The X-Series, the 7th Generation IGBT Modules


Thomas Heinzel at Fuji Electric Europe

Better heat dissipation and higher reliability need to be realized by applying new innovative technologies. In order to achieve further downsizing, the X-Series IGBT modules have been developed considering simultaneously improvements of the chips as well as the packaging. The X-Series IGBT module brings not only high compactness and high power density, but also the possibilities to work with 175°C operating condition. This capability enables higher output currents compared to the previous generations of IGBT modules which are only able to cope with 150°C operating temperature.

The 7th generation chip technologies for 1200V 

For the 7th generation IGBT modules, IGBT and diode characteristics have been improved and at the same time die size reduced but even though higher efficiency was achieved. These module technology improvements are leading to a new generation of highly compact and efficient power conversion systems.

The IGBT chip

The cross-section of the 7th generation IGBT is shown in figure 1. In the surface of the 7th generation IGBT chip, basically a trench-gate structure was applied similar to the 6th generation IGBT. On the backside of the chip a Field-Stop (FS) layer was adopted on a thin FS-IGBT wafer. The drift layer thickness of the 7th generation IGBT was reduced by applying a thinner wafer compared to the 6th generation. Thus the trade-off relationship between on-state voltage drop and turn-off losses was further improved by optimization of the surface structure.  

Figure 1: Cross-section of the 6th generation IGBT (left) and the 7th generation IGBT (right)

The output characteristics of the 7th generation IGBT are shown in figure 2. The on-state voltage drop of the 7th generation IGBT was reduced by 0.5V compared to the 6th generation at the same current density by a thinner drift layer and an optimized surface structure  (Tj=150°C). If the 7th generation at Tj=175°C is compared to the 6th generation at 150°C the reduction of the on-sate voltage drop is still 0.4V. The 7th generation IGBT´s turn-off losses were reduced by 10% compared to the 6th generation. This turn-off loss reduction was realized by the thinner drift layer applied as well as by the enhanced IE effect and the reduction of the miller capacitance. The trade-off relationship between on-state voltage drop and turn-off losses is shown in figure 3. On-state voltage drop was reduced by 0.55V from the 6th generation IGBT at the same current density and turn-off losses. 

Figure 2: The output characteristics of 7th gen. IGBT and 6th gen. IGBT. Tj=150°C, 175°C (7th Gen.), VGE=+15V

Figure 3: The trade-off relationship between turn-off losses and on-state voltage drop. Tj=150°C, VCC=600V, VGE=+15V/-15V, (same current density) 

The Diode chip

The forward voltage of the 7th generation diode was reduced by reduction of the drift layer. Comparing to the 6th generation diode, forward voltage was reduced by 0.15V at the same current density (Tj=150 °C). The reverse recovery switching waveform is shown in fig. 4. Compared to the 6th generation diode, the 7th generation diode realized a much softer switching waveform by optimization of the local lifetime control. This softer switching waveform contributed to 21% reduction of the reverse recovery peak current and finally to a reverse recovery losses (Err) reduction of 15%. 

In recent years requirements for IGBTs are not only loss characteristics but also low EMI (Electro-Magnetic Interference) noise. By the improving the trade-off between the dv/dt and the switching losses, both low EMI noise and high efficiency of power conversion systems can be achieved. The trade-off was improved compared to the 6th generation due to the reduction of the dv/dt by softening the reverse recovery current. 

The 7th generation package technologies

Size reductions of the IGBT and diode chips are required in order to achieve a higher current density of the IGBT modules. Actual result of the chip shrinking is an elevated power density within the modules which leads to increasing junction temperatures and related degradation of the reliability. For the 7th generation the packaging technologies were further improved developing a highly heat dissipation, reliability and heat resistant package. This package realized not only the suppression of rising junction temperatures, but also enables 175°C operating temperature at the same high reliability level.

Figure 4: Reverse recovery switching waveforms of 7th gen. diode and 6th gen. diode. Switching conditions: Tj=150°C, VCC=600V, IC=100A (1x Inom), VGE=+15V/-15V.

