Metallized film capacitors Flashcards
List the main components of film cap basic construction and draw the schematic (Thomas pag 20)
- Coating/box
- Protective film
- Metallization or metal foil (electrode)
- Plastic film (dielectric)
- Contact layer
- Terminals
List the main components of electrolytic cap basic construction and draw the schematic (Thomas pag 19)
- Anode (+), (etched)
- Paper
- Electrolyte
- Cathode (-)
Why is metallization of contact layer important in basic construction of film cap? (3 ideas)
- Good metallization of contact layer is crucial to reduce ESR
- Important for high dv/dt (pulse) applications
- It determine the quality of contact between electrodes and terminals, if contact is weak resistive losses increase
Write stored energy density equation and derive it
W/V = 1/2(E0ErE^2)
W: stored energy
V: volumen A*d
E0, Er: vacuum and relative permittivity
Classification of film capacitors based on structure of electrodes
- Metal foil [F]
- Metallization on dielectric
M]
2 general types of dielectric used in film caps
- Paper
- Plastic film
Common polymers used for metallized film caps
- Polypropylene BOPP
- PET
- PEN
- PPS
Dielectric constant, Dielectric strength and max temperature of polymers (BOPP, PET, PEN, PPS) used in film caps
- BOPP: 2.2, 640MV/m, 105°C
- PET: 3.3, 570MV/m, 105-125°C
- PEN: 3, 400MV/m, 150°C
- PPS: 3.1, 550MV/fm, 140°C
Materials used to construct the electrodes of film caps
- Aluminium and zinc
Why aluminium and zinc are used for electrodes in film caps instead of copper (3 reasons)
- Lower cost and melting point than copper
- Very hot film of copper would melt the plastic film (i.e. polypropelene)
- Using aluminium and zinc gives the flexibility to create PETA metallization or metallization with different etching profiles
3 Advantage and 3 disadvantages of using zinc and aluminium instead of aluminium alone as metallization material
- ZnAl has higher breakdown strength
- ZnAl has more reliable self-healing (due to Zn)
- ZnAl has better C stability (due to Zn)
- ZnAl has higher resistance (bad)
- ZnAl has worse high frequency impedance (bad)
- ZnAl has worse resistance to air, heat, humidity (bad)
Draw the different connections of metallized films to form serial capacitors (Thomas pag 32)
Draw on paper
Which factors influence the capacitance of film capacitors
- Temperature
- Frequency
- Voltage
Describe the behaviour of PP, PPS, PEN, PET film caps as a function of temperature (draw the curves)
- PP: Capacitance decreases with temperature
- PPS: capacitance decreases with temperature (with smaller slope than PP)
- PEN: capacitance increases with temperature (as x^3 curve)
- PET: capacitance increases with temperature (as x^3 with larger slope)
Describe the behaviour of film capacitor as a function of frequency (PP, PPS - PET PEN) (draw the curves) (Thomas pag 37)
- Capacitance of all types PP, PPS, PEN, PET decreases with frequency
- Capacitance drop for PET and PEN is more pronounced
- PPS and PP is almost stable up to 100kHz
Describe behaviour of film capacitor (PP, PET, PEN, PPS) regarding voltage derating (draw curves)
- All types suffer from V derating.
- As T increases rated V decreases
- PEN and PPS show constant rated V up to 105°C
- PP and PET show constant rated V up to 85°C
Explain self discharge of film capacitor and define self-dicharge time constant
- Film cap discharges through its internal insulation resistance
- The product between insulation resistance and capacitance is the time constant (self-discharge time constant)
- The bigger the time constant, the higher the quality of the dielectric
List the processes required for manufacturing film caps
- Metallization of plastic film
- Film slitting
- Winding
- Impregnation (with silicone)
- Healing (by applying voltage)
- Contact layer
- Flattening
- Terminal welding
- Coating
- Test
8 Failure modes of metallized film caps
- Capacitance loss due to self-healing or oxidation of metal
- ESR increase
- Short circuit
- Breakdown
- Open circuit
- Explosion
- Corrosion
- Swelling of mould due to expansion with temperature
9 Failure modes in electrolytic caps
- Capacitance loss
- ESR, leakage current increase
- Short circuit
- Breakdown
- Open circuit
- Explosion
- Corrosion
- Dry out
- Opening of valve
3 Failure modes of ceramic caps
- Crack
- Avalanche breakdown (cracks)
- Thermal runaway (imperfections)
Briefly explain corrosion process of metals
- Metals oxidize when exposed to oxygen and an electrolyte (humidity)
- Electrons move from the metal to the oxygen molecules during this process. Negative oxygen ions then generate and enter the metal, leading to the creation of an oxide surface
6 Effects of corrosion in film caps (Thomas pag 60)
- ESR increases (less conductive area)
- tan delta/loss factor increases (ESR increases)
- T increases (as ESR increases)
- as T increases, pressure increases leading to opening or leakage current.
