Next step towards higher power density with new IGBT and di-ode generation and influence on inverter design
Ciliox, Alexander, Infineon Technologies, Max Planck Str. 5, 59581 WarsteinVogel, Klaus, Infineon Technologies, Max Planck Str. 5, 59581 WarsteinDr. Niedernostheide, Franz-Josef, Infineon Technologies, Am Campeon 1-12, 85579 Neubi-bergDr. Haertl, Andreas, Infineon Technologies, Am Campeon 1-12, 85579 Neubiberg
Abstract
Power density has become an important performance indicator for inverters. From this trend,two requirements arise for power modules containing IGBTs and diodes: First, the powermodules must have a sufficient long lifetime even under higher thermal stress. One solutionto fulfill this requirement is the implementation of the .XT technology [1]. Second, the IGBTsand diodes have to provide better performance in terms of static and dynamic losses. Thispaper presents for the first time the common figures of merit of the new 1200V IGBT and di-ode generation of Infineon Technologies.Looking at the whole system, the other inverter components have to be suitable to cope withthe demands of an increased power density as well. In the second part of this study, meas-urements with different inverter designs are performed to evaluate their thermal manage-ment. Based on these results design rules for high power density inverters can be extracted.
1. The new 1200V IGBT and Diode generation
1.1. Expanding the power rangeThe PrimePACK� power module housing (Fig. 1) has set a standard for power modulehousing in the power range up to 700kW. Because of its success it is self-evident that thelaunch of the new 1200V IGBT and diode generation will also take place in thePrimePACK�2 housing.Currently, the maximum available current rating in the PrimePACK�2 housing in half-brideconfiguration is 900A. By using the new 5th IGBT and diode generation the current rating canbe pushed up to 1200A nominal current.
Fig. 1. (left): State of the art PrimePACK� equipped with IGBT5. (right): Application of IGBT variantsin the PrimePACK�2
1.2. 1200V 5th IGBT generationThe trend to increase the power density in power modules is directly supported by the in-crease of the operation temperature of the semiconductor devices.Infineon has presented the concept of the trench-field stop IGBT for 1200V devices in 2000for the first time. This so called 3rd IGBT generation was qualified for an operation tempera-ture of 125°C [2]. The next step towards higher power density was the introduction of the 4thIGBT generation. This generation is suitable for a junction temperature of 150°C [3]. The 5thgeneration of 1200V IGBTs and diodes is designed for an operating temperature of Tjop=175°C. The IGBT5 is based on the successful trench field-stop concept as well.Electrical performanceFurthermore, the IGBTs designers work constantly to improve the device performance byreducing both the switching losses, and the stationary on-state losses. All this optimization isdone under the “constraint” that the well-established short-circuit withstand time of 10 µs atoperating temperature has to be guaranteed. The new approach to overcome this challengeis depicted in Fig. 2.
Fig. 2. Comparison of IGBT4 (left) and IGBT5 (right). The IGBT5 has a reduced die thickness and anew cell design which, however, is still based on the successful trench field-stop technology.Moreover, copper is used as front-side metal instead of aluminum. The front-side copper thick-ness is significantly increased compared with the front-side aluminum thickness of the IGBT4.
The used approach in the IGBT development can be understood by investigating the eventof short circuit: In the short circuit condition the device temperature is increasing rapidly dueto the massive power dissipation, since power is equal voltage times current. The devicetemperature itself is then given by the short-circuit energy divided by the thermal capacity ofthe IGBT. This thermal capacity is defined by the silicon volume itself and by the metal toplayers. The increased thermal capacity of the new front-side metallization of the IGBT5 ena-bles the successful short-circuit turn off by preventing the thermal runaway. It is worth men-tioning that a high precision in the alignment of the copper layer and the IGBT trenches isneeded to assure that functionality, namely that every single IGBT cell needs to be well con-nected to its additional thermal capacity.One measure to increase the performance was the reduction of the device thickness to110µm. The trade-off relationship between the turn-off losses and the on-state saturationvoltage is shifted to lower values (see Fig. 3). Normally the on-state losses increase with in-creasing operation temperature. However, due to the better performance of the IGBT5 theon-state voltage drop at 175°C is the same as for the IGBT4 at 150°C.
