Report

Capacitors for RF Applications Michael P. Busse Vice President Dielectric Laboratories, Inc 2777 Rte. 20 East Cazenovia, NY 13035 315-655-8710 315-655-8179 www.dilabs.com Purpose • To familiarize users with the basic properties of Ceramic Capacitors and • To demonstrate “CapCad”, a modeling and selection methodology. Outline • • • • • • • • • Application of Capacitors Capacitor Structures Terminology and Definitions Electrical Properties Physical Characteristics Mounting Considerations Capacitor Models CapCad Conclusions Applications • Ceramic Capacitor technology covers a wide range of product types, based upon a multitude of dielectric materials and physical configurations. All are basically storage devices for electrical energy which find use in varied applications in the electronics industry including the following: • • • • • • Discharge of Stored Energy Blockage of DC Current Coupling of Circuit Components By-Passing of an AC Signal Frequency Discrimination Transient Voltage and Arc Suppression Structures • Single Layer SLC – Two plates separated by a dielectric. – Simple to fabricate – Area/thickness limited – Cap Ranges of .05 pF to 2000 pF • Multi Layer MLC – A parallel array of capacitors in a common structure. – High C/V can be achieved – More complex to manufacture – Cap Ranges of .10 pF to 5100 pF Definitions • Capacitor – A device for storing electrical energy. The simplest form is two separate parallel plates with a non-conducting (dielectric) substance between them. The amount of energy that can be stored depends on the Area (A), Dielectric Constant (K), and the Thickness (t) of the dielectric. C=KA(.2246)/t (.2246 is a conversion factor in English, for Metric 0.0884). The area can be manipulated by the structure. • Capacitance – A unit of measure describing the electrical storage capacity of a capacitor. Capacitance is measured in farads, microfarad (millionth of a farad), nanofarad (billionth of a farad or 10-9), or in picofarad (trillionth of a farad or 10-12). • Dielectric – Any material which has the ability to store electrical energy. In a DLI capacitor, it is non-conducting ceramic between the plates. In general, capacitors can utilize any dielectrics such as air, or naturally occurring dielectrics such as mica. Definitions • Classes of Dielectrics – Two basic groups (Class 1 and Class 2) are used in the manufacture of ceramic chip capacitors. Class 1 dielectrics display the most stable characteristics of frequency, voltage, time and temperature coefficients (TC). TC is expressed as a % of capacitance change from a reference or parts per million per degree C (ppm/ºC). Class 2 dielectrics offer much higher dielectric constants but with less stable properties with temperature, voltage, frequency, and time. TC is expressed as a % change from a reference (+- 15% over some range of temperature) Common Dielectrics Vacuum Air 1.0 1.004 Mylar 3 Paper 4 to 6 Mica 4 to 8 Glass 3.7 to 19 Alumina 9.9 Ceramics 5 to 18000 + Definitions • Dielectric Constant (K) – The calculated measurement of a material which defines its capacity to store electrical energy. A higher “K” signifies a higher capacitance per unit at the test temperature. • Electrode – The metallic plates that are the top and bottom of a single dielectric layer. In a SLC (Single Layer Capacitor), the outer metallized plates form the electrodes. In an MLC (Multi Layer Capacitor), the metal print that alternates between the ceramic layers form the electrodes. Electrical Properties • IR = Insulation Resistance – DC Resistance which is a function of the dielectric. It is the ability of the capacitor to oppose the flow of electricity at a given direct voltage. • DF = Dissipation Factor – Loss Tangent is the ratio of energy “used up” by a working capacitor divided by the amount of energy stored over a definite period of time. It is a measure of the capacitors operating efficiency. • ESR = Equivalent Series Resistance – The effective resistance to the passage of RF energy Electrical Properties • Dielectric Withstanding Voltage (DWV) is a measurement of the electrical strength of the dielectric at 2½ times the rated voltage. • Temperature Coefficient (TC) is a measure of how the capacitance changes with temperature. • Tolerance is the amount of variation allowed from a target value. It is normally expressed as an Alpha character, for example a “J’ tolerance would be + 5%. • Voltage Conditioning is a test that applies heat and voltage to the parts for a set number of hours to accelerate failure mechanisms and identify rejects. Q • Q = Quality Factor is a numeric expression of the relative loss of a capacitor. Most commonly described as the storage factor of a capacitor and is the reciprocal of the Dissipation Factor. • Q is defined as – Q=1/2πFC(ESR) • F=frequency • C=capacitance • For any given capacitance at a given frequency, the highest Q part will have the lowest ESR Physical Considerations • Size equates to Voltage Rating – Larger case sizes have greater voltage capabilities – Smaller case sizes have higher series resonance characteristics • The separation between the internal electrodes dominates voltage rating • The dielectric has to be an insulator • The dielectric will determine the properties of the capacitor Mounting Considerations Capacitor Models • Reasonable prediction to the first series resonance • Predicted behavior above series resonance doesn’t match observed results. Transmission Line Model • Treats the capacitor as an open circuited transmission Line • Results closely match measured data CapCad V3 • Modeling software to simplify the selection of the right capacitor. • Easy to use graphical interface • Export and Import s2p files • Smith chart graphing • Includes Spice Modeling • Link:CapCadV3 and CapCal Conclusion • Capacitors present more of a challenge to selection than just the capacitance • The Physical as well as the Electrical properties must be taken into consideration • Proper Modeling Tools can simplify the selection • Thank You !