Aluminium electrolytic capacitors Flashcards
State-of-the-art capacitor technology for DC link use
Al electrolytic caps
Characterize Al-El cap in terms of energy density, storing time, and power density
- Medium energy density (0.01-1 Wh/kg)
- Medium power density (10-100kW/kg)
- Short storing time (360us-36ms)
Compare Al-EL caps with EDLC, batteries and power caps in terms of energy density, and power density
- Higher power density than EDLC and batteries
- Lower power density than power caps
- Higher energy density than power caps
- Lower energy density than EDLC and batteries
at least 5/9 types of Al-El caps in terms of terminals
- Screw terminal
- Snap in
- Axial
- Solder lug
- Photoflash
- Central mounting
- PCB pins
- Single ended
- SMD
Main components of Al El caps (construction)
- Aluminium can
- Electrolyte: solid, liquid, paste
- Dielectric: oxide layer film on anode
- Paper/spacer
- Anode Al foil, etched, covered with Al oxide
- Cathode Al foil etched
- Rubber seal
- Terminal tabs
- Pressure vent
- Leads
How does ESR changes with frequency in an Al El cap?
- it decreases with the frequency (why?)
T dependence of ESR and C of Al El caps
- C increases with T, until it reaches an almost constant value (why?)
- ESR sharply decreases with T in the range -60°C to 0°C (why?)
Draw the impedance curve of Al El cap (compare with MLCC) and describe main characteristics
- Draw on paper
- Impedance decreases with 20dB/decade up to given f (i.e. 100kHz)
- Lower part of the curve is flatter compared to the MLCC curve due to higher ESR of Al-El cap
- For high f impedance keeps increasing due to parasitic L
- Resonance is less marked compared to MLCC and effectiveness of Al El cap has reduced f range.
List the 4 phases of etching (Thomas)
- Etching start - pit distribution - pit density
- Tunnel growth
- Tunnel length/”dead” of growing process
- Tunnel enlargement
What is pitting potential?
- Electrochemical potential in an environment above which a corrosion pit initiates on a metallic surface
- least positive potential at which the metal salt of the aggressive ion in solution is in equilibrium with the metal oxide.
Which chemical process is used to etch the Al anode foil of Al El caps
Corrosion
Why oxide layer is not added to the top of cathode foil
- Only high conductivity is needed
- Some defects are allowed (low quality Al)
At least 7 requirements of electrolyte for Al El caps
- High forming capability to heal defects on dielectric oxide
- Chemically stable
- High conductivity
- High sparkling voltage
- Compatibility with relevant materials
- High thermal stability
- High viscosity
- High impregnation capability
- Low toxicity
- Low crystallization ability
Explain self-healing in Al EL caps
- Oxide is regrowth by applying small voltages that increase slowly
Why does high voltage Al El caps are not practical?
- Tunnels cannot go deep enough into the thicker foil
6 main requirements of paper used in Al El caps
- High electrolyte storage capability
- High impregnation capability to permit maximum retention of electrolyte
- Compatibility with all relevant materials
- High breakdown strength
- Thermal stability
- Low toxicity
List the fabrication steps of Al El caps
- Foil, paper and tab slicing
- winding
- Impregnation
- assembling of winding
- curling/closing can of cap
- Ageing (low voltage is applied for self-healing)
- Test and packing
List 6 applications of Al El caps
- DC-link
- Energy-storage
- Filtering, PFC
- Motor start (bipolar construction, short time AC)
- When used for snubber, high ESR can be used to damp oscillations
- widely used in automotive applications
Which type of cap can be used for medium/high power DC link applications instead of Al El caps?
Metallized film caps
Solution for using Al El caps for high voltage applications and disadvantage
- Caps are connected in series
- Voltage balancing issue
What is a disadvantage of SMD standard electrolyte, polymer and hybrid polymer caps?
They are only available for low voltages
Main parameters (4) of Al El caps that need improvement in automotive industry
- Higher C values
- higher ripple I
- wider T range
- wide V range
Failure modes of Al El caps
- Valve opening (inner pressure is good for keeping impurities outside of cap, but can lead to valve opening)
- Electrolyte leakage
- Electrolyte loss - dry out (electrolyte tends to evaporate over time)
- Breakdown (short circuit and open circuit)
- Capacitance loss
- ESR increase
- Leakage I increase
Why is ESR rise more critical than C loss?
