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Posted on 01 May 2019

System Integrated Drive for Hybrid Traction in Automotive

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ECPE Demonstrator Programme

The pre-competitive research in power electronic systems in ECPE is based on long-term research roadmaps, and has its focus on automotive and industrial power electronic systems. With ECPE it could be managed to jointly formulate research topics in a European industrial consortium of main players in power electronics

By Dr. M. März, Fraunhofer Institute of Integrated Systems and Device Technology (IISB), Center for Automotive Power Electronics and Mechatronics

 

The ECPE research activities funded by the ECPE partner industries is focussing at so-called demonstrator projects where new ambitious power electronic systems or sub-systems are developed and realized by leading European Competence Centers. This article describes results of the “System Integrated Drive for Hybrid Traction in Automotive” Demonstrator Programme performed by the Fraunhofer Institute of Integrated Systems and Device Technology (IISB) in Erlangen, Germany.

Electric drive with the power electronics, both integratedinto the clutch-box of a car

The integration of the power electronic inverter with an electrical machine in the automotive powertrain is in the focus of the automotive Demonstrator Programme where the existing cooling circuit from the internal combustion engine is used also for the direct liquid cooling of the power electronics. The mechatronic integration of motor and power electronics leads to an ultra-high power density of 75 kVA/l for the inverter. The high temperature of the cooling medium of up to 115°C in combination with this high power density poses a unique challenge.

An optimised thermal design is necessary to master this thermal challenge. All temperature-sensitive components, passives and semiconductors, have to be thermally connected to the cooling system which provides a sufficient cooling capacity although working on a very high temperature level.

On the module level, the optimized thermal design has to provide a homogeneous distribution of the chip temperature of the power semiconductors.

Towards an Integrated Inverter Drive

For the latest generation hybrid drive there was a strong request for a solution that completely fits into the existing drivetrain of a passenger car. The clutch-box was designated as the housing, in which an electrical drive unit with a mechanical output power of 50kW and a maximum torque of 220Nm had to be integrated together with the complete inverter. But the conical, tuba-shaped housing with internal studs and ribs considerably complicated the integration challenges.

The basic internal arrangement of the components is shown in Figure 2. A permanent magnet excited synchronous machine (PM machine) provides a very high power density and a high efficiency even in the low speed range. By using a whole-coiled winding (single teeth coils), the winding overhang could be greatly reduced, resulting in an increased active motor length. The torque disadvantage, caused by the conical housing and the thereby restricted motor diameter, could be equalized this way. The electrical circuitry of the motor windings corresponds to that of a classical three phase machine. Thus only three power interconnections between motor and inverter are necessary. Multifunctional winding interconnections prevent a heat transfer out of the hot windings into the power electronics.

X-ray view on the second generation inverter drive (50kW), integrated into the clutch-box of a passenger car

In order to achieve an optimum usage of the available package volume, a ring-shaped dc-link capacitor has been developed in cooperation with EPCOS AG. This capacitor provides a capacitance of 500µF (450V). Its concentric, nearly coaxial terminals form the dc-link busbar and allow the realization of a very low parasitic dc-link. The ripple current rating of this capacitor is several hundred amperes and thus far beyond the actual ripple current load. The self- heating, caused by the inverter ripple current, is therefore negligible. However, in order to protect the capacitor against the high ambient temperature of up to 140°C, it is thermally coupled to the cooling jacket.

As can be seen from Figure 2, three half-bridge power modules are placed at the periphery of the water cooling jacket that surrounds the whole electric machine. The remaining sections at the periphery are used for the control board, the current sensors and the EMC filter. Since the system is also intended as a test platform for different power module designs, special attention has been put on a modular and assembly friendly construction.

Special attention has been paid to an optimized thermal design of the power modules. The inverter drive is inserted in the existing coolant cycle of the ICE. The available coolant flow is about 8 litre/min and the maximum allowable total pressure drop 200 mbar. An optimization of the back-side structure of the modules is necessary in order to minimize the thermal resistance between the power semiconductors and the coolant, but not to exceed the maximum allowable coolant pressure drop. The cooling efficiency of a finger-structure varies with its geometry - namely the finger thickness, shape, height and surface roughness - as well as the spacing and arrangement of the fingers. An important issue in this context is that the parameters of the coolant, especially the viscosity of the water-glycol mixture, greatly vary along with the temperature.

The realized power modules showed a specific thermal resistance junction-coolant (Rth,jc) of 0,45Kcm2/W at a coolant flow rate of 8 litre/min and a pressure drop of 60 mbar, both of which were close to the predictions from simulation. With respect to the coolant flow, the three power modules are connected in series, thus the specification of a total pressure drop of 200 mbar was achieved.

The prototype was built on a machined Cu base plate. This, of course, is no solution for series production, partly because of cost issues, but mainly because of reliability issues of the DCB to base plate solder joint. Due to the passive temperature cycles to be sustained (ca. 11.000 cycles with a mean temperature swing of ca. 100 K), an AlSiC base plate could be a choice to reduce the thermal mismatch between base plate and DCB substrate to an acceptable low level at a small expense of thermal efficiency.

Conclusions

Simultaneous engineering (electrical, mechanical, thermal) is an imperative prerequisite for a mechatronic integration of power electronics. Consistently following this way, an inverter drive for hybrid traction could be integrated into the clutch-box of a passenger car, i.e. in an environment and package volume that had been considered as absolutely useless for high power electronics so far. 3D integration, new components and sophisticated thermal management solutions opened the way to a system with unique power density. The next steps aim on a further optimization of the system reliability, manufacturability, and modularity.

 

Results of the ECPE Research Programme

ECPE Image

ECPE European Center for Power Electronics is driving precompetitive research in power electronics jointly financed from an industrial research fund. Three Demonstrator Programmes have been started in 2003/2004 involving leading Competence Centers in Europe.

Experts from the Fraunhofer Institute of Integrated Systems and Device Technology (IISB) will present the Integrated Hybrid Traction Demonstrator during the PCIM Europe 2007 exhibition. Presentations will take place at the ECPE Joint Stand 12-466 just vis-à-vis the PCIM Forum.

The integration of the power electronic inverter with an electrical machine in the automotive powertrain is in the focus of the automotive Demonstrator Programme where the existing cooling circuit from the internal combustion engine is used also for the direct liquid cooling of the power electronics. The mechatronic integration of motor and power electronics leads to an ultra-high power density of 75 kVA/l for the inverter. The high temperature of the cooling medium of up to 115°C in combination with this high power density poses a unique challenge.

An optimised thermal design is necessary to master this thermal challenge. All temperature-sensitive components, passives and semiconductors, have to be thermally connected to the cooling system which provides a sufficient cooling capacity although working on a very high temperature level. On the module level, the optimized thermal design has to provide a homogeneous distribution of the chip temperature of the power semiconductors.

 

References:

1) M.Marz, M. H. Poech, E. Schimanek, and A. Schletz, "Mechatronic Integration into the Hybrid Powertrain . The Thermal Challenge," International Conference AUTOMOTIVE POWER ELECTRONICS, APE, Paris (France), June 2006.

 

 

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