Fig. 3. Turn-off losses vs. saturation voltage for different IGBT generations at Tj=150°C.
Also linked to the thickness of the IGBT is the softness of the device. Softness is guaranteedby a sufficient amount of tail current. Looking again at the current IGBT4 variants aPrimePACK� module with 1200A has to be equipped with the softer, so called High Powervariant. The new IGBT5 Medium Power variant has a better softness than the IGBT4 Medi-um Power variant. This optimization allows the designer to use the IGBT5 MePo variant witha 1200A PrimePACK�2 module. Thus, he is also able to take advantage of reduced switch-ing losses compared to the IGBT4 High Power variant. Fig. 4 shows the switching curves ofan IGBT5 which was turned-off by the nominal current (Inom=1200A) at 175°C and by dou-ble nominal current (2400A) at 25°C with an external gate resistor of zero ohms.
Fig. 4. (left):Turn-off curves at 2xInom= 2400A, 25°C without an external gate resistor and withoutany external clamping and (right) turn-off curves of nominal current at 175°C. (Green: Vge,black: Vce, red: Ic)
The controllability of the IGBT’s turn off is also of interest. The simplest approach to controlthe switching speed of an IGBT is to adjust the gate resistor connecting a gate source volt-age and the gate terminal of the IGBT. In Fig. 5, the dU/dt controllability of the IGBT4 MePovariant and the IGBT5 MePo variant are compared by plotting the 10%-90%-dU/dt value as afunction of the total gate resistor. The 10%-90% dU/dt value was calculated from the meas-ured collector-emitter voltages at 10% and 90% of the maximum collector-emitter voltageduring the turn-off period and the corresponding time difference. The total gate resistor is thesum of the internal and external gate resistor. For the IGBT5, we found a distinct monotonicdecrease of the dU/dt on a large resistor range. In contrast to the IGBT4, this distinct mono-tonically decreasing dependence of the dU/dt value with the total resistor is valid even for
very small resistors. Consequently, the range of controllability of the IGBT5 is enhancedcompared to the IGBT4.
IGBT4MePo
IGBT5MePo
IGBT5MePo
Tjop 150 150 175
Esw/A [%] 100% 96% 110%
Vcesat [V] 2,1 2 2,1
Tsc [μsec] 10 >10 10
I nom PP2 [A] 900 1200 1200
Fig. 5. (left): 10%-90%-dU/dt value as function of the total gate resistor for the IGBT4 and IGBT5 atmaximum operating temperature and (right) table summarizes the most relevant electrical cha-racteristics of the IGBT4 and IGBT5.
1.3. 1200V emitter controlled diode 5th generationIn power modules for inductive switching applications, the IGBT and diode are operated asone unity. Consequently, the diode has to be designed in such a way that its electrical be-havior matches the IGBT behavior. The new emitter controlled diode (EC5 diode) is also de-signed to support a maximum operating temperature of Tj=175°C.The further optimization of the device in terms of thickness reduction and field stop designenables an improved electrical performance of the EC5 diode, as depicted in Fig.6: it obtainsa better Vf- Erec tradeoff, while on the other hand maintaining a switching softness as leastas good as the previous diode generation EC4.
Fig. 6. (left): Dependence of the reverse-recovery energy on the forward voltage of the 1200V EC5diode at 150°C. (right): Turn-off characteristics of the same diode at 1/10 nominal current atroom temperature illustrating the superior softness of the new diode generation.
The most critical operation mode for the free-wheeling diode is the surge-current condition.The surge-current capability is typically reduced by ~5% for an increase of the operatingtemperature by 25K. Our measurements of the EC5 have shown that we achieve at least thesame I²t-value at Tj=175°C as the EC4-diode features at Tj=150°C.This remarkable im-provement is again based on the increase of the thickness of the front side metal stack,providing the necessary additional thermal capacity.
Power cyclingThe new 5th generation 1200V devices from Infineon are based on the .XT assembly tech-nology. Therefore, the excellent .XT power cycling performance is also obtained by the 5thgeneration. Fig. 7 shows the target power cycling curve [4].
