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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
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Purpose
• To familiarize users with the basic properties of Ceramic Capacitors and
• To demonstrate “CapCad”, a modeling and selection methodology.
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Outline
• Application of Capacitors• Capacitor Structures• Terminology and Definitions• Electrical Properties• Physical Characteristics• Mounting Considerations• Capacitor Models• CapCad• Conclusions
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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
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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
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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.
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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)
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Common Dielectrics
Vacuum 1.0
Air 1.004
Mylar 3
Paper 4 to 6
Mica 4 to 8
Glass 3.7 to 19
Alumina 9.9
Ceramics 5 to 18000 +
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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.
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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
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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.
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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
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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
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Mounting Considerations
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Capacitor Models
• Reasonable prediction to the first series resonance
• Predicted behavior above series resonance doesn’t match observed results.
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Transmission Line Model
• Treats the capacitor as an open circuited transmission Line
• Results closely match measured data
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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
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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 !
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