www.abb.com/semiconductors POWER MODULES …

1 7 2011 POWER Electronics Europe IGBT Press-Packs for theIndustrial MarketThe standard Press-pack IGBT (PPI) basically uses the same packaging concept as Bipolar high powersemiconductor devices. The main difference is that the high POWER semiconductor content in the package issquare IGBT chips arranged on a round molybdenum disc instead of large, round bipolar devices matchedin size to the molybdenum disc and ceramic housing. The Press-pack IGBT comes under the nameStakPakTMin a pseudo-square frame package that allows for much better utilization of the squared IGBT chips and for a modular platform in POWER scaling of the device. B lent Aydin, Franc Dugal, EvgenyTsyplakov, Raffael Schnell, Liutauras Storasta and Thomas Clausen, ABB Switzerland Ltd,Semiconductors, Lenzburg, SwitzerlandOriginally, it was thought that the PPIswould replace Gate Turn-Off Thyristors(GTOs) in both medium voltage drives andtraction drives and be an alternative to theinsulated IGBT MODULES .

2 In 2011, PPIshave become an established part of thehigh POWER semiconductor marketsegment serving the industry in mediumvoltage drives, but PPIs have onlymarginally penetrated the tractionsegment. For propulsion drives, thedevelopment of rugged and reliableinsulated IGBT high POWER MODULES haveallowed for new platforms and topologiesand virtually all new propulsion drivedesigns feature insulated IGBT Semiconductors has many years ofexperience in manufacturing Press-packIGBTs for POWER transmission & distributionapplications. For many years, the StakPakhas proved its outstanding reliability andruggedness by serving as the principleswitching device in the ABB HVDC lightapplication. In the HVDC transmission linkhigh voltage, direct current is transportedeither over long distances or via sea cableswith very low transmission link than 15 of the HVDC lighttransmission links using StakPak devicesfor valves are currently in operationcarrying more than 10 GW of POWER , andfive projects are planned to add an extra 5GW.

3 The first link began operation in 1997and, as of today, no field failures related tothe StakPak device have been proves the device s capabilities inruggedness and StakPak is well suited for seriesconnection. With both and kVswitches, there are a multitude of industrialapplications that can benefit from the useof StakPak devices. Static VARC ompensation (SVC) systems and multi-level high POWER inverters with severaldevices in series to match industrial linevoltages > 6 kV are the most obviousapplications. Also, Pulse POWER applicationsrequiring high voltage level and fastrepetition rates between the pulses outlineWhat makes the StakPak PPI unique as ahigh POWER switch is foremost themodularity of the package combined withthe flexibility of the IGBT/diode ratio in themodule. For high POWER PPI products thepackage size increases to support the highamount of chips.

4 As the number of chipsper unit operation increases, so does theprobability of failures during subsequentdevice manufacturing steps and earlyfailures in the field. The StakPak PPIconsists of a number of standardrectangular sub-units, called number of chips per submodule islimited to 12 and each submodule istested for full functionality before it isinserted into the frame. In this way, poweris configured into the switch based on thenumber of submodules in the frame. Theframe is pseudo-square and can hold 2(Figure 1), 4 or 6 (Figure 2) BiMOS line is designed for highmanufacturability and as a result of recentfactory expansion, high volumemanufacturing is submodule has 12 chip positionsand the layout of the IGBT connections tothe gate pad allows for at least fourdifferent IGBT/diode ratios in thesubmodule. The ratio can be 1:1, 2:1, 3:1or even 5:1.

5 The fifth possibleconfiguration with only IGBT chips iscurrently not possible due to packagerestraints relating to the maximum currentFigure 1:StakPakTM( kV) in a pseudo-square frame package with two submodules20 POWER 7 2011 POWER Electronics through the package. The maximumcurrent flow at present is 2600 A in a 2:1configuration between IGBT and 2600 A maximum rating for thecomplete switch means that one phase legof the medium voltage drive can be builtfrom one stack only without specialconsiderations concerning the diode andspecial design rules with respect to thecooling system and the elimination ofparasitics. The fact that only one StakPakcompact stack is needed per phase leg ishighly beneficial in terms of footprint of thedrive and manufacturability of thecomplete drive. Figure 3 shows thesoldering of a StakPak. The press-pin that contacts the emitterside of the IGBT chip has been designedwith two additional goals in mind besidesconducting the current when the switch ison.