New high power density AlN substrate

In order to improve the heat dissipation from the chip, the X-Series IGBT modules are using an insulated substrate with highly improved thermal resistance. The insulated substrate has the highest influence on the total thermal performance of the module. The standard materials widely used as insulated substrate are the inexpensive Al2O3 substrate and high thermal conductivity AlN. Since such low thermal resistance is necessary there is no way around the high performance AlN. Furthermore the thickness of the AlN substrate needs to be reduced in order to improve the thermal resistance and long-term reliability. By enhancing the material bending strength of the AlN substrate a thinner substrate compared to conventional AlN can be realized. The thinner substrate achieves better thermal cycling capabilities by relaxation of the thermal stress and greater heat dissipation compared to conventional AlN substrate [4]. Finally the thermal resistance of the IGBT modules was greatly reduced and heat dissipation and reliability was increased by applying this new AlN substrate. The thermal resistance of new AlN substrate between junction to case is shown in fig. 5. Comparing to generally used Al2O3 substrate, the thermal resistance of new AlN substrate was reduced by 45%. By applying the new AlN substrate in the 7th generation IGBT modules the temperature rise due to the shrinking of the chip size was successfully solved.

Figure 5: Comparison of the thermal resistance & main characteristics at the same die size

Improvement of dTj power cycling capability

In order to achieve a long lifetime of the power conversion systems, a high reliability of IGBT modules on the long-term is necessary. In particular the power cycling capability against repeating thermal stress is very important. The X-Series IGBT modules expands the operation temperature up to 175 °C which enables working with higher output currents compared to previous generations. Generally, the higher junction temperature reduces the delta Tj power cycling capability by the accelerated degradation of materials around chip. In delta Tj power cycling mode, the wire bonding contacts on the chip and the solder under the chip are facing the greatest thermal stress so they are the first to start deterioration. In the X-Series IGBT modules, wire diameter and length were optimized and a new developed high strength solder material was applied which significantly improved the delta Tj power cycling capability. As shown in fig. 6, by applying these new technologies, the X-Series IGBT modules could achieve 2 times the capability against conventional technologies at Tjmax=175°C. Thus even if the modules are operating at Tj=175°C the highly improved power cycling capability enable them to achieve equal or even better long-term reliability.

Figure 6- 1: dTj power cycling capability curve

Figure 6- 2: Cross-section of solder (under chips) after dTj power cycling test (at the same cycles)

New high heat resistant silicone gel

On the way to ensuring the long-term reliability of the new IGBT modules the deterioration of the silicone gel at higher operating temperatures was the next challenge. Generally higher temperatures are hardening the silicone gels and cracks start occurring. These cracks will lower the isolating capabilities of the silicone gel. Therefore, a new highly heat resistant silicone gel was developed so that a secured continuous operation at a temperature of 175°C can be enabled. The lifetime of new silicone gel at 175°C was further improved compared to current silicone gel, and same lifetime of current silicone gel at 150°C was realized. But the new silicone gel is simultaneously improving the hardening at low temperatures and thus a long-term insulating capability at low temperature could be realized.

Increasing current density of the IGBT modules

By further improvement of the losses of the 7th generation IGBT and diode and by additional improvement of the heat dissipation performance, an enhanced power density could be achieved. For example, modules rated at 1200V in an EP2 package could reach a maximum rated current of 50A by using 6th generation technologies. By applying the 7th generation chips and improved package technologies, a maximum rated current of 75A can be achieved. Comparing to the 6th generation (EP3 package), the X-Series EP2 could reduce the inverter power losses by 10% and IGBT junction temperature by 11°C (fc=8kHz). As the result, the rated current of the 1200V EP2 series was expanded to up to 75A from conventional 50A and the footprint size could be reduced by 36% against the EP3 module. Furthermore, the X-Series IGBT module realized the continuous operation at Tj=175°C by improvement of chip performance, dTj power cycling capability and thermal performance of silicone gel. 

Finally X-Series IGBT modules offer two directions which are a smaller footprint providing same output current as previous generation with higher power cycling capability or increasing the output current with the same footprint up to 35% as shown in figure 7.

Figure 7: Comparison of output current dependency vs. IGBT junc-tion temperature (fc=8kHz switching frequency)

Line-up plan

As a result of the increased current density, Fuji Electric will be able to offer new IGBT modules of the X-Series with higher nominal current capability at the same footprint size. Please see the modules overview in Table 1. 

Table 1: Module line-up for 1200V X-Series modules

Above product range is the first step of the product development with our new 7th generation of silicon chips. Later on, further modules types are under consideration.

 

More information: Fuji Electric Europe    Source: Bodo's Power Systems, May 2015