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Posted on 01 October 2019

Modular IGBT Stacks to Increase Power Density

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Integrating the New PrimePACK

The integration of the new PrimePACK module into an existing inverter platform is discussed. The mechanical features of the module allow an optimization of the thermal management and to increase the usability of the IGBTs. Measurements of the inverter performance are presented.

BY M.Schulz, O.Schilling, M. Wölz, G. Borghoff, and J. Schiele, Infineon Technologies AG, Warstein

 

Whenever a power module is integrated into a converter platform thermal, electrical and mechanical constraints have to be considered. The PrimePACK housing offers a practical interface between IGBT power switches and the converter surroundings [1]. The first integration of the PrimePACK module in a converter is presented based on the well known ModSTACK inverter series [2]. It is shown how an optimised cooling concept results in an improved utilization of the available heat sink area and helps to lower the thermal resistance.

The PrimePACK offers the chance to increase the operation temperature to 150°C. First results covering the performance and the increase in converter usability under these harsh conditions are presented.

Converter architecture

The approved design kit ModSTACK for power converter solutions consists of OEM components for thermal management, electrical and mechanical interconnection and interfaces between power unit and control. Different circuit topologies and expansion of system power range are available using ModSTACK components.

By choosing suitable IGBT halfbridge modules the most common topologies can be realised. High voltage electrolytic capacitors are used for the DC link circuit of the power unit to ensure safe operation up to 1070V. The monitoring and control unit is supplied with signals supervising phase current, DC voltage, on-state voltage and heat sink temperature. The control unit is equipped with EiceDRIVER integrated IGBT driver cores [3]. Figure 1 gives an overview of the functional components of the inverter.

Modular ModSTACK power unit equipped with PrimePACK IGBT halfbridge modules

The ModSTACK was originally designed for IHM modules and is now adapted to the PrimePACK package as depicted in Figure 2. It is compatible with both PrimePACK2 and PrimePACK3 which will be available in the module line-up.

Photo of a ModSTACK power unit equipped with PrimePACK2

Thermal properties and layout optimization for an air cooled heatsink

The PrimePACK offers a practical solution to improve the thermal management by lowering the thermal resistance Rthch between baseplate and heat sink. Due to the rectangular footprint of the PrimePACK a small distance between the screws that tighten the baseplate to the heatsink is achieved even for a large overall contact area between both components. The thickness of thermal grease dg can thus be kept in a very low regime of dg<50μm. The copper baseplate furthermore ensures effective heat spreading through its high thermal conductivity of λ=385W/mK. In an experimental setup Rthch is measured on a water cooled heatsink. This represents a worst case situation because water cooled systems are characterised by less thermal spreading in comparison to their air cooled counterparts.

The heatsink is equipped with a set of thermo-couples that allows measuring both the temperatures at the module baseplate Tc and inside the heatsink close to the surface Ths. The thermocouples TC1 and TC2 are positioned underneath the devices that generate power as shown in Figure 3.

Schematic cross section of measurement setup including position of thermocouples

If the total electrical power dissipated in the power module Pel is known, the following formula can be applied to evaluate Rthch:

Equation 1

The influence of the amount of thermal grease that is dispensed on the surface of the module baseplate prior to mounting it onto the heatsink is investigated. A thermal grease with λ=1W/mK is used. The measurement is done for the PrimePACK2 subsequently for IGBTs and diodes and Rthch is calculated by paralleling both values. The result is given in Figure 4, where Rthch is plotted as a function of the dispensed thickness dg*.

Correlation between Rthch and grease thickness dg, which is measured prior to mounting

The following conclusions are drawn

If a 100μm spacer is applied to guarantee dg=100μm after mounting, the measured value is Rthch~10K/kW. This is close to the calculated value.

If mounting is done without spacers, lower Rthch values of about 4 to 6K/kW are reached that only slightly depend on the amount of grease dispensed as excessive grease is squeezed out during the mounting process very effectively due to the aforementioned small distance of mounting screws. Besides, the heat transfer at the metallic contact between baseplate and heatsink plays an important role and does not depend on grease thickness at all. In general the Rthch of a PrimePACK module is rather insensitive towards the method by which heat conductive grease is dispensed making thermal management more reliable by means of the footprint geometry.

In the range of dg*<50μm, Rthch of ~4 to 5K/kW can be reached.

The thermal resistance between the heatsink and ambient air Rthha depends on how IGBT modules are positioned on the heatsink surface. In order to find an optimised geometry Rthha is determined by measurements using resistors as well defined heat sources. Square shaped resistor modules are mounted to a PrimePACK baseplate instead of IGBTs to create a reference module.

Rthha is calculated by the formula:

Equation 2

The ambient temperature Ta is measured at the entrance of the air flow. To determine the heatsink temperature Ths, small grooves are milled into the surface of the heatsink to place thermocouples close to the module baseplates.

Using the reference modules described allows for the variation of the arrangement in order to find an optimised geometry. It turns out that less thermal stacking and lower Rthha values are obtained if the longitudinal axes of the rectangular modules are aligned in parallel to the heatsink fins in comparison to the geometry with perpendicular orientation of modules and fins. Figure 5 shows a comparison for different footprints. The lowest values are obtained for the PrimePACK3 because it offers the largest contact area.

Measured Rthha-values for different reference modules on an air cooled heatsink

Inverter performance

Measurements were done on the ModSTACK inverter equipped with FF600R17IE3 PrimePACK2 modules under laboratory conditions. In Detail, these were UCC=900V, fsw=2,5kHz, f0=50Hz, cos(Φ)=0, Ta=24°C. The maximum RMS current is calculated as a function of the junction temperature Tvj. For Tvj,max =125°C, Irms=380A respectively for Tvj,max =150°C, 440A were obtained. At 440Arms, the IGBT operates up to its nominal current of IC=600A. The operation point is successfully mastered which is proven by the turn off waveform in Figure 6, that is recorded at the operating inverter.

IGBT turn-off at UCE=900V

The usabilitiy of PrimePACK and IHM dual IGBT modules is compared based on measured Rth values and application conditions that allow driving a 690Vrms motor. The implemented safety margin for over-current capability is 20% for 10s. Since the PrimePACK is designed for operation of up to Tvj=150°C, the calculation is also done for this extended temperature. The result is given in Figure 7.

Usable rms current in a ModSTACK inverter for different 1700V IGBT modules and rated currents.

Apparently the PrimePACK module offers the chance to reach a higher power density even if the maximum junction temperature is kept at 125°C. Assuming that Tvj can be raised up to 150°C, Irms can further be increased by roughly 25%. The future work will focus on qualifying products in the new package for this extended.

Conclusion

The first integration of PrimePACK IGBT module into an existing converter proved to work safely. The optimisation of important issues like thermal management are well supported by the new power module. The potential for higher inverter power density is shown and proven by experimental runs of a laboratory inverter. The most striking benefit will come into play when first products are qualified for operation up to Tvj=150°C.

 

References:

1) O. Schilling et. al., "Properties of a New PrimePACK IGBT Module Concept," Proc. of PCIM, Nuremberg, 2005.
2) P. Zacharias, and J. Schiele, "IGBT ModSTACKs–a Cost Decreasing Approach for Flexible Converter Design," PEE, 2003.
3) Datasheet and Application Note, “Dual IGBT Driver for Medium and High Power IGBTs,” Available at: http://www.eupec.com/eicedriver/boards/2ED300
4) C17-S

 

 

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