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Circuit Designer's Notebook

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Circuit Designer's Notebook CIRCUIT DESIGNER’S NOTEBOOK Capacitor Dielectric Properties Dielectric materials play a major role in Dielectric Thickness: This parameter Grain: A particle of ceramic material determining the operating characteristics of defines the distance between any two internal exhibiting a crystaline or polycrystaline ceramic chip capacitors. Accordingly, they electrodes after the ceramic has been sintered structure. Electrical properties such as are formulated to meet specific performance to its final state. This is a major factor in dielectric strength, dielectric constant, and needs. The following definitions are determining the voltage rating and parallel voltage sensitivity are directly related to provided as a general overview of pertinent resonant frequency characteristics. this parameter. Grain size also affects dielectric design parameters. other electrical properties since it plays Dielectric Constant: Also referred to as ε an important role in the formation of relative permitivity ( r), a dielectric property microstructural characteristics such as that determines the amount of electrostatic 3 ceramic porosity and shrinkage related energy stored in a capacitor relative to a 3 Mg Ti O anomalies. vacuum. The relationship between dielectric Ba Ti O constant and capacitance in a multilayer Temperature Coefficient of Capacitance Air (TCC): The maximum change of capacitance capacitor can be calculated by, C=εr (n-1) A/d, where εr is the dielectric constant, n is the over the specified temperature range is number of electrodes, A is the active governed by the specific dielectric material. Dielectric Strength (Volts) electrode area and d is the dielectric Insulation Resistance (IR): The DC thickness. Dielectric Thickness (mils) resistance offered by the dielectric which Dielectric Strength: The dielectric’s ability is commonly measured by charging the to safely withstand voltage stresses. This is Dielectric Formulations: Formulations capacitor to rated voltage for one minute determined primarily by the dielectric used in the design of ceramic capacitors are and measuring the leakage current flow. formulation and electrode spacing. typically alkaline earth titanates, the most Dissipation Factor (DF): Denotes that common of which is Barium Titanate portion of the total energy in the capacitor Pd (BaTiO3). Electrical properties such as that is lost as internal heat or the ratio of ε voltage rating, dissipation factor, insulation energy dissipated to the energy stored. r resistance, temperature coefficient, as well Dielectric Aging: The gradual decrease of as dielectric constant, are determined by the Pd dielectric constant leading to loss of dielectric formulation. These properties are capacitance over time for certain ceramic tailored to specific applications through the formulations. This loss is logarithmic with addition of appropriate chemical modifiers time and is most pronounced shortly after such as alkaline earth elements and manufacturing. transition element oxides. ATC’s: Design Philosophy - ATC capacitors are designed and formulated in such a manner as to optimize all performance ) Curie Point r 10,000 Excessive voltage ε 9000 characteristics. As an example, using 8000 closely spaced electrodes, i.e., thin gradients in ceramic 7000 capacitors will cause the dielectric to 6000 dielectric sections, will generally increase 5000 lose its insulating properties, resulting in 4000 the parallel resonant frequency but will catastrophic failure. The dielectric voltage 3000 also decrease the maximum voltage rating. 2000 breakdown characteristic is also affected by 1000 In this instance, dielectric spacing and Relative Permitivity ( Relative Permitivity environmental conditions such as operating active electrode overlap areas are balanced temperture, humidity, and atmospheric 25 60 80 100 120140 Temperature (°C) for the best combination for the specific pressure as well as the physical spacing application category. Another example may between the capacitor’s terminations. involve the inclusion of additives to adjust Internal breakdown: An internal failure Curie Point: The temperature at which the the TCC of a given dielectric class. This is condition that occurs when the applied voltage ceramic material will exhibit a peak or accomplished by optimizing TCC while exceeds the dielectric strength, generally sudden increase in dielectric constant is the maintaining good insulation resistance and shorting the capacitor. Curie Point. Chemical agents may be added, dissipation factor characteristics. External breakdown: A failure condition to shift and/or depress the Curie Point. This is that occurs when the applied voltage a major consideration in designing for Richard Fiore exceeds the breakdown path on the outside of specific Temperature Coefficient of Director, RF Applications Engineering the case between terminations. Capacitance (TCC) limits. American Technical Ceramics Corp. Excerpt from complete Circuit Designers’ Notebook, Document #001-927, Rev. E, 1/05 American Technical Ceramics • www.atceramics.com.
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