If we look back over the last 10 years, major changes have taken place in the market perception of power electronics. In the past, neither the public nor the press focused much on power electronics or the use of power electronics in motor, pump, or elevator drives. With the boom of renewable energies, however, this has changed dramatically.
By Thomas Grasshoff
The onset of wind power generation was more than 20 years ago. SEMIKRON was already manufacturing intelligent power modules based on the new pressure contact technology and featuring integrated driver electronics, sensors and cooling unit – pre-tested and optimized systems.
Today, the onus is slightly different. In the fourth generation of SKiiP IPMs featuring integrated driver and heat sink, the main focuses are high power density and overall reliability. This is why every SKiiP module undergoes a 2-hour burn-in test under real 4Q inverter conditions before it leaves our premises.
Now, most wind turbines have an output of 2- 3MW. Work on the 5MW class has already begun, with initial offshore and mainland installations already in the test stages. The next step up to the 7.5 - 10MW class is under development. For power electronics this is a major challenge. Available space in the nacelle is limited but power is increasing. More compact and more efficient solutions are required. The SKiiP4 is the most powerful intelligent power module with an output power of 3600A on the market and is 33% more powerful than its predecessor SKiiP 3.The design was optimized to cope with high DC link voltages and high installation altitudes of up to 4500m above sea level.
There are two ways to reduce the costs of power electronics - either by reducing the thermal resistances between the junction of the chips and the environment or by increasing temperature towards operating temperatures at and above 150°C. Temperature increase has a major impact on module lifetime and new assembly technologies are the key to further improvements in this area. Solder melting temperatures of 250° C are too close to the maximum silicon operating temperatures and lead to delamination and ultimately module destruction. Sinter technology based on a thin silver interface with a melting temperature of 960°C prevents aging and reduces the thermal resistance by 20% compared to traditional solder joints. In the area of thermal efficiency there is one major drawback: the thermal paste layer between the module and the heat sink has 400 times the thermal resistance of copper, is difficult to apply, and is responsible for 60% of the thermal resistance between chip and heatsink. Module manufacturers are working on technologies to combat this “heat blocking paste problem” and are soon to deliver power electronic systems with improved thermal efficiency.
The solar inverter market faces slightly different challenges. Typical topologies used to achieve high inverter efficiency are HERIC or H5 circuits. At the moment, SiC is entering the market and providing an additional 1% efficiency gain. 3-level topologies, known from UPS or traction applications, are now being used in new designs for applications above 10kW. With 3-level inverters, the same efficiency as for SiC topologies can be achieved.
The more pressing call, however, is for greater reliability. If a 500kW solar inverter breaks down, the off-line time affects the business of the solar investor. A solar field operates under entirely different operating conditions: the difference between full sun exposure and shade are rather high. Inverter solutions with monitoring functions for current, temperature and voltages are needed. 3-level topology is currently also entering the market for solar applications of up to 250kW.
These two applications in the area of renewable energies underline the importance of power electronics when dealing with the challenges of energy generation and conversion. Technological developments will continue and make the electrification of other applications attractive too.
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