules used in power electronics – with copper base plate, solder joints, module case and wire bonding - will gradually vanish from the market. For a 6 MW wind turbine about 3000cm² of silicon area (IGBT´s and diodes) are used today. To achieve a high power electronics availability new inverter concepts have to be developed. This means better utilization of silicon, less components and - very important - less mechanical and electrical interfaces. Today high power inverters in the MW range are based on parallelization of modules and/or inverters resulting in the cost increase by reducing the overall reliability. Redundancy concepts are an option to achieve the requested high system availability but only with the disadvantage of high initial investments. The packaging technology - SKiN® - is based on the use of sinter layers instead of solder joints [1]. Using this architecture wire bonds are replaced by a flexible board which is sintered onto the chip surface. Unlike systems with bond wires, the chip has identical metallization (e.g. a silver layer) on both sides, meaning the highly reliable sinter layer on the chip top and bottom is extensively connected to the current path. Wire bonds can only contact about 20% of the potential die contact area. Figure 1 shows a SKiN® contacted IGBT and Diode, where a flexible printed circuit board with Cu-layers on both sides is attached to the dies by Ag diffusion sintering. Springs provide the auxiliary electrical contacts end ensure a solder less and very compact driver interface.

SKiN technology is based on flexible foil used in place of wire bonds

The maximum permissible power dissipation of a power semiconductor is limited by the maximum junction temperature, the temperature of the cooling medium and the thermal resistance between chip and cooling medium. A watercooler based high performance electronic power system shows in its thermal model that thermal paste is a key influencing variable and makes up for approximately 30% of the overall systems thermal resistance. By sintering the DCB directly onto the heat sink, this shortcoming can be eliminated. Bringing base plate materials into a liquid cooling circuit one has to be extremely careful about the long term corrosion effects. Cooling media and base plate material / coating have to match sufficiently. Therefore aluminum is the preferred choice due to its self-passivation (natural aluminum oxide) when liquids contain a small amount of oxygen. The dilemma is that it is not the preferred choice for base plates because of its high CTE and its poor compatibility to soldering. However, there are ways to solve these challenges: The usage of silver diffusion sintering to attach a pure Aluminium small area pin fin cooler to a DBC substrate. A comparison of the thermal resistances of a conventional layered system with base plate and SKiN technology shows that the thermal resistance between the IGBT junction temperature and the temperature of the cooling liquid comes down by 30%. The main terminals are sintered to the DCB as well providing high current contacts to the DC link. A welded joint to the capacitor or the DC link allows a cost efficient, compact and reliable interface. Thus high current density can be utilized to make highly compact and reliable systems in the MW range.

Based on this thermal performance and achieved power density a new design approach has been possible to arrange power electronics and cooler in a different way. The high achievable power density has its biggest benefit in liquid cooled applications. Today most coolers are arranged on the same level as the main assembly direction of modules and bus bars. The small SKiN® units allow different design approach. Three SKiN® elements are mounted onto a common liquid cooler. The resulting base element builds the core of a so called “power blade” and allows practical use of all SKiN® technology advantages, like low stray-inductance, optimal cooling and very compact design. To build up a module, interfaces to DC-link and AC-output, driver functionalities and cooling have to be added. Figure 2 shows a power blade, able to deliver 540amps at a size of approx. 260 x 115 x 45 mm. Despite an integrated water channel the cooler design separates electrical and fluidal areas even in fault conditions. Diagonal liquid flow allows autonomous venting at multiple default mounting positions without additional measures. As shown in [1] SKiN® technology provides very low thermal resistance. The chosen diagonal flow across all identical blades requires comparatively large inlet and outlet diameters. Optimization of the flow area shape and the pressure-drop ratio results in harmonized volume streams even in a nine blade configuration by a total pressure drop less than 400mbar.

SKiiPX power blade

Beside cooling, the liquid cooler with its three power modules acts as robust mechanical basis for the blade, too. A plastic frame on top of the cooler fastens the inserted power terminals which are molded by an elastomer which provides necessary sealing between environment and inner space. DC-link and AC-out terminals are located opposite at the blade’s long sides as shown in figure 3. For a smooth switching behavior the internal and external stray inductances are kept very low. A module suitable for high and medium switching frequencies has to have a low inductive design with a small stray inductance between IGBT and diode. This supports a fast and low loss switching with high di/dt. To prevent higher noise levels on the switching signal the coupling of the main current circuit to the auxiliary circuit has to be low, too. Usage of the flexible SKiN® copper layer enables new design approaches to improve and simplify a half bridge circuit layout. A symmetrical arrangement allows short commutation paths and simplifies the parallel operation thus current sharing between IGBT´s. The existing SKiN design achieves stray inductances in the area of 4 to 5nH, with terminals up to 15nH. A close parallel operation of two modules reduces the stray inductance compared to module designs with bus bars by at least 50% and is a good precondition for a superior current sharing. The terminal contact area has been optimized to allow direct DC-Link connection without any terminal lug by avoiding internal bus bars.

