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Innovating Power Module Packaging - April/May 21
A systemís power delivery network or PDN is made up of passive and active components such as cables, connectors, AC-DC and DC-DC converters and regulators. As power levels increase to enable new More details...
Power Electronics Europe Events
 
Highlights of PCIM Europe Digital Days 2021
May 12, 2021 - June 30, 2021

Addressing the wide range of power electronic applications - from “Mili Watt” power needed for the operation of mobile phones up to the “GigaWatt” power for high-speed trains - all looking into power electronic potential for energy efficiency and sustainable reduction of carbon dioxyde emission in future power conversion systems. Besides the Silicon-based devices like IGBT, Super Junction MOSFET the new type of devices like SiC and GaN transistors are coming up. While SiC devices and GaN transistors are already qualified in many emerging applications, Silicon-based devices are still dominating in most applications. WBG devices are still at the beginning of their production cycle, the learning in material development and device design is ongoing, the benefits on the system level needs to be qualified, long-term reliability issues needs to be proven and cost down programs will come along with high volume production.

Focused on WBG

“And here comes the PCIM 2021 in play”, Prof. Leo Lorenz, General Conference Director, stated. “The slowdown in the economy over the last years shows a diverse picture. Transportation and automation suffered from a significant dropdown, whereas the IT and consumer markets are booming due to Covid regulations such as home office and digitalization. On the other hand stimulus is coming from programs to reduce the global warming such as the European Green Deal. Power Electronics are the driving forces to meet these goals. The main technology drivers in power electronics are material science to elevate temperature behavior in all systems along with increased power density and longer lifetime. Thus PCIM focuses on pioneering work and product innovations to meet the power electronic market trends and cover the main future directions.”

The PCIM Europe digital days featured more than 2700 participants with around 24.000 live chats at 308 presentations of 313 speakers, according to official figures. More than half of the content was related to WBG technologies and it applications.

Material/device developments for power electronics

A presentation, entitled “The Long Journey from Crystal Growth to Power Devices, the Role of Material Development for III-Nitride Semiconductors”, was given by Elke Meissner from Fraunhofer Institute for Integrated Systems and Device Technology (IISB) in Germany (www.iisb.fraunhofer.de). It discussed gallium nitride (GaN) as a semiconductor material for power electronics in terms of its current applicability, it’s potential and recent shortcomings. A review of the way from crystal growth to the wafer, ready for device fabrication, was given and the bow was spanned from materials properties to device performance. “The technology of native GaN-on-GaN devices may be costly but physically right and at the end the target to go for. Defect densities and overall complexity would be drastically reduced in this case. The alternative way of heteroepitaxial growth of GaN-on-Si and the realization of AlGaN/GaN HEMTs has its own issues and limitations. The high number of defects present in the material put fundamental constraints on the reliability and final performance borders. The technology however is paving the way for the implementation of GaN devices on the market due to the much better cost efficiency and compatibility of existing fabrication equipment. A fundamental understanding of material production is essential in order to better understand and define device sensitivity related to defects and to find ways to analyze and monitor such defects in early stages of fabrication. The only way to achieve that is through a stringent correlation of material defects and device performance to be able to optimize technological processes such that production yields can be improved,” Meissner concluded.

Another presentation “Challenges of New Packaging Solutions for Power Modules” was given by Ronald Eisele from Kiel University of Applied Sciences in Germany (www.fh-kiel.de). “As many new materials solutions are developed for power electronics packaging, the material mix will continue to grow. Given the wide choice of materials, new packaging concepts can be realized to keep up with new requirements in power module packaging. Accordingly, standard applications as well as those having specific requirements can now be served by cost or performance driven demands. It is particularly important to consider the full stack of materials in the power modules to reach better thermal resistance, higher current-carrying capability, high operating temperature and improved reliability. Just a single weak material in the complete stack can deteriorate the performance of the whole module significantly. Both Si3N4-based substrates and sintering technology allow for high performance regarding reliability and heat dissipation. Thick copper-based die top connections and new inorganic encapsulation materials will complete the material stack to enable for even higher junction temperatures and power densities,” Eisele summarized.

