Inverter compatible with 800V systems for the Porsche Taycan (assumed)

Introduced by Hitachi Automotive Systems; from the Spring 2020 Congress Technical Sessions

2020/08/05

Summary

システム電圧800V対応インバータとパワーモジュール
Inverters and power modules compatible with 800V systems
(Displayed at the Hitachi booth, Tokyo Motor Show 2019)

  The Spring 2020 Congress of the Japan Society of Automotive Engineers (JSAE) was scheduled to be held in May 2020 but was canceled due to the new coronavirus (COVID-19) global pandemic. The JSAE has instead released the technical papers that were scheduled to have been presented during the symposium’s technical sessions.

  This report introduces new technologies for 800V compatible inverters from the "High Voltage and High Power Density Technologies for Inverter in Vehicle" paper by Ayumu Hatanaka et al of Hitachi Automotive Systems (Hitachi AMS).

 

  Hitachi AMS has been promoting downsizing the inverter by using a double-sided direct water-cooled power module, which was adopted for the inverter of the Audi e-tron released in March 2019. The newly developed inverter is compatible with 800V battery-voltage systems, was developed for the Porsche Taycan EV, and appears to have gone into production from Q2 of 2019.

  To make the 800V-compatible inverter possible, Hitachi AMS increased the withstand voltage of each component typified by changing the power device from a 700V to 1200V withstand voltage product, ensuring that the size of the device remained compact while developing technology such as high voltage insulation structural components, optimizing the terminal shape of the power module, and adopting conductor laminated insulation sheets.

 

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Outline of high-voltage and high-output inverter

  The system voltage of an electric vehicle is generally around 400V, but if the voltage of the battery is doubled to 800V, the charging time required for a battery of the same capacity can be reduced in half, and high motor rotation and output can be easily achieved. However, the components required for an 800V system have different withstand voltage assumptions, so they must be newly developed.

  In the inverter developed by Hitachi AMS, the overall insulation design was reviewed and the company developed a power module compatible with 800V systems that realizes a power density of 94kVA/L at 800V, which is about twice as high as that of conventional inverters and with a power density that is 2.75 times higher. In addition, to support two products with different specifications and outputs in the market, as well as to minimize costs, Hitachi AMS developed the key components such as the power module, smoothing capacitor, motor controller board, and current sensor to be shared as standardized parts.

  The range of the power supply voltage is common between 450V and 850V, but the “Inverter 300” is equipped with three new power modules with a maximum phase current of 335A for a 2-second rating, a continuous power supply voltage of 190A, and an output density of 62.3kVA/L. The "Inverter 600" is equipped with two new power modules connected in parallel for a total of six, with a maximum phase current of 670A for a 2-second rating and 380A continuously, with an output density of 94.3kVA/L.

インバータの構成部品とスペック 標準部品による車種展開要望への対応
Inverter components and specifications Responding to vehicle model development by using standard parts

(Source:Hitachi AMS: from the proceedings of the 2020 JSAE Annual Congress (Spring) Technical Sessions)

 



To accommodate high voltages for components and packaging

沿面距離を考慮した絶縁樹脂の構造
Structure of the insulation resins that considers creepage distance
(Source:Hitachi AMS: from the proceedings of the 2020 JSAE Annual Congress (Spring) Technical Session)

  Since the system voltage changes from 400V to 800V, it was necessary for Hitachi AMS to increase the withstand voltage of each component, while changing the design of the inverter and the insulation structural components in a manner to avoid increasing the size of the power module.

  As a method of thermal insulation, in addition to completely covering the electrodes to which a high voltage is applied with an insulator for insulation, it is also possible to insulate by providing an insulating distance. To do so, it is necessary to secure both the spatial distance (the shortest distance passing through the space between the electrodes) and the creepage distance (the shortest distance along the surface of the insulator between the electrodes).

  Since the voltage is doubled in 800V systems, the creepage distance must be doubled when using a material with the same comparative tracking index (a value indicating the difficulty for leakage to occur, the phenomenon leading to dielectric breakdown due to repeated microdischarges at the surface of the insulator). For this reason, ribs and grooves for extending the creepage distance of the insulating resin component are added, and a chamfered shape is provided at the end of the bus bar to secure the creepage distance while suppressing any increases in component size.

  The figure shows a cross-sectional view of the resin insulation portion of the bus bar to which 800V direct current voltage from the battery is applied, where the copper bus bar shown as hash marks is separated by the insulation resin that is shown in gray. To increase the creepage distance indicated by the dotted line, in addition to increasing the height of the resin at the end vertically, it can be seen that the creepage distance is secured by notching the area where the insulation resin contacts the bus bar.

