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Posted on 25 February 2019

Power Semiconductor Development

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The Influence of Power, Power Density and Lifetime Demands

In 1983, GROWIAN, the world’s largest wind energy conversion system of its era went live with an output power of 3MW and an overall conversion efficiency of about 80%. With the absence of suitable power electronics, the necessary energy conversion was achieved by rotating machines consuming a tremendous amount of material, several cubic meters of space and a vast amount of money. This way of mechanical conversion became obsolete with the introduction of modern power semiconductors which massively increased the conversion efficiency, too. Despite the impressive capabilities they reached by today, the market demands for future generations of power modules to be developed remain challenging.

Dr. Martin Schulz, Infineon Technologies AG

Why power density matters even in wind power plants are recognized as huge entities. The space available to integrate the necessary subsystems is heavily limited. Inside a windmill’s nacelle, the mechanical setup consumes most of the space, especially if a gearbox adds to the drive train’s volume. Besides the obvious space restrictions, losses that heat up the nacelle have to be considered. Air condition is often needed to keep the ambient temperature inside the housing at tolerable limits during operation.

Given a system efficiency of 95% from rotor shaft to the power inverter output, 5% of losses in modern wind energy of up to 6MW need to be handled. If, additionally, solar load at the installation site needs to be considered, air conditioning inside the nacelle may have to cope with up to several hundred of kW of losses generated. The basic components of a wind energy converter (WEG) are sketched in Figure 1.

Subsystems of a wind mill showing sensors (1), communications (2), temperature control (3), pitch- and azimuth control (4), drive train with generator (5) and power electronics (6)

Modern power electronic converters today feature an average volumetric power density of about 1kW per liter; a converter for handling 1MW thus consumes about 1m³ of space. This includes the bare power section with the heat sinks as well as inductors, filter- and DC-link capacitors. In this scenario it becomes obvious, that power density is of utmost importance for this application.

Throughout the last decades, power semiconductors have grown in both, current carrying capability and efficiency. Figure 2 summarizes 25 years of development in power semiconductors.

Power semiconductor development

The figure also includes the fact that the growth in current density reached the factor 4. At the same time, the losses were reduced by about 50%. As a consequence, the power loss density increases and thus the temperatures in a given environment grow as well. At a first glance this seems to be a drawback, as higher chip temperatures and higher temperature swings are considered detrimental in regards of lifetime. As a rule of thumb, an increase in chip temperature by 20K reduces the predicted lifetime of a given power semiconductor constellation by 50%.

An increase in power density can only be considered positive if the lifetime and the reliability of the application are not decreased. Moreover, the market expectation is that power density and lifetime increase simultaneously. With this requirement, only a holistic approach to enhance power semiconductor modules promises substantial progress. This is especially true for the application specific overload conditions in various modes of operation in windmills. These may lead to short periods of time with very high peak power demand.

To tackle the challenges described above Infineon developed the .XT technology: a combination of new interconnection processes and technology changes featuring the newest power semiconductors available. This new setup allows achieving higher power density levels while even increasing the cycling capability as depicted in Figure 3.

Cycling capability comparison between IGBT4 and .XT

Keeping the same temperature levels as today, an increase in lifetime in a range of factor 10 is achieved. Trading lifetime for output power for the cost of higher power density, additional 25% of output power can be gained without enlarging the current design’s cabinets. A state of the art 6MW offshore wind mill today carries up to 12 racks with power electronic components. The power density increase demonstrated by .XT leads to a noteworthy reductions; the same output power can now be handled by only 10 racks. Besides the savings in space inside the nacelle, further benefit arises for the application. Less material is in use, less weight needs to be shipped and a lower number of units has to be handled.

Resources spent per kW installed

System cost within energy conversion systems is a complex issue. It is often stated that the power semiconductors contribute a noteworthy part to the financial aspect. An increase in the power density that can be delivered by modern power modules also influences other parts of the overall systems. From Figure 3 it can be taken that even at higher temperature levels, the same output power can be achieved while keeping the lifetime.

New materials like copper will help in achieving high market demands for power density, lifetime and reliability

For the designer, this leads to possible reductions in the size of cooling systems or heat sinks in use. Increasing the switching frequency and shifting the thermal budget to switching losses in turn, allows reducing the physical size of grid filters, saving material in wound goods as well as in grid connected filter capacitors. Eventually, inverters based on the new power components make more efficient use of the resources needed per kilowatt installed, thus leading to a reduction in size, weight and – most important – in system cost.

Dedicated to the application

Paradigms in building power electronic components have just recently seen a transition. Traditionally, new developments were driven by new chip technologies. After introducing a lead type, the new technology subsequently was migrated to different power module families and various power ranges or frame sizes. This approach became less helpful when modern IGBT modules reached a maturity that made other influences than bare switching behavior reach higher priority. A rethinking took place to start improving power electronic components as an inherent part of an overall system instead of a standalone entity. Intense discussions about the most urgent issues were part of the change process. Additionally, experts were asking how to eliminate the root causes of troubles specifically related to dedicated applications. For the semiconductor manufacturer, deep system understanding was now necessary – and attained – to create a technology platform for new generations of power devices that solve these major issues in power electronic designs.

Future prospects

The face of power electronic components is about to change. The most visible feature will be the replacement of aluminum materials and surfaces by copper, new materials in interconnection technologies and ultimately new designs for power semiconductor modules. With its .XT technology, Infineon introduced the next step in highly reliable power devices that will continue to serve a demanding market. Besides these enhanced power semiconductor components, a new philosophy in optimizing these parts will ensure that Infineon Power Semiconductor remains a seal of quality and innovation.

 

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