- T increase might lead to dielectric melting
- Capacitance decrease (due to corrosion of metallization)
Explain the relation between corona discharge and self-healing
- Corona discharge leads to short circuit between the metallization foils.
- During the short circuit there is high current density and therefore high temperatures (and probably sparks)
- The high temperature vaporizes the metallization around the corona discharge point.
- The point with weak dielectric strength is removed as metallization doesn’t exists anymore
What is safety concern of capacitors using PP as dielectric
- Melting of PP will produce flammable gas
- When gas concentration is high, there is risk of explosion
- gas detectors can help to monitor propilene gas concentration and increase safety
3 Advantages and 1 disadvantage of BOPP compared to PP for film caps
Advantages
- BOPP is state-of-the-art dielectric
- BOPP has excellent self-healing properties
- BOPP shows low losses
- BOPP is cheaper
Disadvantages
- Limited performance at high T
- Tmax: 105-125°C
A problem with PP due to new WBG technologies
- It’s reaching its limits due to rising demands of new WBG semiconductors especially in high-temperature applications
2 alternatives to PP
Not PPS, PEN, or PET
- Semi-crystalline PP: easy to process but temperature limitation
- Copolymer: high T operation but difficult to process into thin films
From the combination of these two material a new technology blended film has emerged.
4/7 advantages of CoC-PP compared to BOPP
- Higher operation T (up to 125°C)
- Higher power density
- Less cooling needed
- Less restricted junction T
- Lower voltage derating
- Ageing and failure mechanism similar to BOPP
- Similar self-healing properties
5 Causes of increasing ESR (Thomas pag 91)
- ESR rises with rising frequency, increasing power losses (standard caps), skin effect
- Inhomogeneous impedance and internal resonances
- Negative electromagnetic interaction
- Winding geometry and metal profile
2 Advantages of MKP (metallized foil) technology (Thoma pag 95)
- Self healing ability: Long lifetime, High reliability
- Good ampacity
4 Advantages of using BOPP in MKP (metallized foil) technology (Thomas 95)
Includes effect of V, f, T on C and loss factor
- Low loss factor with low f and T dispersion
- stable C with low f dispersion and linear T dependence
- C and loss factor independent of V
- low leakage current
How does dielectric loss of PP films changes with T? (Thomas 111)
- PP films have low dielectric loss at T up to 85°C
- Above 85°C dielectric loss increases significantly (leakage, polarization)
Solution to the high dielectric loss of PP films at high T (draw the schematic)
- As heat conductivity of PP is low, another material (ceramic) layer is inserted in the middle for heat dissipation.
- This is known as multilayer stack metallized film cap
which factors (2 factors) do the transport mechanism (leakage current) depends on? (Thomas pag 115)
- Defect density
- Defect energy levels
Why is oil used in paper dielectric used for film caps? (2 reasons)
- It is used to reduce partial discharge
- it is impregnated in paper to fill air voids and increase dielectric constant
2 alternatives to replace oil to impregnate paper for film caps
- Use gas as it is more effective
- It’s also preferred over air sice it is composed of oxygen and nitrogen which are easy to ionize.
