This article describes aluminum electrolytic capacitors types, features, characteristics and behaviour.
The primary strength of aluminum electrolytic capacitors is their ability to provide a large capacitance value in a small package, and do so for a relatively low cost.
Additionally, they tend to have good self-healing characteristics; when a localized weak spot in the aluminum oxide dielectric layer develops, the increased leakage current flow through the weak point in the dielectric causes a chemical reaction similar to that used during the initial formation of the dielectric layer, resulting in a thickening of the dielectric at the weak point, and a consequent reduction in leakage current.
The shortcomings of aluminum capacitors are mostly related to
- the chemically-reactive nature of the materials used in their construction
- the conductive properties of the electrolyte solutions
- the volatility of liquid electrolytes.
The chemically reactive nature of the materials used in aluminum capacitors is problematic on two points; the stability of the dielectric layer and the long-term mechanical integrity of the device. Since the aluminum oxide dielectric layer in these devices is formed through an electrochemical process, it can also be eroded by an electrochemical process simply by reversing the applied voltage. This is why most aluminum capacitors are polarized; application of voltage with the wrong polarity causes rapid erosion & thinning of the dielectric, resulting in high leakage current and excessive internal heating.
From a mechanical integrity standpoint, mixing a highly reactive metal (aluminum) with a corrosive electrolyte solution is a delicate proposition; errors in electrolyte composition can result in premature failure, as evidenced by the “capacitor plague” of the early 2000’s.
Another shortcoming of aluminum electrolytic capacitors is the fact that the electrolytes used aren’t particularly efficient conductors, because conduction in electrolyte solutions is achieved through ionic, rather than electronic conduction; instead of loose electrons moving between atoms serving as the charge carriers, ions (atoms or small groups thereof that have a charge due to a surplus or deficit of electrons) are moving about through the solution. Since ions are more bulky than electrons, they don’t move as easily and hence ionic conduction generally tends to be a higher-resistance proposition than electronic conduction. The extent to which this is the case is influenced significantly by temperature; the lower the temperature, the more difficult it is for ions in an electrolyte solution to move about through the solution, which translates into a higher resistance. Thus, electrolytic capacitors tend to have a relatively high ESR that exhibits a strong inverse correlation with temperature.
The third major downside to aluminum capacitors (with the exception of the solid polymer types) is that the liquid electrolyte solutions tend to evaporate over time, eventually being lost to the atmosphere by diffusion through the rubber sealing plug, leaks in safety vent structures, or similar phenomena.
There are more types of aluminum electrolytic capacitors construction and termination styles:
- SMDs (V-chip) for surface mounting on printed circuit boards or substrates
- Radial lead terminals (single ended) for vertical mounting on printed circuit boards
- Axial lead terminals for horizontal through hole mounting on printed circuit boards
- Radial pin terminals (snap-in) for power applications
- Press-fit terminals
- Large screw terminals for power applications
The most common styles are wound foil capacitors packaged in aluminum can as leaded or SMD termination styles. See Figure 1. and 2.
Electrolyte can be wet, gel (TCNQ salt), solid (conductive polymer) or hybrid (combining wet and conductive polymer) based:
Wet Liquid Types
- 4 ~ > 500V
- high ESR
- poor temp performance
- dry out, cap decrease with life
- low cost
Solid Conductive Polymer
- 2.5 ~ 100V
- low ESR, high ripple current
- stable high and low temperatures
- leakage current stability issues
- higher sensitivity to humidity
- higher cost
Hybrid Wet + Polymer
- Up to 125V
- similar ESR as polymer capacitor
- more stable then liquid type
- better leakage current stability vs polymer cap
- higher cost comparing to wet
Panasonic, one of supplier of all aluminum and tantalum polymer capacitor technologies provide comparison of its technologies as follows:
- OS-CON is a TCNQ salt electrolyte
- Hybrid combine wet and polymer electrolytes
- SP-Cap is a solid polymer chip capacitor
- POSCAP is a tantalum polymer capacitor
The following chart on Figure 3. is demonstrating lifetime with temperature comparison of wet vs polymer vs hybrid aluminum capacitors that can be helpful for a specific application selection guide.
DCL and balancing
Aluminum electrolytic capacitors leakage current and balancing is explained in more details in a paper below:
DCL of Aluminum Electrolytic Capacitors – by Dr. Arne Albertsen from Jianghai Europe Electronic Components GmbH
The production process starts with mother rolls. First, the etched, roughened and pre-formed anode foil on the mother roll as well as the spacer paper and the cathode foil are cut to the required width.
The foils are fed to an automatic winder, which makes a wound section in a consecutive operation involving three sequential steps: terminal welding, winding, and length cutting. In the next production step the wound section fixed at the lead out terminals is soaked with electrolyte under vacuum impregnation.
The impregnated winding is then built into an aluminum case, provided with a rubber sealing disc, and mechanically tightly sealed by curling. Thereafter, the capacitor is provided with an insulating shrink sleeve film. This optically ready capacitor is then contacted at rated voltage in a high temperature post-forming device for healing all the dielectric defects resulting from the cutting and winding procedure.
After post-forming, a 100% final measurement of capacitance, leakage current, and impedance takes place. Taping closes the manufacturing process; the capacitors are ready for delivery.