Gauss's Law and Boundary Conditions Lecture 7 Gauss's Law and Boundary Conditions Dielectric Strength and Insulation Breakdown Md Arafat Hossain Department of Electrical and Electronic Engineering, Khulna University of Engineering and Technology, Khulna, BANGLADESH

Outlines Capacitor Dielectric Materials Typical Capacitor Constructions Dielectrics: Comparison Piezoelectricity, Ferroelectricity, and Pyroelectricity Piezoelectricity Piezoelectricity: Quartz Oscillators and Filters Ferroelectric and Pyroelectric Crystals

Capacitor Dielectric Materials Typical Capacitor Constructions The selection criteria of dielectric materials: Capacitance value, Frequency of application, Maximum tolerable loss, and Maximum working voltage, Size and cost (additional external constraints) Requirements for high voltage power capacitors are distinctly different than those used in small integrated circuits. Large capacitance values are more easily obtained at low frequencies because low-f At high-f, it becomes more difficult to achieve large capacitances and at the same time maintain acceptablem low dielectric loss, inasmuch as the dielectric loss per unit volume is SoE coT} tan 5.

Examples of dielectrics that can be used for various capacitance values. Examples of dielectrics that can be used in various frequency ranges

Two polymer tapes in (a), each with a metallized film electrode on the surface (offset from each other), can be rolled together (like a Swiss roll) to obtain a polymer film capacitor as in (b). As the two separate metal films are lined at opposite edges, electroding is done over the whole side surface.

Capacitor Dielectric Materials Dielectrics: Comparison The capacitance per unit volume CVoi, which characterizes the volume efficiency of a dielectric, can be obtained by dividing C by Ad It is clear that large capacitances require high dielectric constants and thin dielectrics. We should note that d appears as d2, so the importance of d cannot be understated.

Comparison of dielectrics for capacitor applications

Piezoelectricity, Ferroelectricity, and Pyroelectricity Certain crystals, for example, quartz (crystalline Si02) and BaTiOs, become polarized when they are mechanically stressed. Appearance of surface charges leads to a voltage difference between the two surfaces of the crystal. The same crystals also exhibit mechanical strain or distortion when they experience an electric field. The direction of mechanical deformation (e.g., extension or compression) depends on the direction of the applied field, or the polarity of the applied voltage The two effects are complementary and define piezoelectricity. piezoelectric crystal with no applied stress or field. The crystal is strained by an applied force that induces polarization in the crystal and generates surface charges. An applied field causes the crystal to become strained. In this case the field compresses the crystal The strain changes direction with the applied field and now the crystal is extended

Piezoelectricity: Quartz Oscillators and Filters

Ferroelectric and Pyroelectric Crystals