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1、14 PASSIVE COMPONENT INDUSTRY SEPTEMBER/OCTOBER 2004By Marianne VancleemputVishay BCcomponentsMetallized film capacitors satisfy a large variety of elec-tronic applications, because available dielectricshave excellent fundamental electrical characteris-tics, high stability and longevity. Capacitor m
2、anufacturersare constantly improving the breed to deliver greatercapacitance within smaller package dimensions. Engineersneed to be aware of current technologies in order to selectthe optimal device quickly. This allows them to focus onaspects of design that will differentiate their product in thema
3、rket for maximum value. If there are special require-ments relating to capacitor performance, help from thedevice manufacturer can minimize time needed to com-plete a design.Film TechnologiesCapacitor manufacturers can optimize the characteris-tics of metallized film capacitors for specific applicat
4、ions byselecting a suitable dielectric. For example, polyester filmsdisplay good properties for general-purpose applications.These include high dielectric constant (allowing them toachieve the highest capacitance per unit volume of metal-lized film-type capacitors), high dielectric strength, self-he
5、aling properties and good temperature stability. Poly-ester capacitors display the best volumetric efficiency of allfilm capacitors at moderate cost and are a popular choicefor DC applications like decoupling, blocking, bypassingand noise suppression. Figure 1 shows the cross section ofa generic met
6、allized film capac-itor, illustrating the dielectric,connection to electrodes andflame-retardant packaging.Capacitors made from metal-lized polypropylene film displaylow dielectric losses, high insula-tion resistance, low dielectric ab-sorption, high dielectricstrength and deliver a robust,space-eff
7、icient solution. Long-term stability is also good. Thesecharacteristics make metallizedpolypropylene film capacitors a strong choice for mains-attached applications like AC input filters, electronic bal-lasts and snubber circuits. Polypropylene film capacitorsare available rated to 400VAC and beyond
8、 for industrial 3-phase applications and specialist equipment. They are alsoused in switch mode power supplies, frequency discrimina-tion and filter circuits, as well as in energy storage and sam-ple-and-hold applications.AC and pulse capacitors are optimized for applicationswhere steep pulses occur
9、, like electronic ballasts, motorcontrollers, SMPS, CRT televisions and monitors or snub-bers. These applications feature double-metallizedpolypropylene film construction with low-loss dielectric, capable of withstanding high voltage and high-pulse loadapplications at high operating frequencies. One
10、 example isthe MKP383 series Figure 2. Designing with film capacitors is usually straightforward.For example, an EMI filter for a main-powered product follows a well-established topology and capacitance values areeasily selected. X-capacitors, placed between the main andneutral phases, have no theor
11、etical upper limit but are usu-ally selected between 0.1F and 1.0F. Y-capacitors, placed between the main or neutral and chassis ground, are chosenas small as possible to minimize AC leakage to ground. Formost designs, 4700pF is optimal. These devices must meetsafety and performance standards applic
12、able to mains-con-nected components, including UL 94 V-0, European ENECstandards and EN 132400. The American UL1414, CanadianCSA C22.2 No.1. IEC 60384-14, IEC 60384-14 (2nd edition),and IEC 60065 have been proposed as international guide-lines concerning mains-connected filtering capacitors.Often, t
13、he most significant design activity is to select appropriate capacitor technology and ensure package di-mensions and styles meet the applications needs. Stability ofcapacitance includes adequate self-healing capabilities, a keyperformance criterion. After satisfying these criteria, specialtest requi
14、rements, supply and logistical issues become chiefconcerns.Applications and Technical Consideration forMetallized Film CapacitorsFigure 1Figure 2METALLIZED FILM CAPACITORSPASSIVE COMPONENT INDUSTRY SEPTEMBER/OCTOBER 2004 15PackagingMiniaturization (including achieving higher capacitance,maintaining
15、insulation and isolation characteristics, voltagerating and other key electrical parameters within smallerpackage dimensions) gives greater design freedom to engi-neers facing board space and mounting height limitations.With reduced package dimensions, development of lead-freepackages has become a p
16、rime concern. Suitable devices areemerging, enabling new productsto meet forthcoming guidelines.As an example, new compact, lead-free EMI suppression film capaci-tors such as the 339 X2 series (Fig-ure 3) fulfill these requirements,with values from 1.0nF to 4.7F insmaller case sizes and pitchesdown to 7.5mm. In addition, 339x2 capacitors below 100nFare between 1mm and 2mm lower in height and thickness.Alternative PackagesAs a rule, engineers need to complete designs like EMI fil-ters
