Electronic component miniaturization and operation in harsh environmental conditions are growing trends in applications, such as on-board chargers, energy meters, capacitive power supplies, including connection in series with the mains, motor drives, wind and solar inverters. Current EMI (Electromagnetic Interference) X2 class suppression and DC-link power box film capacitors need capability improvement to meet these requirements.
Very high capacitance and dissipation factor stability are required during operational life in severe ambient conditions such as high temperature and relative humidity, while still meeting European and other Electrical Norms (ENEC and CQC), the criteria in the standard for automotive application (AEC-Q200) and the international safety requirement (UL). The moisture absorbed into the capacitor leads to corrosion of the electrode and accelerated degradation of the capacitor by increasing of capacitance loss. Temperature-Humidity-Bias (THB) is a standard test for accelerated stress testing of corrosion and other moisture-driven mechanisms for degradation. In this paper, we have studied the characteristics and performance under high temperature and humidity conditions of new capacitor designs in a miniaturized version of first to the market metallized EMI X2 class suppression and DC-link power box film capacitors. Three advanced KEMET series of metallized film capacitors have been stressed under an applied rated AC or DC voltage at 85°C and 85 %R.H. and the drop of capacitance and change of the dissipation factor have been monitored with the time for 500 and 1000 hours, respectively.
MINIATURIZATION CHALLENGES. FAILURE CAUSES AND FAILURE MODE MECHANISMS
In the latest film technology developments, manufacturers of film capacitors are trying to achieve excellent protection of film elements by utilizing new advanced humidity protective coatings, metallization treatments and improving the overall capacitor manufacturing processes. In this way, products could withstand severe operating conditions that would otherwise lower their reliability and performance. However, enhancing the reliability levels under high temperature, humidity, and bias (THB) conditions in miniaturized capacitors, for both DC and AC applications, can be particularly challenging [1-7].
Self-healing phenomena The self-healing property of metallized film dielectrics is the ability to recover from an internal drop of an insulation resistance. This property ensures a safe failure mode in AC frequency applications where electrical noise and peak voltages are added repeatedly or occasionally to the fundamental signal. So, when the self-healing operates the temporary break down which results in a clearing of a small area causing a minor loss of capacitance and a restoration of the capacitors’ initial electrical properties. In the metallized film capacitors, the metallized area is very thin and in case of dielectric break down the energy released by the arc discharge in the break down channel is sufficient to totally evaporate the thin metal coating close to the channel which results in a restoration of insulation and a small capacitance drop which can be between 1÷2 %. This property makes film capacitors highly suitable technology for safety applications.
Table 2 presents a comparison of different film dielectrics properties like Polypropylene (PP), Polyethylene Terephthalate (PET), Polyphenylene Sulfide (PPS) and Polyethylene Naphtholate (PEN) highlighting the main advantages, disadvantages and characteristics of each of these dielectrics and showing the suitability for different applications.
As this article concerns mainly applications with suppression and DC-link power box capacitors, it can be highlighted that the polypropylene is the most suitable dielectric due to its internal structure and its excellent self-healing property, shown in Figure 2.
Metallized polypropylene film technology is currently the main solution for EMI suppression and DC-link capacitors due to its excellent high voltage per micron and ultra-low, stable dissipation factor capabilities. Perhaps most importantly, it also has the best self-healing properties compared to other film dielectric technologies. However, combining high temperature and humidity conditions often has a drastic effect on the metallized polypropylene material when an AC or DC voltage is applied, resulting in accelerated degradation and potentially catastrophic failures of the capacitors. The root causes for this failure mode can be due to atmospheric corrosion or oxidation, corona effect and electrochemical corrosion of the metallization.
Mechanism of Atmospheric Corrosion or Oxidation Phenomenon An oxidation or atmospheric corrosion of metal electrode alloys is mainly caused by the chemical destruction of metals due to the presence of H2O, O2 and corrosive media. Taking zinc electrode as an example, zinc reacts with oxygen and moisture in the environment at a very fast rate to form hydroxides and oxides. The corrosion reaction of zinc can be expressed by the following chemical reactions:
Atmospheric corrosion is often related to the ingress of atmospheric moisture and requires the presence of water and usually progresses from the ends of the capacitor cylinder in an axial direction toward the middle. Figure 3 shows the conditions of two metallized films before and after the oxidation phenomenon.
