IP&E SOURCING


Dielectrics aDvance capacitor functionality


Howtowork through the price/performance equationwithMLCCs By Paul Staubli, Kemet


One of themost noticeable evolutions inmodern engineering has been the inclusion and then rapid growth of electronic content in products and systems thatwere once purelymechanical in nature.


Perhaps the prime example is vehicles,where electrical and


electronic systems once only supported lighting and instrumentation functions,whereas nowthey forma crucial part of the functioning of thewhole vehicle.


In earlier vehicles, electronicsmodules to controlmany functions


where situated in the passenger cabin. However, with pressure on manufacturers to launch vehicles that offered themaximum amount of space for the driver, his passengers plus their luggage and other possessions,manymodules have been required to relocate to less ‘cosseted’ locations in the vehicle. In the case of enginemanagement, braking and steeringmodules and several others, this has oftenmeant finding a new home ‘under the hood’ – often attached to or close to the function that they are tasked with monitoring and controlling.


The operating under bonnet environment is not a friendly one,


and so electronics located there have had to advance significantly in order to operate reliably and satisfactorily over the long potential lifetime of the vehicle.


Electronicsmodules for systems such as enginemanagement,


braking and steering comprise several blocks of functionality that provide elements such as sensing, signal conditioning and processing. Supporting all of these will be passive components which will include devices such asMultilayer Ceramic capacitors (MLCCs). Although passives are not generally the high BOMvalue items in a vehiclemodule, their correct functioning is crucial to the overall performance and reliability of the system.


a newgeneration of dielectricmaterials


HiCV COG MLCC 24 | June 2010


Recognizing the growing demand for stable, higher temperature range devices for applications in under-the- hood automotive, capacitor specialists such as Kemet have invested time and expertise in developing dielectricmaterials that provide appropriate levels of performance.


One of themost recent


advances has seen the introduction ofMLCCs that use X8L and X8R dielectrics. These offer an operating temperature range of -55°C to +150°C. Importantly this wide temperature range performance is achieved with a high degree of capacitance stability. For designers an ongoing problemis theway that quoted capacitance values fluctuatewith changes in temperature. This change is typicallymoremarked as the operating temperature of the device approaches its upper limits. For example if a capacitor is utilized in a circuit to provide bulk storage, the amount of charge storedmay be several tens of percentage points less than the specified device capacitancewhen the operating temperature is high. Thismay result in the capacitor being unable to performthe function forwhich itwas originally designed. Typically the onlyway to overcome this potential problemwas for designers to specify device capacitance valuesway abovewhatwas actually required. In addition to being a crudeway of achieving a design, it oftenmeant a larger case size and footprint capacitor needing to be specifiedwhich could also bemore expensive.


Kemet’s Paul Staubli Aswell as offering a high operating temperature specification, X8L


and X8R dielectrics crucially offer tight capacitance shift tolerances up to the 150°Cmaximum.


Kemet offersMLCCs that utilize an ultra-stable X8R dielectric. This


gives the same temperature capability as conventional X8R but also provides additional stability in both temperature and that it does not exhibit any capacitance loss due to applied DC voltage. This characteristic canmake ultra-stable X8RMLCCs suitable for replacing higher capacitance, larger footprint conventional X8R devices.


X8R and X8L dielectricMLCCs can bemore expensive than


comparable capacitance value devices in less stable dielectrics and with narrower operating temperature ranges. However, the costmay be offsetwhen designers—and buyers-- are facedwith the need to choose eithermultiple parallelMLCC or larger, higher capacitance deviceswith larger case sizes in the lower performance dielectrics. The potential space saving achieved by not needing to select larger, higher than needed capacitance devices is an additional benefit in a sectorwhere the amount of available PCB real estate is shrinking.


Paul Staubli is director, application engineering, Europe, for Greenville, N.C.-based based Kemet Electronics Corp.


www.kemet.com www.electronics-sourcing.com


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