- ESR leads to increased T.
- Increasing T usually leads to higher R
- It can result in positive feedback and thermal runaway
Explain a process by which electrolyte can evaporate
It can diffuse through the rubber and terminal connections if there is a bad sealing
6 future challenges of Al El caps
- Higher life-time requirements (8000 to 30000h)
- Higher switching frequency requirements due to GaN and SiC
- Higher T requirements
- Higher reliability
- Lower cost
- HIgh quality of materials
2 Advantages of electrolyte PEDOT compared to conventional electrolyte
- PEDOT:PSS has higher conductivity
- PEDOT has high T stability
Explain frequency dependence of ESR of electrolytic caps, compare PEDOT:PSS (solid) with commercial HV liquid electrolyte (Thomas 123)
- ESR decreases with frequency
- For high frequency ESR of liquid electrolyte caps is almost constant
- For high frequency ESR of PEDOT:PSS caps continues decreasing
Alternative to Al El caps that uses Al, 3 advantages, and application
Al polymer caps
- High reliability
- Long life
- Low ESR and ESL
- Application: battery charger
Write breakdown voltage equation for a Al El cap and explain terms
Vb = aB+bB*log(p)
- p: electrolyte resistivity
- aB, bB: constants for given metal and electrolyte composition
1 Parameter that determines the electrolyte conductivity (Thomas 134)
- Density of anions in the bulk electrolyte
Does an Al El cap behave as a diode? If yes explain
- Yes it behaves as a diode allowing current to pass in only one direction
- Blocking V is small about 2V, when reverse V is applied the “diode” breaks and C is destroyed
Why does capacitance of Al El cap decreases as T decreases?
- Because electrolyte starts freezing and stops conducting I (distance between plates increases)
Why does the C drops with frequency in an Al El cap?
Due to the incomplete change of polarization of dipoles, leading to less stored energy.
Explain the process by which oxide layer is created in the anode Al foil
- Anode Al foil is sank into a corrosive solution like boric acid
- Solution is contained in a metallic container
- Voltage is applied between the anode and the container. Anode is connected to + and container to -
- This creates a chemical reaction in which H2O molecules are separated into H and OH (hydroxide ioins)
- Then OH combines with Al to create the oxide layer or Al2O3
- It’s important to control the purity of Al as any impurity like copper will cause breakdown
Is the C value of an Al El cap constant with time? (Class notes)
- No, it decreases with time
- When about 8% of C is lost, the slope of C drop becomes higher (C drops faster)
5 Main advantages of AL El caps
- High C value and small size compared to other types
- Cheap
- V range: 10-800V
- High-energy storage
- Self-healing
Main disadvantages of Al El caps
- High ESR
- Low ripple current
- Electrolyte vaporization
- High loss factor
- Bad frequency and T characteristics (not good for high frequency)
- Resistive losses from high electrolyte resistivity
- Finite capacitor life
- Polar
7 applications of electrolytic caps
- DC power supplies
- Batteries
- Power filters
- LF filters
- Coupling
- Energy storage
- Input caps for power inverters
5/6 Types of electrolytic caps
- Wet Al El caps
- AC Al El caps
- Dry Al El caps
- Wet tantalum caps
- Dry tantalum caps
- Tantalum polymer caps
Dielectric constants of Al oxide, tantalum pentoxide, and niobium oxide in electrolytic caps
They are ordered
- Al oxide: 8.5
- Tantalum pentoxide: 27
- Niobium oxide: 41
What do we need to consider regarding storing of Al El caps?
- Oxide insulating layer will tend to deteriorate in the absence of rejuvenating voltage
- cap will lose ability to withstand V if the V is not applied for long time
- Cap can be reformed by applying V through a R until it is charged to full V
4 Reasons of high C of Al El caps
- High dielectric constant
- High breakdown field strength
- Rough surface
- Extremely small, uniform thickness of anodically formed metallic oxide
Reason by which El caps have uniform dielectric stress and can operate at high E fields
- Anodization (formation) is performed on electrode at fixed V, and dielectric oxide layer grows all over the area to the thickness required to support that V
- C electrolyte continues healing work of the original forming electrolyte on electrode foil, repairing and thickening the dielectric locally as required
Which factor drives the healing process in Al El caps
- DC leakage I, drawn whenever DC voltage is applied
Explain dielectric formation
- It is produced from anode metal itself (forming process)
- Current flows from anode metal (Al, tantalum, or niobium) through conductive bath to the bath cathode.
