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Capacitors for Power Electronics: Latest Technology Trends

  One of NICHICON CORPORATION’s focuses of its capacitor business is the field of power electronics. It is using its strengths of reliability and advanced technologies to boost sales chiefly in inverter applications for solar power generation, wind power, electric vehicles (EVs), hybrid vehicles (HVs), rolling stock, industrial equipment, and home electronics. We can develop and mass-produce aluminum electrolytic capacitors and film capacitors for these markets, and provide the specific technologies and products that respond to customers’ demands. Let’s look at the latest technology trends in aluminum electrolytic capacitors and film capacitors for the field of power electronics.

Aluminum Electrolytic Capacitors for Power Electronics

  With energy-efficient operation crucial to today’s social infrastructure, inverters have become essential standard equipment on power electronics devices. Along with the high-efficiency and energy-saving benefits that inverters bring to systems, their parts are required to be increasingly smaller and have higher withstand voltage. Compactness, high withstand voltage, and long life are also increasingly important criteria for the aluminum electrolytic capacitors used in the control circuits of items like inverter power sources. Higher withstand voltage is particularly important in renewable energy fields like solar and wind power generation. And in products like AC servo motors, high reliability is absolutely essential due to the need for repeated rapid charging and discharging.
  In response to the need for greater parts performance in various industrial equipment in the field of power electronics, Nichicon’s development efforts have focused on high withstand voltage, high ripple current, and stability under repeated charging and discharging. Here is an example of this: the NU Series (figure 1) lineup, which was expanded last year.

NU Series of screw terminal-type aluminum electrolytic capacitors

Figure 1: NU Series of screw terminal-type aluminum electrolytic capacitors

  The NU Series is a screw-terminal-type, compact, high-withstand-voltage product guaranteed to operate at 105°C. It was developed with (1) electrode foil utilizing high-withstand-voltage oxide film, (2) long-term-stability, electrolyte solution with enhanced oxide film restoring capacity at high voltages, (3) low density, high withstand voltage, electrolytic paper, and (4) a structure that enhances heat dissipation. With these advantages, compared to the standard screw-terminal-type, 105°C NT Series, the NU Series realizes higher withstand voltage (rated voltage of 525V) and longer life (5,000 hours of operation). It is also compact—20-30% smaller than our product with a rated voltage of 500V—and thus contributes to making equipment smaller and lighter. It also incorporates our highly appraised rapid charge-discharge stabilizing technology used in industrial equipment applications subject to large load fluctuations. This makes the NU Series suitable for use in applications such as AC servo motors, which require repeated rapid charging and discharging over short periods of time.
  And with the high rated voltage compared to standard capacitors, the NU Series can enjoy extended use in power source smoothing for machinery in regions with poor voltage and power source conditions; for example, input voltage fluctuations. It is also ideal for designing circuits of reduced size and cost, since the number of components can be reduced if multiple aluminum electrolytic capacitors are connected in a series to meet the needs of the system’s required voltage.
  Elemental technologies of the NU Series have made possible the following characteristics.

(1) Compactness
The NU Series uses highly reliable, high-capacity electrode foil made possible by highly reliable, high-withstand voltage oxide film formation and an etching pit shape ideal for high voltage and high capacitance.
(2) Enhanced Heat Dissipation Structure (Compoundless Structure)
Conventionally, fixation agents such as thermoplastic resins were used as the element fixation method for screw-terminal-type electrolytic capacitors. The NU Series, however, uses a compoundless rib structure for fixation of the element. This compoundless structure gives greatly enhanced heat dissipation, making it possible to minimize the temperature rise of the element when a ripple current is applied.
(3) Low ESR
In the past, it was common to use electrolytic paper with relatively large fiber width, but the NU Series uses low-density electrolytic paper with small fiber width in order to reduce heat generation. As well, by increasing the number of lead tabs coming off the electrodes, we have decreased the resistance within the electrode foil. The result is low ESR.
(4) Protective Foil Structure for Stability Under Rapid Charging and Discharging (hold three associated patents)
In applications such as AC servo amps, large voltage fluctuations (ΔV) and rapid charge and discharge loads can quickly cause a breakdown. With Nichicon’s patented protective foil structure technology, this problem is completely eliminated. As inverters become more widespread in power electronics industrial equipment, the performance made possible by this technology is becoming increasingly important for applications with severe loads that require high levels of reliability; for example, in various types of power sources, servo amps, and inverter air conditioners.

Film Capacitors for Power Electronics

  There has been increasing demand in recent years for film capacitors in energy-efficiency-related fields like EVs, HVs, and solar and wind power generation. Let’s look at some of the latest technologies in this area.

Features of Film Capacitors

  Film capacitors uses a plastic film that is only several microns thick as their dielectric material. In the power electronics field, the main type used is a vapor-deposited film (metalized film) in which metal is vapor-deposited on a polypropylene film to create an internal electrode. Film capacitors have the following outstanding electrical characteristics, safety, and reliability.

  • (1) Stable capacitance over a wide temperature range
  • (2) Superb withstand voltage, ideal for high-voltage applications
  • (3) Low loss for greater energy efficiency
  • (4) Exceptional high frequency characteristics and superior filtering
  • (5) High ripple current capacity for greater current density per unit of volume
  • (6) Self-healing function for greater safety
  • (7) Maintenance free for at least 10 years under high-temperature conditions

  Conventional film capacitors using polypropylene film had superb electrical characteristics but were limited in how small they could be made and how much capacitance they could achieve per unit of volume. As a result, they were limited to certain applications. But today’s film capacitors have thinner films and improved vapor-deposition technology, putting them in greater demand in power electronics applications.

EV and HV Applications

  Film capacitor voltages have been rising in recent years in order to realize higher output and greater efficiency for the motor drive systems of EVs and HVs. Film capacitors with outstanding electrical characteristics such as high withstand voltage, low-loss, and high permissible ripple current are being used mainly for smoothing capacitors (figure 2.)

Film capacitor for HVs

Figure 2: Film capacitor for HVs

  Key to the proliferation of EVs and HVs in future will be how low-cost and compact systems can be made, and in this respect film capacitors must become smaller and be able to handle higher ripple currents and higher temperatures. Further development of film capacitors, therefore, must focus on making them smaller through use of thinner dielectric film; giving them higher withstand voltages and lower ESR through vapor-deposition technologies; making them even more reliable; and improving their heat dissipation through material development and optimal structure.

Solar and Wind Power Applications

  There is a great need for long-life, highly reliable direct current filter capacitors in industrial equipment fields such as solar and wind power and other renewable energy sources, and general purpose inverters. To meet these needs, Nichicon has developed the ER Series of DC-filtering cylindrical film capacitors (figure 3).

ER Series of DC-filtering cylindrical film capacitors

Figure 3: ER Series of DC-filtering cylindrical film capacitors

  Cylindrical film capacitors use an aluminum exterior casing, which offers a grounding function as well as improved heat dissipation. And because they are constructed of a single large element, they are better suited than conventional capacitors of the same rating to meet customers’ needs for competitive pricing and quick delivery. Nichicon makes the most of the various strengths of aluminum electrolytic capacitors and film capacitors to develop and manufacture products with the specifications that keep our customers satisfied.


From the Dempa Shimbun, Jan. 30, 2014

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