The converted metal in its oxidized state is not enable electronically conducting, leading to loss of capacitance in the areas affected. This process is active, without the voltage being applied, but its magnitude in encapsulated capacitors is most often negligible as the reaction rate is limited by diffusion of oxygen into the part and is strongly limited by the compact film winding and the thermosetting resin encapsulation.
Partial discharges or corona effect phenomenon The term partial discharge is used to describe a localized dielectric breakdown of a small portion of a solid or fluid electrical insulation system under high voltage stress that does not completely bridge the gap between the electrodes, for example discharges occurring within solid insulation systems with gaseous voids. In film capacitors, there are small air gaps inside the film windings at which partial discharge may easily occur under high electric field and it can lead to dielectric degradation. The discharge can take place in the air gap in series to the film layer or in the air surrounding the active layers. Sometimes called ‘corona,’ this is the breakdown of micro-voids in the bulk of the dielectric material or air gaps between insulating layers. The effect is to insert a ‘partial’ short circuit into the insulation, effectively shortening the insulating path and locally reducing the breakdown threshold voltage. Each short places an extra stress on the remaining insulation, and as they accumulate over time, a tipping point is reached, and total breakdown occurs.
The film capacitors operating at mains voltage can suffer progressive loss of capacitance as corona discharges cause local vaporization of the metallization. The observed loss of capacitance is caused by ionization / corona, i.e. the air enclosed by the coil becomes ionized and this makes it more conductive. This means in turn that partial discharges on the metallized film surface can occur, causing damage to the metallized layer. The potential difference between two points on the film surface creates a small arc which vaporizes the metallization over a small area. There is a small capacitance loss, which after many such events becomes measurable. The behavior with respect to ionization is affected by ambient climatic conditions such as humidity and temperature. The potential difference decreases with temperature because the air pressure is lower at low temperature, therefore making it more likely to have ions or electrons oriented in the field direction for the discharge mechanism. The potential difference decreases with the humidity level because the dielectric strength of humid air is lower than dry air. This phenomenon can be limited using a minimum dielectric thickness and protecting the film internal element from humidity, with good enclosure materials as box and resin.
Due to geometrical factors, in single-metallized film construction, the corona effect usually starts very near to the armatures edge, in the free margin area. This is due to the fact that, in that zone, the electric field magnitude increases due to the tip effect. In Figure 4, two films where the corona effect has occurred are shown. During the corona effect, high temperatures are reached in proximity of the electrodes and the metallization is vaporized. Therefore, the active area is progressively reduced in size with a corresponding capacitance drop. It is easily observable that the two films start de-metallizing from the metallization edge, close to the free margin, on both sides. If the voltage is kept higher than partial discharge extinction voltage, the erosion will continue until the electrodes are eroded so that they are not overlapped anymore.
Mechanism of Electrochemical Corrosion Electrochemical corrosion is an electrochemical reaction at the interface between metal and electrolyte. The reaction is a combination of an anode reaction and a cathode reaction forming circuit by ionic flow in electrolyte and electron flow in metal. Taking aluminum electrode as an example, the corrosion reaction of aluminum can be expressed by:
Aluminum or zinc aluminum alloys with an oxide layer on the surface may suffer electrochemical corrosion. Especially when the surface of the oxide layer has defects such as scratches, cracks, impurities or alloy phases, intergranular precipitation, local corrosion is more likely to be induced. The local corrosion is likely to expose the base metal, because the metal is in the active state (constitutes the anode), and the undamaged oxide layer is in the passive state (constitutes the cathode), thus forming an active-passive corrosion battery, so that the corrosion can be produced.
In the area of the electrochemical reaction, there will always be a certain amount of moisture in the polypropylene, and the moisture in the external environment will also penetrate inside of the capacitor. Since the electrode potential of aluminum is very low (the standard electrode potential of aluminum is -1.662 V), the free energy of forming Al2O3 is negative. Therefore, a certain amount of alumina must exist at the interface between polypropylene and aluminum. If the gap between the alumina and film layers is small enough, the water will form an electrolyte, connecting the alumina to the polypropylene and then forming an electrical double layer at the Al2O3/electrolyte and electrolyte/polypropylene interface.
In addition to the reduction of the capacitance, the corrosion of the metal electrode also increases the equivalent series resistance (ESR) of the capacitor and increases the dielectric loss. The main cause of the deterioration of the metallized film capacitor in the AC circuit is the decrease of the capacitance caused by electrochemical corrosion. The electrochemical corrosion rate of metallized film capacitor is affected by factors such as operating voltage, operating temperature, ambient humidity and electrode material.