  In the case of 800V system voltages, if there exist restrictions that prevent changing the shape of some 400V components, then creepage distance can be secured by switching to the use of materials with a high comparative tracking index.

  These insulation properties are ensured in the assembled state as an inverter as well as the individual parts.

 



Double-sided cooling power module

パワーモジュール冷却方式の比較
Comparison of power module cooling methods
(Source:Hitachi AMS: from the proceedings of the 2020 JSAE Annual Congress (Spring) Technical Session)

  As shown in the comparison table, cooling in the power module of the inverter for electric vehicles was initially a single-sided indirect cooling in which grease for heat dissipation was applied to a general base plate in an industrial power module and connected to a heat sink. However, the design of the power module evolved into a single-sided direct cooling type power module, and then to the new double-sided direct water-cooled power module with a wider cooling area, where the heat-dissipating grease layer has been eliminated and heat resistance has been reduced.

  In the double-sided direct water-cooled power module, the heat generated by the power device is radiated directly from the fin base through the insulation layer, resulting in a 50% reduction in thermal resistance compared to the single-sided indirect water-cooled power module.

  The inverter supplied by Hitachi AMS for the Audi e-tron is also equipped with a double-sided direct water-cooled power module. In Hitachi’s new inverter, the breakdown voltage of the IGBT (insulated-gate bipolar transistor) chips built into the power module has been changed from 700V to 1,200V. In addition, AC/DC terminals and the signal terminals laid out at the opening of the power module have two positive and two negative poles alternately arranged. By canceling the magnetic flux caused by the transient current in opposite directions, the parasitic inductance has been reduced, and the inductance causing the surge voltage has been reduced by 30%.

  The circuit configuration is comprised of a 2-in-1 package in which two power elements are connected in series, with components such as the power semiconductors and lead frames such as the IGBT and FWD (freewheeling diode) sealed in a transfer mold, and further housed in an aluminum module case, completely sealing them against the coolant.

直接水冷型両面冷却モジュール パワーモジュールの端子構造
Double-sided direct water-cooled power module Alternating terminal structure of power module

(Source:Hitachi AMS: from the proceedings of the 2020 JSAE Annual Congress (Spring) Technical Session)

 



Conductor laminated insulation sheet

  Although ceramic materials such as aluminum nitride are often used for insulation of the conductor portions and radiating fins to which high voltages are applied in general-purpose power modules, the Hitachi AMS power module uses conductor laminated resin sheets to insulate the lead frames and radiating fins to which power devices are soldered.

  The resin sheets are highly filled with a ceramic filler that plays a role in heat conduction to the heat radiating films as well as insulation, and high heat dissipation performance can be ensured by thinning the sheets, but this amount would be small when bonding to the insulation sheet and the lead frame or heat radiation fins. In this case, if a small void is generated, the dielectric strength may decrease, so conductor foil is laminated inside the insulation sheet.

 

絶縁シートの比較
Comparison of insulation sheets
(Source:Hitachi AMS: from the proceedings of the 2020 JSAE Annual Congress (Spring) Technical Session)

  As shown in the comparison table, partial discharges can be prevented by increasing the thickness of the insulation sheet by 1.5 times when the system voltage is doubled to 800V, but thermal resistance increases and the output must be reduced. Therefore, the double-sided direct water-cooled power module this time uses an insulation sheet with conductor foil laminated inside to prevent partial discharges and to ensure the same thermal resistance as the previous power module.

絶縁シートの電解解析結果
Electric field simulation results of insulation sheet
(Source:Hitachi AMS: from the proceedings of the 2020 JSAE Annual Congress (Spring) Technical Session)

  Partial discharge occurs when the voltage applied to the air layer of the void is higher than the dielectric breakdown voltage of air, but the electric field of the void can be reduced by dividing the voltage applied to the insulation sheet with the conductor foil.

  The upper side of the figure shows an insulation sheet without conventional conductor foil lamination, and the lower side of the figure shows the electric field analysis result when there is conductive foil in the center of the insulation sheet. The electric potential difference between the lead frame and the heat radiating fin is divided by the conductor foil into the upper and lower insulation sheets to fix the potential of the conductor foil to the intermediate electric potential, thereby reducing the electric field applied to the void. The electric field applied to the void is reduced to 80% of the conventional structure by this division, and the partial discharge characteristics of the insulation sheet are improved.


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Keywords
Hitachi Automotive Systems, inverter, 800V system, power module, ECU, Audi e-tron, Porsche Taycan, EV, Electric

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