Mission-Critical Applications Require High-Voltage Capacitors to Meet Stability and Durability Requirements

Aug 1, 2010 12:00 PM
Joe Moxley, Senior Engineer , Custom Electronics, Inc., Oneonta, NY

The user usually specifies the capacitor packaging based on their application. If the capacitor is to be left exposed to atmospheric conditions, then a range of options are available. Tape-wrapped and end-filled, fiberglass/plastic/metal potting form or epoxy molded units (Fig. 2.) offer varying levels of environmental protection, structural strength and mounting options. If the final application entails potting the capacitor into an assembly, then a bare section design offers the best economy and smallest size. The choice of potting or casting materials whether epoxy, silicone, polyurethane or some other polymer, needs to be appropriate for the capacitor’s end use. The glass transition temperature, rated temperatures, Shore hardness, dielectric strength, volume and surface resistance, water absorption, shrinkage and thermal conductivity are some of the criteria that need to be addressed. The design of mold or potting forms should avoid sharp edges, sharp points or projections and allow for adequate coating of the capacitor pack. Fiberglass cloth may be used to wrap the capacitor pack to remove sharp edges and add structural strength in some applications. Some materials require that they be transferred to the mold under vacuum for best effects, and most require a heat cure for optimum performance

The typical dielectric stress voltage is 200 percent of the rated voltage up to voltage ratings of 8 kVdc, at which point the stress is reduced linearly to 110 percent at >45 kVdc. Capacitance is again checked after soldering, assembly and burn-in operations. Capacitance, df, and dielectric stress are once more 100 percent tested at the completion of packaging options. Other customer-specified testing might include, but is not limited to: burn-in voltage/temperature and length of time, corona inception and extinction voltages, capacitance, df, ESR, IR at various temperatures, thermo cycling/shock, shock and vibration, moisture resistance, fungus resistance, solderability, x-ray and resistance to various chemical. At a minimum, all capacitors should be tested 100 percent for capacitance, dissipation factor and dielectric stress at the bare single element stage to ensure that all the infant mortalities have been weeded out.

Designs of 1300 V/mil are the industry standard for the dielectric stress between foils associated with reconstituted mica paper capacitors with 26 V/mil for the margin area. These stress levels will meet the requirements for 100,000 + hours of reliable use. Different winding techniques such as straight winding with embedded foil, series winding with embedded foil and extended foil can be used to maximize desire characteristics. The number and location of tabs (terminals) are calculated to obtain the best current, voltage and induction performance. Many times, capacitor requirements are such that multiple capacitors must be connected in parallel, series, or a combination of parallel/series connections to achieve the desired capacitance/voltage rating.

RMPCs

Besides all the internal options available to users of RMPCs, designers can also choose various shapes for these capacitors (see Fig. 3.). The user must maintain the required volume for the capacitance and voltage rating, needed, but otherwise, the user is free to define the physical dimensions to fit their needs. For example, if a capacitor required 1 in. x 1 in. x 6 in. for a specified capacitance and voltage rating, it can be also be designed with almost any variation of dimensions that contribute to a final form measuring 6 cubic inches.

Download the story in pdf format here.

Want to use this article? Click here for options!
© 2007 Penton Media, Inc.