Dielectrics- Capacitance Flashcards
What are capacitors?
Insulating barriers in electric circuits. They block the flow of current. Current only flows when the charge on one side becomes large enough to breakdown the barrier and cross it. Used for charge strorage, block dc current, ac-dc conversion
Capacitance equation
C=q/V
Charge of over volume
Why are ferroelectics ideal to sit between plates of capacitors?
They are insulators so prevent charge flowing. They polarise in the field created by the plates, further stabilising the charge and allowing a greater charge to build up on the plates
Why are ferroelectics ideal to sit between plates of capacitors?
They are insulators so prevent charge flowing. They polarise in the field created by the plates, further stabilising the charge and allowing a greater charge to build up on the plates
How to increase capacitance
Shrink the gap between plates. Increase the area of plates. Increase the permittivity.
C=ε0A/d
C’=ε0εrA/d
εr=C’/C
Relative permittivity of BaTiO3
About 10,000 at Tc
What is volume efficiency?
Capacitance over volume (Fmm^-3)
C/V=ε0εr/d^2
How to improve volume efficiency
Use thin layers (e.g polymer dielectrics 1-10μm thick but small εr).
Oxidise surface of a metal foil (electrolytic capacitors) to get 0.1-1μm thick insulating surface.
Tape cast ferroelectric BaTiO3-based ceramics (0.8-1μm thick layers) and stack layers together to form multi-layer capacitor (normally 100-400 layers)
Design of multi-layer ceramic capacitors
Two main external electrodes parallel to each other. Internal electrodes perpendicular to them coming out into gap (alternate from which external electrode). Between internal electrodes they are separated by ceramic. Each layer contributes to overall A while separation is only very small gap between each electrode
Why can capacitance vary with temperature?
The polarisation of materials varies with temperature. Phase changes in BaTiO3 cause shifts in permittivity
Ideal behaviour of capacitors
Electrical engineers want capacitors with same behaviour (capacitance) across a temperature range. Permittivity stabilised. Capacitor design often focussed around optimising temperature coefficient of capacitance (TCC) for certain temperature ranges
How thick is a MLCC?
0.8-2 microns with 100-800 layers
What is charge stored efficiency?
Volume efficiency times working voltage
Units Cmm^-3
Electrolytic capacitors
Could be Al (low cost) or Ta (high cost). Both good CSE. Both high capacitance. Both can only operate at low frequencies (less than 10kHz)
Polymer capacitors
Cheap. Poor CSE. Wide frequency range. But only work at low voltage (less than 1V)