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Complete Circuit Designer's Notebook

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Complete Circuit Designer's Notebook ) Curie Point r Ω ZTR (1 10,000) ε L1 L2 b 9000 LIN c 8000 7000 Ccb C1 C2 C3 R PIN 6000 IN R 5000 C IIN RS 4000CIRCUIT ZIN (50 Ω) i1 i2 i3 3000 e 2000DESIGNER’S 1000 Relative Permitivity ( NOTEBOOKS 25 60 80 100 140 Ceramic 120ATTENUATOR Element Temperature (C) SHORTING PLUNGER DUT CENTER CONDUCTOR PRECISION SOURCE VOLTAGE PROBE RESISTOR LOOP TNC 3 O 3 RF SIGNAL MILLIVOLT O GENERATOR METER Unloaded Line Mg Ti Ba Ti Line With DUT Air Dielectric Strength (Volts) f fa' fo' fb' fa fo fb Dielectric Thickness (mils) CIRCUIT DESIGNER’S NOTEBOOK Table of Contents Capacitor Dielectric Properties ......................................................................................1 ESR Loss Factors...........................................................................................................2 Capacitor ESR Measurement Technique .........................................................................3 Understanding Temperature Coefficient of Capacitance .................................................4 Understanding Insulation Resistance .............................................................................5 Dielectric Aging Phenomena .........................................................................................6 Piezoelectric Effect in Ceramic Capacitors ......................................................................7 Effective Capacitance vs. Frequency ..............................................................................8 Capacitors in Coupling Applications ...............................................................................9 Capacitors in Bypass Applications ................................................................................10 High Q Capacitors in Matching Applications .................................................................11 American Technical Ceramics • www.atceramics.com CIRCUIT DESIGNER’S NOTEBOOK Capacitor Dielectric Properties Dielectric materials play a major role in Dielectric Thickness: This parameter defines Grain: A particle of ceramic material determining the operating characteristics of 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, needs. The following definitions are provided determining the voltage rating and parallel resonant frequency characteristics. and voltage sensitivity are directly related as a general overview of pertinent dielectric to this parameter. Grain size also affects design parameters. other electrical properties since it plays Dielectric Constant: Also referred to as an important role in the formation of relative permitivity (ε ) , a dielectric property r 3 microstructural characteristics such as that determines the amount of electrostatic 3 ceramic porosity and shrinkage related energy stored in a capacitor relative to a Mg Ti O anomalies. vacuum. The relationship between dielectric Ba Ti O Temperature Coefficient of Capacitance constant and capacitance in a multilayer Air (TCC): The maximum change of capacitance capacitor can be calculated by, C = εr (n-1) A/d, where ε is the dielectric constant, n over the specified temperature range is r is the number of electrodes, A is the active Dielectric Strength (Volts) governed by the specific dielectric material. electrode area and d is the dielectric Dielectric Thickness (mils) Insulation Resistance (IR): The DC resistance thickness. offered by the dielectric which is commonly Dielectric Strength: The dielectric’s ability measured by charging the capacitor to rated Dielectric Formulations: Formulations used to safely withstand voltage stresses. This is in the design of ceramic capacitors are voltage for one minute and measuring the determined primarily by the dielectric typically alkaline earth titanates, the most leakage current flow. formulation and electrode spacing. common of which is Barium Titanate Dissipation Factor (DF): Denotes that portion (BaTiO3). Electrical properties such as voltage of the total energy in the capacitor that is Pd rating, dissipation factor, insulation lost as internal heat or the ratio of energy resistance, temperature coefficient, as well as dielectric constant, are determined by the dissipated to the energy stored. εr dielectric formulation. These properties are Dielectric Aging: The gradual decrease Pd tailored to specific applications through the of dielectric constant leading to loss of addition of appropriate chemical modifiers capacitance over time for certain ceramic such as alkaline earth elements and transition formulations. This loss is logarithmic with element oxides. time and is most pronounced shortly