- Current causes insulating metal to grow out of and into the surface of the anode.
- Applied potential must be above the oxide breakdown V before significant I flows
Which factors determine dielectric strength of oxide layer?
- Thickness and composition
2 Reasons for using Al in El caps
Apart from good conductivity
- flexibility in use, etching, electrode formation, winding
- cost
How does a El cap act when reverse voltage is applied?
Acts as a short circuit
How AC electrolytic caps are formed?
- Built with two electrolytic caps connected in series back to back.
- One cap works in any given polarity while the other is a short circuit
2 Advantages of Al El caps compared to tantalum
- Al El caps rated voltage is higher
- Lower cost per unit
2 Types of Al El cap and features
Based on electrode (foil) structure
- Plain foil type: higher AC current capacity
- Etched foil type: anode and cathode foils chemically etched to increase surface area and permittivity (higher C value), high tolerance (20%)
Disadvantage of using paper/separator
- add to the loss factor as it restricts free flow of I through electrolyte
1 disadvantages of dry El caps
Not breakdown voltage nor leakage I
- Electrolyte (i.e. manganese oxide) tend to have lower volumetric efficiency
Basic fabrication process of Al El caps (3 steps) (Deshpande 170)
- Anode and cathode foils are interleaved with electrolytic paper separator and wound into cylindrical shape
- The foils are impregnated with electrolyte
- No capacitance exists and anode and cathode are electrically connected until V is applied and Al oxide layer is formed. Actually, controlled I is passed through the foil in stages, until V on foil reaches forming V (above working V)
- Aluminium cans act as the second electrode in DC.
Advantage of electrolytic caps compared to most film caps in terms of dielectric constant
- About three times larger for Al El caps
3 Main properties to characterize anode foil
- thickness
- forming voltage
- C density (uF/unit area)
Which applications need larger foil thickness and lower gain? and 3 properties
- High current applications (i.e. motor start caps)
- better performance
- longer AC voltage withstand time
- lower power factor
5 Characteristics of cathode aluminium foil
- Thinner than anode
- Shows much higher C than anode, as cathode C appears in series with the anode C to yield total C (very low formation V is needed)
- Etched, but not formed, although there is a thin layer of hydrous oxide due to exposure to air
- better frequency response than anode
- cathode’s voltage capability is only about 1V
What is the effect (2 effects) of applying reverse V for few seconds?
- I drawn during reverse V can reach hundreds of amperes
- cause significant heating of electrolyte and anode oxide
Why is more than one tab connected to each plate of Al El cap?
- Tabs connect the conductive plates and connection terminals in the winding
- To keep L and R low
Which factors are affected by electrolyte-spacer combination?
- ESR (power losses, heat, ripple I, loss factor)
- Cap frequency response
- Breakdown V
5 main functions of electrolyte
- Generate oxide layer
- Maintain layer in service
- Heal the depletion of oxide layer
- keep desired properties of dielectric intact
- conduct electrical I from electrodes with low R
Problems of too tight or too loose seal
- too tight causes pressure build up, causing explosion
- too loose shortens the life by permitting drying out of electrolyte
Explain ageing process during manufacturing of Al El caps
- Caps are dipped in electrolyte bath
- Voltage over maximum rated V, but below anode formation V is applied, at maximum rated T
- ageing grows oxide on areas on the anode foil, repairing any oxide film which may have been damaged during slitting, winding, and assembly processes, reducing leakage I to an acceptable level
Alternative to Al El caps for motor start capacitors and 7 advantages
- Metallized film caps
- Very thin film, so size is comparable to electrolytic caps
- Comparable cost
- Impeccable reliability
- Lower loss factor
- Withstand AC voltage for longer time
- Lower tolerances (5%)
- Self-healing