Fig. 7. (left): Power cycling target curve of the .XT technology. (right): Comparison of the thermal per-formance of .XT assembly technology with the IGBT4 solder technique. In the time range larg-er than 0.2 sec, the Rth junction-case of .XT is about 15% better.
Rth decrease based on .XTThermal simulations were performed in order to compare the thermal resistance (Rth) of thecurrent solder standard technique in comparison with .XT. This simulation was done for a ful-ly equipped PrimePACK�2 module. First we simulated the FF900R12IP4D powered by theIGBT4, and for the second case we simulated the FF1200R12IE5D; Fig. 7 shows theachieved results. An Rth junction-case improvement of about 15% for the same silicon sizewas calculated, and this value was also confirmed by measurements.
2.2. Inverter output current calculationThe IPOSIM tool from Infineon is used to calculate the maximum inverter output currentbased on the presented values in this paper. In Fig. 8 the achieved result is depicted.
Fig. 8. Calculated inverter output current for IGBT5 &4 at max. junction temperature. Conditions: Aforced air cooler Rth h-a of 0.098 per arm, cosine phi=0.8 modulation factor= 0.8, DC link volt-age 600V, same silicon placement
Looking at a typical switching frequency of 4 kHz the inverter output current based on theIGBT5 MePo can be increased by ~23% as compared to the IGBT4 MePo, and by 33% as
compared to the IGBT4 HiPo variant. The IGBT5 operated by 150°C leads to a power densi-ty increase by ~14% compared to the IGBT4 HiPo.
3. Consideration on different inverter designsBesides the development of the IGBT module technology to fulfill the requirements of highpower density and lifetime, the inverter thermal management will become more important toobtain the best performance for such type of semiconductor device [5, 6]. The increase ofthe operation temperature of the switching silicon leads to a higher heat sink temperature atgiven cooling conditions. Considering this, it is very likely that compact air cooled systemswill suffer from an elevated temperature of the components neighboring the power electronicmodule. To find out more about the influence of different thermal designs, two commerciallyavailable 4kW inverters from drives applications are examined. The focus is the temperaturebehavior of temperature sensitive components of the control electronics and DC-Link athigher IGBT operation temperature up to 175°C. Figure 9 shows the different inverter con-cepts schematically. While one concept has a dedicated air flow implemented to cool theelectronic components, the other inverter has its PCBs cooled by convection. At the sametime, the latter is thermally coupled to the heat sink through the power electronic terminals.
Design 1: 4kW Inverter with 2,1kW/l Design 2: 4kW Inverter with 1,6kW/lFig. 9. Different inverter design and different power density for drives application.
Design 1 has the heat sink completely inside the housing, while in design 2 is the heat sink apart of the housing and larger in volume by circa 70%.
Two benefits of the 25K higher maximum operation temperature compared to IGBT 4 areexamined in two tests: one with an increase of the output current by 20% and the other witha decrease of the cooling efforts by 25%. The temperature of various electronic componentsis monitored with thermocouples and the results are shown in figure 10. Both inverters aretested under load and overload conditions up to Tvj of 175°C.
Fig. 10.Temperature behavior of the electronic components by increasing the IGBT operation tempera-ture up to 175°C in two different inverter designs. Dotted lines represent the maximum allowedtemperature for the respective components.
As can be recognized easily, the measured temperatures inside of the inverter 1 are lower inall operation points in comparison to inverter 2. Due to the fact that Inverter 1 has the higherpower density, this is a good indication for a better thermal design compared to 2. The tem-perature spread over the different measurement points is larger for design 1 in comparisonto design 2. Especially at Tvj_op = 175°C the captured points of all measured componentsare distributed in the whole Inverter 2 within 12K. For this operation point, the temperaturespread in Inverter 1 is 49K. Due to the inverter design with a small heatsink close to the airoutput side, Design 1 has “cold” areas inside the housing and the IGBT junction temperaturedoes not influence all areas of the inverter evenly. In design 2 the large heatsink is heated upand through convection and conduction along the module terminals, the above situated elec-tronic components are heated up and stressed uniformly. Using Design 1 the developmentengineer has the possibility to place the thermal sensitive components in the “cold” areas ofthe inverter, while this is not possible with design 2.