after manufacturing. ) Curie Point ATC’s: Design Philosophy - ATC capacitors r 10,000 ε 9000 are designed and formulated in such a 8000 7000 manner as to optimize all performance 6000 characteristics. As an example, using closely Excessive voltage 5000 gradients in ceramic 4000 spaced electrodes, i.e., thin dielectric 3000 capacitors will cause the dielectric to 2000 sections, will generally increase the parallel 1000 lose its insulating properties, resulting in ( Relative Permitivity resonant frequency but will also decrease catastrophic failure. The dielectric voltage the maximum voltage rating. In this instance, 25 60 80 100 120140 breakdown characteristic is also affected by Temperature (°C) dielectric spacing and active electrode environmental conditions such as operating overlap areas are balanced for the best temperture, humidity, and atmospheric Curie Point: The temperature at which the combination for the specific application pressure as well as the physical spacing ceramic material will exhibit a peak or category. Another example may involve the between the capacitor’s terminations. sudden increase in dielectric constant is the inclusion of additives to adjust the TCC of a Internal breakdown: An internal failure Curie Point. Chemical agents may be added, given dielectric class. This is accomplished condition that occurs when the applied to shift and/or depress the Curie Point. This is by optimizing TCC while maintaining good voltage exceeds the dielectric strength, a major consideration in designing for insulation resistance and dissipation factor generally shorting the capacitor. specific Temperature Coefficient of Capacitance (TCC) limits. characteristics. External breakdown: A failure condition thatoccurs when the applied voltage exceeds Richard Fiore the breakdown path on the outside of the Director, RF Applications Engineering case between terminations. American Technical Ceramics Corp. American Technical Ceramics www.atceramics.com 1 • CIRCUIT DESIGNER’S NOTEBOOK ESR Loss Factors The summation of all losses in a capacitor is called dielectric and metal losses for a 22pF ATC180R Equivalent Series Resistance, (ESR) and is typically series capacitor. The losses are tabulated at various expressed as milli-ohms. ESR losses are comprised of frequencies and are added together to yield ESR. Note both dielectric loss (Rsd), and metal loss (Rsm). that the dielectric losses are predominant at the lower ESR = Rsd+Rsm frequencies and diminish at higher frequencies. The converse is also true for metal losses. Other capacitor Dielectric loss (Rsd) is determined by the specific values have the same pattern with different splits characteristics of the dielectric material. Each dielectric between Rsd and Rsm. material has an associated loss factor called loss tangent. The loss tangent is numerically equal to the Catalog ESR curves typically denote ESR values for dissipation factor (DF) and is a measure of loss in the capacitor’s dielectric at RF frequencies. The effect of Frequency Capacitor Rsd Rsm ESR (MHz) (pF) (m-ohm) (m-ohm) (m-ohm) this loss will cause the dielectric to heat. In extreme 1 180R220 145 7 152 cases thermal breakdown may lead to catastrophic failure. The dissipation factor (DF) provides a good 3 180R220 48.2 7.8 56 indication of the dielectric loss, and is typically 30 180R220 4.82 9.18 14 measured at low frequencies e.g. 1MHz, where this loss 300 180R220 0.48 28.51 29 factor is predominant. frequencies at or above 30 MHz, where the losses are Metal loss (Rsm) is determined by the specific predominantly due to Rsm. At these frequencies the conductive properties of all metallic materials in the dielectric losses are virtually transparent and do not capacitor’s construction. This includes electrodes, significantly influence the overall ESR. terminations plus any other metals such as barrier layers etc. The effect of Rsm will also cause heating I I ESR =Q = DF =Xc = of the capacitor. In extreme cases thermal breakdown X c × DF 1/DF 1/Q 1/2Π × F × C may lead to catastrophic δ X /Q X /ESR ESR/X ESR/DF failure. These losses c c c encompass ohmic losses as X c × tan δ 1/tan δ tan δ ESR × Q well as ‘skin effect’ losses (ESR) at frequencies typically above 30 MHz for most Table 2: Relationship between ESR, Q, DF and Xc MLCs. In most instances it is important to consider ESR Example: and Q for designs at high frequencies and DF for Given a 100pF capacitor with an ESR of 18 milli-ohms (due designs at lower frequencies. The general rule is to Rsm) @ 30 MHz, what is the ESR of this that DF is a factor that will help the design engineer capacitor at 120 MHz? evaluate Rsd losses at low frequencies, usually well Solution: below 10
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