One of the three keynotes entitled “Next Generation of Power Electronics Module Packaging” by Hannes Stahr from Austrian AT&S (www.ats.net) covered embedded packaging on the example of a SiC half-brigde. “A major benefit of WBG devices is their ability to switch faster than Silicon, thereby increasing switching frequency and reducing passive component size. Standard power packages are not conducive to fast switching and cannot take full advantage of the superior performance of WBG semiconductors,” Stahr stated. Embedding of IGBTs and diodes up to 1,2 kV has been shown on demonstration level with the potential of loop inductance reduction. Furthermore, SiC-based modules with embedded MOSFETs show even lower loop inductance values. “Embedding die allows a design for short power and gate loops, if the circuitry is integrated into the same PCB, but only a few modules have explored this advantage. An embedded half-bridge module with integrated SiC-MOSFETs operates at 1,2 kV and offers excellent cooling capabilities. On the topside of the module DC-link capacitors are mounted to reduce the loop inductance below 3 nH which is about 5 times lower in comparison with existing conventional SiC-modules. The maturity of Embedding Technology means integration of WBG-devices into PCB material, is reflected in the working group of Automotive Qualification Guideline AQG-324 by implementation of this technology,” Stahr concluded.

Also drive inverters can take advantage of WBG devices, as the keynote “Next-Generation SiC/GaN Three-Phase Variable-Speed Drive Inverter Concepts” by Johann W. Kolar from Power Electronic Systems Laboratory, ETH Zurich in Switzerland (www.ethz.ch) confirmed. Today, variable-speed drives (VSDs) are core elements of industrial automation and robotics. Typically, a three-phase IGBT-based PWM inverter stage with voltage DC-link (voltage source inverter, VSI) is employed for supplying the electrical machine. The switching losses of the IGBTs and anti-parallel free-wheeling diodes are limiting the switching frequency to around 16 kHz, which is still within the audible range. Furthermore, a relatively large total chip area / power module footprint is required and the constant, i.e., current-independent, on-state voltages of the bipolar power semiconductors result in relatively low part-load efficiency. In contrast, novel SiC or GaN power MOSFETs, which are competitive with IGBTs when considering system-level costs, feature small chip areas, internal free-wheeling diodes and enable synchronous rectification with ohmic conduction characteristics and hence high part-load efficiency. Moreover, the significantly higher switching speeds enable switching frequencies above 100 kHz. This facilitates an integration of a single-stage or two-stage LC output filter into the inverter system housing, i.e. the generation of continuous (sinusoidal) output voltages.

Novel monolithic AC power semiconductor switches (monolithic bidirectional switches (M-BDSs), with bipolar voltage blocking capability and bidirectional current controlability require only a slightly larger chip area compared to a single (unidirectional) switch; e.g., 600 V drain-drain M-BDSs or SiC M-BDSs for higher blocking voltages that are currently under development. The DC/AC stage can then again be realized with only 6 switching devices and the overhead remains limited to a doubling of the number of gate drives and the implementation of a four-step commutation scheme. GaN M-BDSs, which in addition to a normally-off variant also exist in a normally-on variant that is advantageous regarding the realization of protection concepts, form the general basis for the future use of three-phase DC/AC or AC/AC current DC-link converters.

“It is important to highlight that the AC/AC converter topology also is of clear advantage compared to direct or indirect AC/AC matrix converters, because the latter are inherently limited to buck operation and require 3 filter inductors to form a continuous output voltage. We are at the beginning of a fascinating new chapter of power electronics research, comprising the characterization of novel M-BDSs, the identification and evaluation of optimum three-phase current DC-link converter modulation schemes with respect to switching losses and EMI, and the multi-objective optimization and the realization of industry-like demonstrator systems. Furthermore, a comprehensive comparison with voltage DC-link converter systems is mandatory, which also should consider overload requirements as well as protection aspects. Such analyses will close the gaps in the current knowledge base and prepare the future industrial application of the impressively low-complexity current DC-link converter systems”, Kolar concluded. AS

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