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www.us-tech.com
June, 2018
Bubble-Free Potting of High-Voltage Capacitors for Medical Engineering
By Karin Prechtner, Marketing Communication, Scheugenpflug AG
n increasing demand for high-voltage capac- itors in medical devices is causing manufac- turers, such as Fischer & Tausche Capacitors (FTCAP), to reexamine their potting processes.
A
In medical technology, capacitors with nomi- nal voltages of more than 120 kV are often used in equipment, such as CT scanners or X-ray machines. These capacitors smooth the alternating current in high-voltage cascades to ensure consis- tent ray quality.
A reliable potting process protects these com- ponents from damaging external influences, including moisture, dust and aggressive chemicals. Vacuum potting prevents air bubbles from being trapped in the potting medium around the capaci- tors. Air bubbles can cause partial discharges that damage the surrounding insulation and eventual- ly lead to flashovers.
Capacitors are indispensable in both indus- trial and everyday products, such as cameras, pacemakers, hybrid vehicles, and solar-powered ships. In electrical systems, they smooth voltage
Vacuum potting prevents air bubbles from being trapped in the potting medium around the capacitors. Air bubbles can cause partial discharges that damage the surrounding insula- tion and eventually lead to flashovers.
peaks and eliminate fault currents. Capacitors serve as electrical energy storage devices for flash applications, such as radar traps and professional
Collaborative practical solutions are developed at Scheugenpflug’s technology and application center.
reduce EMC emissions.
Fischer & Tausch is one of only a few compa- nies that manufactures its capacitors exclusively in Germany. Founded in 1948, it specializes in film and aluminum electrolytic capacitors for special applications in small and medium quantities. In addition to its standard product range, the compa- ny offers special solutions for more complex requirements, such as durable film capacitors for converters in wind turbines. In order to meet new demand, the company recently expanded its man- ufacturing facilities by adding a 6,458 ft2 (600m2) production hall. A new vacuum potting process
flash units in photography. At a certain voltage, they are able to power appliances with high, short- term energy requirements.
In aircraft, these passive components are used to supply energy to the brake and onboard systems. Capacitors are also used in the field of e- mobility, where they power electric motors and
was also added for the production of high-voltage capacitors for medical equipment.
Film Capacitor Production
The basis of film capacitor production is the “master roll” of plastic foil. This foil is coated on one or both sides with a layer of aluminum or zinc only a few atoms thick. The dielectric strength of the future capacitor depends on this layer. For higher voltages, a thicker dielectric is needed. The metallized master roll is then cut into smaller rolls. Depending on the design of the capac- itor, the width or height of these smaller rolls defines its dimensions. Two metallized foil rolls are wound one on top of the other with an offset so that one edge of the metallization protrudes from each side of the winding. The film ends on either side are then spray metallized. This is called “schoopage.”
During the burn-in process, voltage is applied to the schooped roll, which burns out any existing defects in the film. This step is followed by welding or soldering terminal pins to the schooped surfaces of the windings. The component is placed in a housing and then encapsulated in a vacuum. This encapsulation is critical to preventing moisture from penetrating the windings.
In the last step of the process, the component is tested. Each capacitor is tested to meet its spec- ified capacitance, loss factor and impedance. To prevent leakage currents during measurement, FTCAP carries out these measurements in fully air-conditioned rooms with low humidity.
Picking the Potting
Before deciding on process technology for encapsulation, the company had to choose the
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