Wide bandgap transistors place high demands on DC link capacitors. TDK has developed a dielectric for film capacitors to be used up to +125 °C and a novel winding design. We asked Dr. Lucía Cabo, Manager Basic R&D Film Capacitors, and Fernando Rodríguez from TDK’s Application and Modelling Group.
DESIGN&ELEKTRONIK: Wide bandgap power semiconductors pose different requirements to DC link capacitors. What are these requirements?
Dr. Lucía Cabo: With the new wide bandgap power semiconductors we see a much higher degree of integration of the inverter system. This encloses also the passive components, especially the DC link capacitor, which poses a couple of challenges. Although the overall switching losses of these new inverter systems are lower, the temperature stress for the DC link capacitor is much higher. This has several independent causes. First, due to the higher degree of integration the DC link capacitors are much closer to the transistors, the main source of heat in the system. Therefore, the heat generated by the semiconductors is transferred via the busbar to the DC link capacitors. Second, as the inverter is much more compact, the current density is much higher. And third, wide bandgap power semiconductors are able to operate at higher temperatures.
Another challenge for DC link capacitors in these new inverter systems are the high and fast transients they have to cope with. As wide bandgap power semiconductors are able to switch much faster as silicon devices, the frequency response of the capacitors in general and especially the frequency curve of the ESR (Equivalent Series Resistance; editor’s note) have to be much more even up to the megahertz range.
In a lot of applications aluminum electrolytic capacitors are used in the DC link. Which disadvantages have aluminum electrolytic capacitors?
Dr. Lucía Cabo: Aluminum electrolytic capacitors are widely used in lower power applications. If you increase the power, both technologies are feasible. To find the best solution there you have to look at other parameters like power and current density as well as frequency response.
Fernando Rodríguez: I want to add that aluminum electrolytic capacitors are generally spoken a great solution, because it is the capacitor technology with the highest energy density today. However, it has some drawbacks. On the one hand, the performance at high frequencies is not very good, they poorly conduct higher-frequency currents. On the other hand, the capacitance is highly temperature-dependent. Therefore, film capacitors based on biaxially oriented polypropylene are the better choice, especially if wide bandgap power semiconductors come into play. First, the capacitance is very stable with temperature. Second, film capacitors are very good at high power densities as they have low losses over their frequency range. And wide bandgap power semiconductors are usually implemented in applications with high power densities. And third, film capacitors have excellent self-healing properties and are cheap.
However, film capacitors are not very robust against high temperatures. What has TDK done to reduce this issue?
Dr. Lucía Cabo: Conventional film capacitors based on biaxially oriented polypropylene can only operate at full power up to +105 °C. Beyond this temperature you have to derate. Alternative dielectric materials have only limited self-healing capabilities, are much more difficult to process as polypropylene and are much more expensive.
To reduce this issue we at TDK have developed a different film dielectric. It is a blend of polypropylene and amorphous cyclic olefin copolymer. By this, we are able to combine the favorable properties of those two materials (see Fig. 1). As I mentioned, polypropylene is easy to process into thin films, but has temperature limitations. Amorphous cyclic olefin copolymer on the other hand is capable to operate at higher temperatures, but it cannot be process into thin films. In order to manufacture the film we mix the pellets of these two ingredients in a defined ratio and extrude these to a film of down to 3 µm.
Which advantages has this new film dielectric?
Dr. Lucía Cabo: With this blend it is now possible to operate these capacitors up to +125 °C without any voltage derating. Additionally, the new blend is almost as easy to process as normal polypropylene, so we can manufacture this film in existing polypropylene lines with almost the same setup. And not to forget, the blend retains the main beneficial properties of the polypropylene material like a low dissipation factor and the self-healing ability.
Another advantage is that this new blend has a higher mechanical stability as biaxially oriented polypropylene as the shrinkage in transverse direction starts at higher temperatures beyond +130 °C. At this temperature, polypropylene shrinks 1 % (see Fig. 2). Also the specific leakage current is lower at higher temperatures. At +120 °C and electrical field strength of 250 V/µm, the new blend has some 20 µA/µF, but polypropylene some 70 µA/µF. The specific leakage current is often not considered adequately as it is the cause of some of the failure modes of film capacitors. It can promote corrosion which can lead to hot spots and later on to breakdowns and avalanche effects. So capacitors with our new film dielectric are much more reliable.
Last but not least, this new blend can absorb much higher electrical field strengths as traditional polypropylene, so you don’t need to derate such film capacitors as much as components with polypropylene. For example, at +120 °C polypropylene can withstand 130 V/µm, but our blend 170 V/µm (see Fig. 3). This gives you a lot of headroom for your application.