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Capacitors > Physics of dielectrics and applications > Flashcards

Physics of dielectrics and applications Flashcards

1
Q

Formula of ESR and impedance magnitude of capacitors

A
  1. ESR = tan(d)/(wC)
  2. Z^2 = ESR^2+(wL-1/wC)^2
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2
Q

Derive charge and discharge equations of capacitor (Thomas pag 9)

A

Write on paper

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3
Q

After a time constant what is the percentage of voltage/charge drop in the capacitor?

A

63.2%

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4
Q

Maximum electric field of: Al2O3, SiO2, BOPP, PET, BaTiO3 (Thomas 17)

A
  1. Polypropylene (BOPP): 450V/um
  2. Polyester (PET): 280V/um
  3. BaTiO3: 10(60) V/um
  4. SiO2: 600 V/um
  5. Al2O3: 600 V/um
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5
Q

Order these common dielectrics based on energy density: polypropelene, polyester, BaTiO3, SiO2, Al2O3

A
  1. Polypropylene (BOPP): 2.0 J/cm^3
  2. Polyester (PET): 1.1 J/cm^3
  3. BaTiO3: 0.5-1 J/cm^3
  4. SiO2: 6 J/cm^3
  5. Al2O3: 14 J/cm^3
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6
Q

What is the reason for aluminium oxide to have large energy density?

A

Due to the etching of the Aluminium layer, which increases effective area and capacitance and hence energy density

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7
Q

Mention one characteristic in the basic construction of Electrolytic, film and MLCC capacitor

A
  • Electrolytic: effective area of anode foil is large due to etching.
  • Film: distance between plates is very small.
  • Ceramic: dielectric constant is high.
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8
Q

Describe equivalent circuit of capacitor

A
  • Consists of series connection of inductor, ESR and capacitor.
  • It is also possible to connect a resistance across the capacitor to account for the leakage current.
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9
Q

What is the cause of leakage current?

A
  • It occurs because dielectric is not a perfect insulator and therefore a path for the leakage current can be established.
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10
Q

Describe briefly the breakdown process of a material

A

When a very large E field is applied to the material, some free/loose electrons might be accelerated. As they are accelerated their speed can increase a lot before it hits another atom, it might knock an electron off that atom. This released atom is accelerated under the strong E field and contribute to the generation of more ions and knock off electrons.

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11
Q

Describe the tunnel current - Fowler-Nordheim tunneling phenomena

A
  • Tunnel current is an electrical current that occurs through quantum mechanical tunnel effect and also through non conducting materials
  • Described by quantum mechanical wave functions. Wave function does not become zero after hitting a barrier (non-conductor) but decreases exponentially, so there is a probability of finding the electron on the other side of the barrier.
  • Wave function decreases exponentially with the width of the barrier
  • Tunnel current only occurs when electrons find a free place on the other side of the barrier
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12
Q

Describe briefly Poole-Frenkel effect

A
  • Model that describes the mechanism of trap assisted electron transport in an insulator and how in a large E field electrons move more frequently
  • Electrons can move slowly through the insulator.
  • Electrons are generally trapped at certain atoms, eventually, thermal fluctuations give an electron enough energy to leave the localized state and move to conduction band.
  • Once in the conduction band, electrons can move in the crystal for a short time before relaxing into another atom.
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13
Q

Main components of classical electrolytes of AL capacitors

A
  1. Solvents
  2. Conduction salts (ammonia salts of inorganic and longer chain organic acids)
  3. Additives
  4. Long chain fatty acid or additives adsorbed at the anode surface and prevent smaller ions to penetrate surface during anodization
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14
Q

Describe briefly breakdown with cond. polymers

A
  • Electric breakdown is determined by injection of electrons from PEDOT into oxide
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15
Q

Explain corona discharge, problems, and solutions

A
  • Air enclosed in the winding becomes ionized and thus more conductive, allowing partial discharges on metallized surface film and causing local vaporization of metallization
  • Repeatedly corona discharge events can remove metal from surface of the film leading to a decrease of the capacitance (not breakdown) and increase of ESR
  • It can be prevented by using special oils that fill up the air voids or lowering the voltage across the dielectric
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16
Q

Different types of voltage ratings in electrolytic capacitors

A
  • Rated voltage
  • Working voltage
  • Surge voltage (can be applied for a short time)
  • Transient voltage (can be applied only for very short time)
  • Reverse voltage
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17
Q

Why is it important not to apply voltage above the rated voltage in a capacitor and 1 example

A
  • Voltage above rated voltage will cause some degree of damage and reduce lifetime and reliability
  • In Al El caps it causes chemical reactions (formation of dielectric) on the anode foil with the leakage I rapidly increasing, producing heat and gases and increasing the inner pressure
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18
Q