6 The first of these goals is to make thepin assembly consisting of 12 pins on acarrier independent of each other and thesecond goal is to bring a failing IGBT chipinto short circuit failure rather than opencircuit failure as is the case for insulatedIGBT MODULES . The underlying principle inorder to achieve the first goal is theindependent suspension principle. Thisprinciple originates in the automotiveindustry and is considered to offer thehighest quality and load characteristics forthe individual pins as well as for the pinassembly. The individual pins are designedfor lowest possible unsprung weight andeach individual pin spring contacts thecarrier without affecting the other pinspring contacts in the submodule (Figure 4).IGBT and diode technologyThe IGBT chips inside the StakPak aremanufactured using the Soft-Punch-Through (SPTTM) technology platform thatis considered among the best high voltage(HV) chip and which performs excellent inboth hard switching and soft switchingapplications.

7 The HV IGBT SPT+ chips usethe same rugged planar cell design as theformer SPT chips, but the SPT+ chips havea much lower on-state voltage drop. Thelower on-state voltage drop has beenachieved by introducing an enhancementlayer within the IGBT cell that increases thecarrier concentration on the emitter sideduring turn-on of the device. The SPT+chips, besides having a much lower on-state voltage in comparison with SPT chips,also show an improved turn-off SOA as aresult of the advanced diode chips inside the StakPak needto have a fast recovery when they go fromthe conductive state to the blocking state,so that the diode does not become alimiting factor with respect to the overalllosses of the switch. In principle, the diodeis a simple pin type structureFigure 2:StakPakTM( kV) in a pseudo-square frame package with six submodulesFigure 3:Soldering ofa StakPakFigure 4:Press-pins contact the emitter side ofthe IGBT chip manufactured using the same technologyas is used for the SPT+ IGBT chips.

8 Inorder to match the performance of theIGBT chips, there is a trade-off needed inthe on-state voltage versus the switch-offlosses. This trade-off curve increases theon-state voltage by placing 7 2011 POWER Electronics Europe designed carrier lifetime control layers inthe bulk and close to the pn-junction ofthe diode, but in turn makes the recoveryfast and significantly lowers the diodeswitching losses. The localized carrierlifetime control technique gives a muchbetter diode performance in comparisonwith conventional carrier lifetime controltechniques, where the carrier lifetime inthe whole bulk is lowered by , electronirradiation. With localized carrier lifetimecontrolling, the minority carrier lifetime iskept high in the bulk, which in turn lowersthe leakage current in the device. Whenswitched off, the localized layers act as acarrier recombination drain for fastrecovery, and it shapes the tail of thereverse recovery current for benefits There are several other functionaladvantages of the StakPak PPI overstandard PPIs and insulated IGBT benefits are all functionalities thatmake the design of medium voltage drivesmore straightforward or help to reduce theneed for POWER semiconductor protectivecircuits in the the most important parameterthat will greatly benefit the designer ofmedium voltage drives is the unsurpassedthermal properties of the StakPak.

9 Besidesthe possibility of cooling both the emitterand the collector side, the IGBT and diodethermal resistance (junction to case) is aslow as 4 K/kW, but since only ~20% ofthe heat flows to the emitter side whendouble side cooling is employed, it iscrucial that the main emphasis is oncollector side cooling. Cooling only fromemitter side must be avoided. When taking care in the design of theheatsink (mostly the surface roughness), itis possible to achieve a thermal resistancefrom case to heatsink for both the IGBTand Diode of only 1 K/kW. The thermalresistance from junction to heatsink is upto 50% lower compared to standard PPIsand insulated IGBT MODULES . The excellentthermal properties are obtained by havingthe chips soldered directly onto themolybdenum submodule disc. With thechip-to-case alloyed design, the short-timethermal overload capacity, which is anotherimportant parameter for the designer ofthe medium voltage drives, is also muchimproved in comparison with other typesof IGBT module case of an event outside the safeoperating area (SOA) for the switch or achip in the switch, the press pin alloy tothe IGBT chip because of the localtemperature increase at the failing point.

10 Ifthe alloyed pin/IGBT is not conductive, theswitch fails into open circuit, which is adisaster for series connected devices. Thedesign thus has to be so that the pin/IGBT alloy is conductive in the event of a current path is thus short circuited bycreating a low current path between thecollector and emitter. The built-in pin shortcircuit failure mode (SCFM) is a keyfeature of the trend in medium voltage drives is tomove away from bulky protective circuitelements in the design; reduce thenumber of passive elements. This basicallyput the protective control scheme on thesemiconductor switch itself or alternativelyon the on/off gate control unit. Anotherimportant protection feature of the StakPakis the surge current capability, which is upto 27 kA (w/o reapplied voltage)dependant on the diode content in another element of built-inprotection, which is very difficult to test, butis an important parameter for designingmedium voltage drives, is case non-rupturerating.