Transcript Capacitors for RF Applications

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 !