Explosion view of a SKiiPX® power blade

The secondary side driver board is fixated by the plastic frame, too. Specified landing positions of the power module spring connectors establish reliable electrical contacts. Above the PC-board a supporting grid acts as countermeasure to the spring forces. Thermally sensitive electronic parts are closely attached to the cooler to extend their lifetime. On top of the blade a flat plastic sheet covers the inner structure and seals it against environmental impacts.

Basically the overall system consists of multiple blades stacked on top of each other plus two end parts. Those are providing water inlet/outlet as well as mounting functionality. One essential idea of SKiiP®X concept is to use identical power blades regardless of the overall system configurations, e. g. paralleled half-bridges or 3-phase configurations.

SkiiPX® System assembly with nine blades for a 1.5MW 3-phase configuration

Figure 4 shows a complete SKiiP®X system consisting of nine power blades including the top sided interface plane to the controller as well as AC-outlets at the front side. Climate protection has been increased to climate class 3K4 and pollution degree 3 supporting long life operation in harsh environments. Compared to SKiiP® 3 (launched to the market in 2000) the footprint area has been reduced by 70% for the same achievable output power which leads to a significant reduction of the inverter size. A table below shows several IPM configurations. A four-blade configuration needs far less than 300mm width and is capable to control about 2200kW output power in a three phase system. The largest 3-phase type, consisting of nine blades, is able to handle 1650kW power and is mountable in an 600mm cabinet. There will also be a six blade configuration for a 3-phase one MW configuration available. This scalability supports versatile cabinet and power configurations. Weight of a 9-blade configuration is targeted below 25kg.

Blade version in a stack configuration

Using the 9-blade version in a stack configuration each of the three AC-out bus bars are collecting the AC outputs of 3 blades. An appropriate current sensor can be easily attached externally to them. A sealed interface connection (SUB-D) is located at the bottom front side to avoid water intrusion in case of condensation.

Five of this units can be mounted into a 600mm wide cabinet at standard height of 2000mm building a four-quadrant 3.3MW inverter plus brake chopper unit. Today systems need approximately twice the space.

Gate driver and performance

Each power blade includes its own 2nd side driver, fully insolated to the primary side. The driver board within each blade provides sufficient power to drive the paralleled IGBT gates of all three blade power modules. New low cost ASIC designs for primary and secondary side using digital protocol provide bi-directional communication across the high voltage insulation. Thus the logic interface connector at the blade front side uses voltages below 30volts while the overall jitter remains below 10ns. Different primary planes (= interface boards) are able to control up to nine blades, depending on the desired system configuration. They connect all blade signal outlets and provide a control interface to the system controller connected via a sealed SUB-D connector.

Double pulse test switching up to two times of Ic nominal (1100amps) for one blade shows a very good behavior in terms of current sharing, switching behavior and transient overvoltage. The overall rising current mismatch remains always below 10%. The important variance of switching delays does not exceed 50ns while transient overvoltage stays at any time below 1700volts @ 1300volts DC-Link voltage. This is valid for the whole operation temperature range without additional snubber capacitor.

Switching performance via temperature @ 1100A per blade

Figure 5 gives an overview about switching behavior as function of the operational temperature range. The overvoltage for all conditions remains well below 1700volts. The new assembly concepts sets a benchmark for reliability. SKiN ® Technology increases the power cycling capability by a factor 10 compared to traditional soldered and bonded modules thanks to the low temperature diffusion sintering attach. Part count of driver components is reduced by 50% as well the number of critical electrical plugs and water connections. Compared to e. g. SKiiP® 3 based assemblies the fit rate is decreased by half.

Summary

SKiN®-technology is a revolutionary progress in technology. It combines a reduction of low thermal resistance and reduced internal parasitic inductance and enhanced reliability of a wire bond free package technology platform. However in order to exploit the advantages emerging from this new technology, the module outline and system configuration are different to this new packaging platform.

Due to elimination of thermal interface materials and integration of a high performance pin-fin heat sink it is possible to withstand a doubled power dissipation in comparison to traditional designs. Just the elimination of thermal grease layer exhibits an improvement of 30% of the total thermal resistance junction to water. The SKiN® base unit is a building block supporting a compact assembly. This is ensured by a screw less connection of the main terminals and a spring contact interface to the driver board. A new design approach using a 500kW building block for use in low voltage converters shows that twice the current density can be achieved compared to a solution based on standard modules. Regulatory requirements in wind inverter applications have demanding requirements for the grid support like voltage and low frequency ride through conditions. The power module has to support these conditions by ability to work in an overload or high voltage situation. With this module concept wind power inverter from 2MW onwards can be realized easily accepting the nacelle space limitations. This opens the 6MW+ market for low voltage inverter using all the known benefits in terms of less restrictions and lower costs compared to medium voltage systems.

References
[1] P. Beckedahl et al.: Performance comparison of traditional packaging technologies to a novel bond wireless all sintered module; PCIM Europe 2011

 

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