- Vacuum is another option but is difficult and expensive
Method to reduce parasitic inductance of capacitors
Includes 1 method for mutual cancelling of M fields
- Place the + and - terminals as close as possible in parallel configuration to cancel M fields
- Use multiple connections in parallel between the foils and terminal
Common applications of film caps
-EMI
-DC link
4 Main electrical properties of film caps
- Low capacitance
- Low ESR
-High ripple current - high V
- high power
List the 2 types of film caps
- Metail foil: dielectric and metal are not in direct contact
- Metallization on dielectric: metal and dielectric are in contact
3 list disadvantages of metallized film caps
- Resistance of metallized foil electrode is higher increasing loss factor
- Lower current carrying capability
- Loss of capacitance due to repetitive self-healing
3 disadvantages of metal foil film caps
- Bigger size
- Minimum two layers of film to increase reliability of high voltage caps
- No self-healing
Why does the roughness of dielectric film can affect dielectric breakdown?
Because during metallization the metal can fill voids in the dielectric and reduce breakdown voltage.
What is the disadvantage of using zinc in AC conditions regarding corrotion
We have circular corrosion, meaning that defects grow in all direction (circular).
Draw the schematic showing how we can internally create serial connections of caps
Draw on paper
Equation of lifetime estimation for film caps and explanation
- L = Lo*2^(deltaT/10)
Every 10°C rise of temperature halves the lifetime of the capacitor - This is due to the increasing electrochemicals reactions with T
3 types of plastic dielectrics materials used in film caps
Doesn’t matter if it is film or metallized
- dielectric: PET, PP, PEN
at least 6 Main characteristics of polypropylene film
Includes 1 from metallized capacitors
- low loss factor (good for AC applications)
- operation up to 100°C
- high stress capability (reducing C size)
- low dielectric constant
- low C variation (3%) over T range and frequency
- self healing capability of MPP (metallized capacitors, good for reliability in AC apps)
- good performance regarding harmonics and high frequency applications
At least 9 main characteristics of polyester film
3 compared to PP
- High loss factor (limitations in AC applications)
- higher dielectric constant compared to PP
- smaller size than PP caps
- cheaper than PP
- higher working T (up to 125°C)
- best suited for DC or low frequency applications.
- high leakage
- losses strongly depend on T and frequency
- large variation of C over T and frequency range
- values from 0.01uF to 10uF
Application where metallized PET film caps can replace electrolytic caps and 5 reasons
Motor start capacitors
- comparable size can be achieved
- low loss factor of film caps
- can withstand higher AC voltage
- smaller value cap can be used
- motor can be redesigned to allow much higher voltage on cap.
Effect of increasing frequency on ohmic and dielectric losses in metallized PP film caps
- Increase due to ohmic losses of lead wires and metallization deposited on the film
- dielectric losses are very small and do not change
draw C variation against T and frequency of PP and PET film caps (Deshpande pag 73)
draw on paper
How can we minimize corona and partial discharge effect in film caps (at least 1 solution)
- Avoid voids or spaces in capacitor winding which can trap air or moisture, since air/moisture have lower E field strength
Explain dielectric absorption effect
- Tendency of a cap to charge itself after it has been discharged, resulting in a V appearing across the cap
- This occurs because dipole energy is not discharged instantly (due to inertia) and dipoles get reoriented after removal of external discharge path.
Applications of PP film and PET film caps
- PP: AC applications (due to low losses), SMPS, precision and low tolerance applications (filtering, timing)
- PET: Coupling, decoupling and bypass.
2 Main characteristics of PPS film
Both related to T
- operation over -55°C to 150°C
- C variation less than 1% over T range
3 advantages of metallized film caps over paper/foil and film/foil caps (Deshpande pag 77)
- Longer lifetime (thanks to self-healing)
- Can withstand higher stress
- Reduced size
3 Important consideration of dielectric and film in metallized film caps to increase breakdown V
- (Dielectric) It has to be of very high purity to work under high voltage stress
- (Dielectric) free of weak spots and pinholes
- Free of dust particles which create weak or conducting points
Materials that compose electrode of metallized film caps and at least 1 reason
- Zinc aluminium alloy
- Al: high stability and cheap
- Zn: higher and more stable C,
- Zn: better healing properties
Parameter of metallization that must be controlled to avoid short circuit in metallized film cap
- thickness of metal deposit, if it fails to evaporate and isolate defective spots, permanent short circuits are formed
which material can be used to improve zinc aluminium alloy adherence?