Increasing of the output power leads to an increase of the inverter temperature. This is,however, not alone due to the higher IGBT operation temperature, but also due to the highercurrent through DC-Link capacitors and PCBs. The results show that, compared with Design2, the PCB and the IGBT driver in Design 1 suffer from a higher temperature elevation whenincreasing Tvjop from 150°C to 175°C by a current increase. This can be explained with astronger coupling of these components with the heatsink in the more compact design.
Nevertheless, the margin to the maximum allowed component temperature in Design 1 ishigher than in Design 2.
Regarding the DC-Link capacitors, the temperature rise is similar in both designs. Design 2requires 105°C capacitors against 85°C in Design 1, a clear disadvantage for design 2 dueto the passive cooling of this part of the inverter.
Design 2 comes in trouble with the reduction of the cooling efforts. This is equivalent to anincrease of the heatsink thermal resistance Rth_ha. Already at a temperature of Tvj = 115°Cmost of the components are above the maximum permissible temperature. A worsening ofthe heatsink leads to an increase of the heatsink temperature, the heat remains in the in-verter housing and is dissipated towards the control part of the inverter trough convectionand conduction via the module terminals. Design 1 can manage this situation better. Allmeasured parts operated inside the allowed temperature up to a Tvj_op of 175°C of theIGBT. The distance to the maximum allowed operation temperature of the electronic com-ponents during the operation of the IGBT at 175°C and the increase compared to operationat Tvj = 150°C is shown in the next table.
PCB IGBT Driver DC-Link
Margin tomax
Delta toIGBT150°C
Margin tomax
Delta toIGBT150°C
Margin tomax
Delta toIGBT150°C
�������� ��� ���������Tvj_op 175°
13K +10K 13K +10K 25K +5K
Design 2 7K +5K 8K +7K 8K +7K
�������� ��� ��Rth_ha =
Tvj_op 175°C
17K +10K 3K +15K 21K +6K
Design 2 x x x x x x
* compared to an IGBT 4 solution
A core element when increasing the power density in inverters is the lifetime consideration ofthe components. Using the same component technology at higher temperature automaticallyleads to a decrease of the lifetime. A smart inverter design with “cold” areas and forced con-vection on electronic level will help to handle this challenge. It is shown that despite of thedimensioning of DC-link capacitors, which has to be adjusted to higher power densities any-way, an inverter design implementing a designated PCB cooling may draw benefits from in-creased junction temperatures straight away. In contrast, considerable redesign efforts haveto be considered if the electronics part is thermally coupled to the main heat sink throughpower terminals of the module.
ConclusionThis paper presents the achievable level of power density increase based on the new 1200VIGBT and diode technology. The achievable inverter output current can be increased by 23%compared to the current IGBT generation. The first available module with the 5th IGBT gen-eration will be a FF1200R12IE5D in the PrimePACK�2 housing. Besides this, the im-portance of the inverter’s thermal design on the ability to provide the higher power densitywith new module packages is presented and discussed. Care must be taken regarding thetemperature raise seen at other system components to guarantee that the frequency invert-er`s lifetime remains constant. A smart inverter design is the best way to handle with anIGBT Tvj = 175°C without the need of expensive components with new technologies forhigher temperature.
4. Literature[1] A. Ciliox et al: New Module generation for higher lifetime, PCIM , Nuremberg, Germany, 2010[2] M. Hornkamp: IGBT Modules, 2011 Infineon Technologies AG, Munich, Germany[3] M. Bäßler et al: 1200V IGBT4- High Power- a new Technology Generation with optimized Charac-
teristics for High current Modules, PCIM, Nuremberg, Germany, 2006[4] K. Guth et al: New assembly and interconnects beyond sintering methods, PCIM, Nuremberg,
Germany, 2010[5] K. Vogel et al: IGBT with higher operation temperature - Power density, lifetime and impact on in-
verter design, PCIM 2011, Nuremberg, Germany[6] K. Vogel et al: IGBT inverter with increased power density by use of a high-temperature-capable
and low-inductance design, PCIM 2012, Nuremberg, Germany