Draw impedance curve of electrolytic and MLCC capacitors against frequency

A

Draw on paper

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19
Q

6 Main properties of leakage current in Al El caps

A
  1. In an electrolytic capacitor, it’s the current flowing through the dielectric when DC voltage is applied
  2. It’s a measure of dielectric quality
  3. It’s voltage, time, and temperature dependent, also depends on history of capacitor
  4. Usually measured after 5 minutes at applied rated voltage
  5. Relevant for lifetime estimation
  6. Depends strongly on previous storage rule (when it has been stored for a long period, we need to repair the electrolytic capacitor by applying a current to reform the oxide layer)
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20
Q

Advantages of capacitors over batteries in terms of response time and charge/discharge cycles

A
  • Capacitors have a better response time than batteries and can be charged/discharged many times more than a battery. - - For short time storage capacitors are preferred.
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21
Q

What type of capacitors are class X and class Y capacitors?

A

Ceramic and metallized film

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22
Q

Why are class X and Y capacitors used? give 3 applications

A
  • Used because they are usually film or ceramic (good HF response)
  • Used in AC line filtering
  • Used to reduce or suppress electrical noise caused by operation of electronic equipment
  • Providing protection against electrical shocks (X capacitors)
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23
Q

How are x and y capacitors connected?

A
  • X capacitors are connected between L and N
  • Y capacitors are connected betweenn L and GND and N and GND.
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24
Q

Typical capacitance of X and Y capacitors (Thomas pag 46)

A

2 to 500 nF

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25
Q

Name one dielectric material used for X/Y caps

A
  • BaTiO3
26
Q

Explain the relation between charge/discharge speed and losses of a Cap

A
  • The faster it charges/discharges the less losses it has.
  • Faster charge process implies a smaller R thus less power dissipation
27
Q

What are the elements needed to produce a dipole moment?

A
  • We need charges of different polarity
  • Charges must be separated by a distance d
28
Q

Name an example of molecules easy and difficult to polarize

A
  • Easy to polarize: water
  • Difficult to polarize: barium titanate BaTiO3 (not sure)
29
Q

Explain cause of losses in a dielectric under AC current

A
  • Polarization of dielectric is a source of losses since the dipoles of the material must reverse its direction in every cycle
30
Q

Region during the discharge of capacitor where big losses occur and how to avoid this

A
  • Big losses occur in the tail of the discharge curve, when the voltage drops to less than 2/3 of rated voltage
  • It is recommended not to discharge the capacitor below 2/3 of rated voltage
31
Q

4 Properties of dielectric used for film caps

focus on dielectric constant and breakdown

A
  • Low dielectric constant
  • Dielectric are carbon based with small dipole moment, thus losses are low.
  • Dipole can be flipped easily thus losses are low
  • High dielectric strength
32
Q

Relation between dipole and melting points

A
  • As dipole moment increases melting point also increases: molecules stick together strongly so more energy is required to break these bonds.
33
Q

What can reduce the distance considered for dielectric breakdown?

A
  • Sometimes the metals is pushed against the dielectric, shrinking the dielectric or penetrating some regions of dielectric.
  • If distance is reduced breakdown voltage is also reduced.
34
Q

Why are film caps suitable for high voltages? (2 reasons)

A
  • Dielectric used has high dielectric breakdown
  • Number of defects in the dielectric is very small
35
Q

Relation between conductivity and breakdown voltage of dielectric

A
  • As conductivity of dielectric increases breakdown voltage drops
36
Q

Effects and causes of partial discharge in a cap

A
  • Usually caused by very thin metal film and humidity or air trapped in voids inside the dielectric
  • Can decompose the dielectric
  • Vaporizes metallization
  • Reduces C and increases ESR
  • Can generate propane gas (PP)
37
Q

Compare electrolytic caps and film caps in terms of ease of impurities to penetrate the casing

A
  • Electrolytic caps incorporate inner pressure which avoids impurities from penetrating the casing.
  • Film capacitors do not have this property
38
Q

Write the equation of a dipole

A
  • P = dq (d is distance that separates charges q)
39
Q

On what depends the proportionality constant of dipole moment of a material and the E field

A

Depends on the ease with which electrons are displaced, in other words on the kinds of atoms in the dielectric

40
Q

What is the proportionality constant between polarization vector of dielectric and E field (Feynman section 10-3)

A

electric susceptibility* permittivity of vacuum (x* e0)

41
Q

By which factor is the capacitance increased when a dielectric is added

A

k = 1+x (x: electric susceptibility of dielectric)

42
Q

Relation between force and dielectric in a capacitor

A

Dielectric increases capacitance by a factor of k, all forces will be reduced by the same factor

43
Q

Why does water shows a permanent electric dipole moment and oxygen doesn’t?

