Self healing capacitors fix themselves

James Lewis

Thursday, September 21st, 2017

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When I first learned about self healing capacitors, I imagined a superhero power. Of course, a capacitor that self-heals does not have magic powers. Instead, materials that allow for self healing capacitors repair dielectric fault sites. Another name for this process is called “proofing.” The simplest way to look at this process is to consider a leaky dielectric.

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When (relatively) high current flows through a leakage site, heat is generated causing the material set to change. The change results in a healed or proofed site. Depending on the dielectric and the electrode materials used, the results may be similar, but the process can differ.

Self healing capacitor types

The ability to self heal depends on the capacitor’s dielectric and electrode system. Capacitor types like ceramic or EDLC do not have a self healing mechanism. Fortuantly they are relatively robust as-is. This post looks self healing capacitors using aluminum, film, and tantalum materials.

Aluminum capacitors

There are two major types of aluminum capacitors: polymer and wet. What we call “wet aluminum” capacitors are traditional aluminum electrolytics. These use an electrolyte to connect the cathode of the capacitor element inside of the capacitor to the negative (or cathode) terminal.

One helpful feature of the electrolyte is that it can provide oxygen to regrow or heal weak areas in the dielectric layer. This effect happens when the electrolyte comes in contact with the aluminum through the damaged dielectric. A reaction occurs enabling the dielectric to rebuild. This healing process makes the capacitor more robust. However, eventually, the electrolyte dries up. This process limits the operational lifetime of the wet aluminum electrolytic capacitor.

What about polymers?

Polymer-based aluminum capacitors sometimes, called “organic aluminum” or “organic polymers,” use a solid polymer material for their cathode connection. The material is not an electrolyte, and it does not dry up in the same way as the wet counterpart. Instead of allowing the dielectric layer to heal, the polymer material will transition to a non-conductive site when a fault in the dielectric causes localized heating. This proofing is sometimes called self-healing, even though the fault still exists in the dielectric layer.

Even though there is no dielectric to dry up, polymer aluminum capacitors can have a limited lifetime. As humidity gets absorbed by the polymer, more areas of the cathode become non-conductive. Eventually, the ESR will become so high that the capacitor is effectively an open.

Both aluminum capacitor types reach end-of-life through a parametric failure.

Film capacitors

Film capacitors have a thin polymer layer with metal electrodes formed either with a thin foil sheet or by spraying (well, vaporizing) the film with metalized metal. The film-foil style capacitors are not able to self-heal. While the metalized film type, which is far more common, tend to have good healing capabilities.

While I lump film capacitors into a single group, there are different dielectrics based on several film materials. For example, you might see polypropylene (PP), polyester (PET/PEN), metalized paper (MP), or polyphenylene sulfide (PPS). These dielectrics all have self-healing capabilities, the amount they self-heal does vary.

When a film capacitor self-heals, a spot in the dielectric allows an excessive amount of current to flow. This extreme leakage generates localized heating causing the film material to melt. As it melts, pieces of the electrode break up, breaking the path for current flow. The melted metal cools, leaving a non-conductive site. One way to consider the film healing process is like a small fuse blowing.

Film’s self-healing process is what makes film one of the most robust capacitor types. It is challenging for a film capacitor to fail short. As the capacitor fails, it turns itself into an open. This capability is one reason film capacitors are a popular choice as X/Y Safety Capacitors in an RFI or EMI filter.

Tantalum capacitors

There are two types of solid tantalum capacitors. Their cathode system defines their names. They are called “MnO2” and “polymer.” Often, you will hear MnO2 referred to as “solid,” but both are solid capacitors. (A third type of tantalum, wet tantalum, is made using a wet electrolyte. KEMET does not sell these types of capacitors.)

Check out previous articles discussing the difference between MnO2 and polymer capacitors.

If there is a fault site in the dielectric, the current flowing through causes localized heating. The cathode material will transition to a non-conductive state. This action heals or proofs the fault site. The next time the same voltage is applied, the capacitor will be more robust.

In MnO2-based tantalum capacitors, the MnO2 cathode transitions to Mn2O3 in the presence of the localized heating. Mn2O3 is orders of magnitude more non-conductive compared to MnO2. With polymer capacitors, the cathode material breaks down creating non-conductive sites.

Conclusion

Understanding self healing capacitors can give you insight into the lifetime and reliability of different capacitor types. Factors that may influence the ability to self heal include humidity, temperature, and applied voltage. If you have questions about how a capacitor will react in your environment, get in touch with a KEMET Field Application Engineer through the chat