Silver
Explain self healing phenomena (draw an schematic)
In the event of a weak spot on the film (dielectric) the energy discharge (current) through the weak spot vaporizes the surrounding metallization so that the weak spot gets isolated and C continues working.
Explain (draw) heavy edge metallization principle and advantages
- Usually metallization is uniform and Al is used, this being the cheapest option.
- In this case the edges are made thick to enable good contact with sprayed metal and to have better I density at I collecting areas.
- This allows to reduce thickness of active area and achieve more efficient evaporation of surface metal (better self healing).
Which factor is a measure of proper coating thickness and purity of metallization
Resistivity as it will increase with thinner coating and oxidation of metal surface by atmospheric effect or moisture.
Draw curves of films resistivity, capacitance, tan delta stability as a function of aluminiun content (Deshpande pag 81)
Draw on paper
Method to improve self-healing
- Segment the metallization into rectangular or diamond shaped patches connected by small restricted paths (fuses)
Explain how segmented films can improve self-healing?
In teh event of weak spot appearing in a section the fuses operate an isolates the entire section while the C continues working normally
Disadvantages of segmented fields regarding capacitance
Self-healing by this mechanism results in progressive loss of capacitance
Reason for using wave-cut films
- When Zn is sprayed on the ends of winding, it makes contact with metallized edge of film.
- But the contact only takes place at a series of points, and current is collected through these points.
- wave-cut films offer a better end contact with sprayed metal layer as sprayed metal enters the valleys of the film.
3 Advantages of using wave-cut films
- better end contact between sprayed zinc layer and metallized edge film
- reduced I density and thermal stress, so increased I capacity and reduced film thickness
- better distribution of film tension layers
Which type of capacitors use wave-cut metallized film
X2 caps
Method in terms of internal film cap structure to increase working voltage
Create a series combination of multiple caps in only one cap by using series metallized (multisection metallized) films
4 Advantages of series metallized (multisection) films
- Ease of construction
- More reliable connections (only extreme ends are metallized)
- more reliable and better life expectancy
- Breakdown V is doubled
6 advantages of metallized film caps compared to film/foil caps (Deshpande pag 87)
- very thin metallized film reduces capacitor volume
- self-healing
- due to self-healing property only one single-layer dielectric is needed
- absence of separate Al foil electrodes, which reduces size
- open-circuit mode failure (film is taken away from sprayed end connections)
- sprayed end connections result in these caps being non inductive
5 advantages of film/foil caps compare to metallized film caps (Deshpande pag 87)
- can sustain higher voltage fluctuations because of lower R of electrodes
- can handle higher surge I
- C value does not degrade with time
- Preferred for close tolerance small value caps
- better insulation R
- Lower dissipation factor than metallized caps
2 Applications of (multilayer polypropelene) MPP film caps (Dehspande pag 88)
Not EMI filters, or DC link
- distribution lines with moderate voltage and load variations
- capacitor banks
5 Situations where MPP caps are not recommended
- loads with widely fluctuating I
- locations with higher incidence of harmonics
- where explosions are not acceptable
- areas with high short circuit levels
- supply systems with wide voltage fluctuations
3 Functions and 1 property of the oil in impregnated plastic/paper film
- to fill up the air voids
- to cool the dielectric and contact area after self-healing discharge
- does not allow the film to melt
- should not interact chemically with film or sprayed metal
3 main types of plastic film caps based on metallization
- metallization: MKT, MKP, MKN
2 main types of plastic films caps based on plastic film
- metal foil: KT, KP
Applications where PPS films are used
Consider T and frequency dependance
- Used in timers, filters, high T applications.
- Used in high frequency coupling, decoupling, high dv/dt applications like SMPS and snubbers.
In which type of capacitors the dielectric absorption effect is stronger, explain? and give 1 solution
- Electrolytic and electrochemical due to high inertia of dipoles
- They need to be kept shorted for long time to reach 0V
Which dielectric material was replaced by PP film a reason?
- replaced paper on oil type caps, which have higher dissipation factors
