Ceramic capacitors have a wide spectrum of applications in today’s analog and digital electronic circuits. The characteristics of ceramic capacitors vary depending on the properties of the dielectric formulations used in their construction. Ceramic dielectric materials are broadly categorized into three classes: Class I, Class II, and Class III dielectrics. These dielectric materials are classified according to their dielectric constants.
The most common Class III dielectrics are Z5U and Y5V. These high-K dielectrics are based on alkaline earth titanates. Chemical modifiers are added to dielectric formulations to produce desired performance properties. The most commonly used modifiers include transition element oxides and alkaline earth elements.
Classification of ceramic dielectrics
Class I ceramic dielectrics have low dielectric constants, and they yield ceramic capacitors with high stability of capacitance, linear and reversible temperature coefficient of capacitance, low dissipation factor (DF), and high insulation resistance. Variations in frequency, time, or voltage have minimal effects on the electrical values of Class I ceramic capacitors.
Temperature compensating capacitors are commonly used in applications that demand high quality factor (Q) and superior stability. Although these passive components offer impressive stability and low dissipation, they have low volumetric efficiency. C0G, also commonly known as NP0, and U2J are the most common Class I dielectric materials. Most Class I dielectric formulations are based on calcium zirconate (CaZrO3).
Class II ceramic dielectrics have high dielectric constants and yield ceramic capacitors with high volumetric efficiency. The electrical characteristics of these passive components are affected by changes in voltage, temperature, frequency, or time. Class II multilayer ceramic (MLC) capacitors are commonly used for bypassing, coupling, and decoupling in applications where stability of capacitance and Q are not of critical importance. Class II dielectrics are made of ferroelectric materials, and most of them are based on barium titanate (BaTiO3). The most common Class II dielectrics include X5R, X7R, and X8R.
Z5U, one of the most common Class III dielectric materials, has a high volumetric efficiency and low stability of capacitance. Class III multilayer ceramic capacitors are mostly used for general purpose applications. These passive components are common in applications where insulation resistance and stability of capacitance are of little importance. Just like Class II dielectric materials, most Class III ceramic dielectrics are based on barium titanate.
Characteristics of Class III dielectrics
Class III capacitors are constructed using ferroelectric formulations, and their performance characteristics are nearly similar to those of Class II capacitors. These formulations have the highest dielectric constants, typically 3500 to 12,500, and yield components with high volumetric efficiency. On the flip side, these ceramic capacitors have the lowest stability. Their low stability makes them unsuitable for many applications.
Ceramic dielectric materials are usually formulated to meet specific performance requirements. Some of the performance characteristics that are greatly dependent on the dielectric material include voltage rating, temperature coefficient, insulation resistance, equivalent series resistance (ESR), dissipation factor, and parasitic inductance. Unlike Class I MLC capacitors, the performance characteristics of Class III ceramic capacitors are significantly affected by variations in temperature, frequency, voltage, and time.
Effects of temperature
Changes in temperature can significantly affect the performance characteristics of some ceramic capacitors. For Class III dielectrics, variations in temperature cause a considerable change in capacitance and dissipation factor. The maximum capacitance change is determined by the temperature characteristics of a dielectric formulation.
The insulation resistance of a Class III MLC capacitor is affected by variations in temperature. An increase in temperature results in a decrease in insulation resistance. Typical Z5U capacitors are suitable for +10ºC to +85ºC applications. For this temperature range, these capacitors have a maximum capacitance change of +22% to -56%. At 25oC, Z5U has an insulation resistance of 10 GΩ and a maximum dissipation factor of 0.04 (4.0%).
Y5V capacitors have an operating temperature range of -30°C to 85°C. For this temperature range, the maximum capacitance change (temperature coefficient) of these capacitors is +22% to 82%. At 25ºC, Y5V has an insulation resistance of 105 megaohms and a dissipation factor of 0.05 (5.0%).
Effects of frequency
The impedance and capacitance characteristics of a capacitor are significantly dependent on frequency. In ceramic capacitors, the effect is more common in dielectric formulations with high dielectric constants. Since Class III dielectric formulations have the highest dielectric constants, their characteristics are considerably affected by variations in frequency. For Class III dielectrics, variations in frequency cause significant changes in the capacitance and dissipation factor of a component.
The total impedance of a typical Class III ceramic capacitor is a function of equivalent series resistance, capacitive reactance, and inductive reactance. An increase in frequency causes a decrease in capacitive reactance. In comparison, the inductive reactance of a Class III capacitor increases with an increase in frequency. The total impedance can be capacitive, inductive, or purely resistive depending on the operating frequency. At self-resonant frequency, the capacitive reactance and inductive reactance cancel, and the total impedance at that frequency is purely resistive.
Effects of voltage
The performance characteristics of some ceramic capacitance are considerably affected by variations in voltage. For capacitors that are based on low-K formulations such as C0G, variations in voltage have minimal effect on their characteristics. For high-K dielectrics such as Z5U and Y5U, changes in voltage have significant effect on the capacitance and dissipation factor of a component.
In high-K ceramic capacitors, application of a DC voltage, typically above 5VDC, causes a decrease in capacitance and dissipation factor. In comparison, when a low AC voltage, typically 10-20VAC, is applied to a Class III capacitor, it tends to increase the capacitance and dissipation factor of a component. Application of a high AC voltage to a Class III capacitor produces the same effect as a DC voltage.
Effects of mechanical stress
When subjected to mechanical stress, some ceramic capacitors exhibit piezoelectric characteristics. As compared to low-K dielectric formulations, this effect is more pronounced in high-K dielectric materials. Since Class III dielectric materials have the highest dielectric constants, they are significantly affected by piezoelectric reactions. When selecting a Class II or III capacitor for coupling applications, it is necessary to consider this effect.
Effects of time
The capacitance and dissipation factor of a Class III ceramic capacitor change significantly with time. This process is commonly referred to as aging, and it is caused by realignment of the crystalline structure of a ceramic dielectric material. This gradual realignment results in a decrease in dissipation factor and an exponential loss in capacitance over time. For Z5U, this predictable and reversible process occurs at a rate of 5% per decade of time. The Y5V dielectric material has an aging rate of 6% per decade of time.
The initial capacitance and dissipation factor of a Class III ceramic capacitor that has been on the shelf for long can be regained. This process, commonly known as de-aging, involves heating a component to a temperature above its Curie Point.
Applications of Class III dielectrics
Class III capacitors are commonly known as general purpose capacitors and are mostly used in applications that demand components with high volumetric efficiency. In applications where stability of capacitance is not a critical factor, Z5U and Y5V capacitors are commonly used for bypassing and decoupling applications. These capacitors are also commonly used to replace tantalum capacitors.
The performance characteristics of ceramic capacitors are greatly determined by the properties of the dielectric materials used in their construction. Ceramic dielectric materials are classified into three classes depending on their dielectric constants. Class III dielectrics have high dielectric constants and a low stability of capacitance. Just like Class II dielectrics, these dielectrics are mostly based on alkaline earth titanates. The performance characteristics of Class III capacitors are significantly affected by variations in frequency, temperature, voltage, and time. Class III capacitors are unsuitable for applications that demand high stability of capacitance. These capacitors are commonly used as bypassing, coupling, and decoupling elements in applications that demand components with high volumetric efficiency.