A
  • Water has a nonsymmetric arrangement of hydrogen and oxigen atoms. Center of gravity of negative and positive charges do not coincide.
  • Water is called a polar molecule.
  • Oxygen has a symmetric pair of atoms in each molecule. Nonpolar molecule.
44
Q

Explain electronic polarization

A

When atoms of a material are under an electric field effect, electrons are pulled one way by the field while nucleus is pulled the other way. This slight net displacement of the centers of charge induced a dipole moment.

45
Q

Explain relation between polarization and temperature (polar molecules)

A

Polarization is inversely proportional to temperature because at higher T the misalignment of molecules by collisions increases in a gas or polar molecule. This is known as Curie’s law.

46
Q

Explain relation between polarization and frequency (polar molecules)

A
  • Due to moment of inertia of molecules, it takes certain amount of time for heavy molecules to turn towards the direction of E field.
  • At high frequencies, polar contribution to dielectric constant decreases because molecules cannot follow.
  • But electronic polarization still remains the same because electrons have smaller inertia.
47
Q

Do the dipoles of a dielectric reorient themselves after external field is removed?

A

No, they must be neutralized externally.
That’s why capacitors are able to store energy even after field is removed.

48
Q

How is dielectric layer created in an electrolytic cap

A

An oxide layer is formed on electrode material as a result of chemical reaction with electrolyte. This layer acts as dielectric.

49
Q

3 parameters that characterize a dielectric

A
  1. Dielectric strength
  2. Dielectric constant
  3. Dielectric loss factor
50
Q

2 Components that contribute to ESR

A
  • Electrode resistance, wire leads, solder, end connections.
  • Dielectric resistance/losses due to dipole movements, it increases with frequency and occurs when AC currents are applied.
51
Q

Formulas of dissipation factor, loss factor, tan delta

include quality factor Q

A

Dissipation factor(tan delta) = ESR/Xc
Loss factor = (I^2ESR)/(I^2Xc)=1/Q=ESR/Xc

52
Q

Loss factor is only proportional to frequency

A

No, it is also a function of temperature and applied voltage.

53
Q

Describe the two types of losses in dielectrics (Deshpande 54)

A
  • Conduction loss: due to flow of actual charge through the material. (due to ESR in AC and Rins in DC)
  • Dielectric loss: due to the movement of atoms or molecules in AC electric field (due to dipole movements).
54
Q

Order these dielectric materials based on dissipation factor and ESR: aluminium electrolytic, polyester, polypropylene, tantalum

A
  1. Aluminium electrolytic: 10% loss factor at 120 Hz, 132.6Ohm ESR.
  2. Tantalum: 4% at 120Hz, 53 Ohm.
  3. Polyester: 1% at 1kHz, 1.59Ohm
  4. Polypropelene: 0.1% at 1kHz, 0.159Ohm.
55
Q

What are the causes and effects of series inductance in a cap

A
  • Cause by inductance of electrodes and leads
  • Defines the limiting factor of how well or fast the cap can decouple noise off a power bus.
  • Defines the resonance point.
56
Q

List at least 5 common dielectric (general classification) materials

A
  1. Plastic films: polypropylene and polyester,
  2. Composite dielectrics:
  3. Ceramics: dielectric constants ranging from 6-10000. Electric field changes the effective dielectric constant of material and thus capacitance.
  4. Mica
  5. Glass
57
Q

Brief description of composite dielectrics (4 ideas)

A
  • They are made of film or paper.
  • They are impregnated with liquids to remove air voids. Liquid must have dielectric constant close to the solid to ensure uniform E field.
  • Moisture is removed to increase reliability.
  • usually two layers are used to increase reliability.
58
Q

Brief 2 characteristics of ceramic dielectrics and classification (3 types) (Deshpande)

Characteristic focus on dielectric and E field effect

A
  • Dielectric constant ranging from 6-10000
  • E field across dielectric changes effective dielectric constant and therefore the capacitance (voltage dependent).
  • Classified into 3 classes
  • Class I are used for resonant circuit and high frequency bypass and coupling and have wider temperature range.
  • Class II are used for bypassing RF, filtering and coupling and have very small size.
  • Class III are used for low-voltage coupling and bypassing
59
Q

Brief description (4 characteristics) of mica dielectric

A
  • Very stable electrically, mechanically and chemically.
  • Dielectric constant in the range 5-7.
  • Stable and chemically inert, does not react with oil, water, many acids and solvents. So ageing does not occur to a major degree.
  • High cost due to careful inspection and classification of the material
60
Q

Brief description (8 points) of glass dielectric

A
  • preferred choise for long-life capacitor
  • highly stable over frequency and temperature
  • low loss factor.
  • low dielectric constant variation.
  • High electric strength.
  • Used in RF circuits offering high level of performance.
  • High cost.
  • Low capacitance value up to 3000pF only.
61
Q

Define partial discharge

A

an electrical discharge that does not completely bridge the space between two conducting electrodes

Capacitors (6 decks)

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