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.
A well-accepted accelerated life test standard for active and passive components in the electronics industry is the Temperature-Humidity-Bias (THB) test, with levels of 85°C and 85% relative humidity under AC or DC bias conditions. For many years, designers in various industries (including automotive, energy, consumer, and industrial) have used this test to ascertain the reliability of their final products for up to 25 years of operation under severe climatic conditions. More recently, the THB test has been recognized as an IEC (International Electrotechnical Commission) standard for EMI suppression film capacitors.
The suppression capacitor is usually a metallized film capacitor, defined as a safety capacitor that meet the requirements of IEC 60384-14. This standard incorporates seven groups of tests that cover standard capacitor properties: resistance to heat, vibration, mechanical shock and solvents, damp heat performance, impulse voltage endurance, charging and discharging properties, RF characteristics and passive/active flammability tests. THB test or Temperature, Humidity Bias test is a reliability test designed to accelerate the degradation process of a safety capacitor and measure the electrical parameters after a certain period of time if they are still within the specific limits which are defined as three different biased humidity grades I (A and B), II (A and B) and III (A and B) in the latest amendment AMD1:2016 of the standard IEC 60384-14, table 3, starting from mild up to harsh to ensure that the capacitors can meet the demanding harsh environmental and can continue to function during the whole product lifetime without having any issues. The acceptance criteria are listed in the table 4.
Designers needing to ensure that their products pass the THB evaluation and emission certification have encountered several challenges; for instance, it can be difficult to obtain the required technology and include multiple EMI suppression and DC-link capacitors into already component-dense circuits. There are also higher power requirements within a limited board space to take into consideration. The THB test ensures that the capacitance and dissipation factor are highly stable during the expected lifetime of the capacitors.
After going through the basic technical overview and requirements regarding the EMI suppression capacitors the next topic will be on the products in the KEMET portfolio. In Table 5 are listed the different types of X and Y capacitors available by the input voltage, temperature, THB grade and end applications. In X1 class there are mainly R49, R47 and PHE845 series defined for different input AC voltages: 330, 440 and 760 Vac respectively. Similarly, in Y2 class there are R41 and R41T series being the highest temperature series with capability to withstand also higher DC voltages. In X2 class there are different series R46 with temperature capability up to 125°C and the R46P which is the miniaturized version of R46 suitable for commercial applications. The F862 V054 and F863 series are suitable for harsh environmental conditions and for automotive applications.
MINIATURIZATION CHALLENGES. DESIGN DETAILS
The EMI X2 capacitors are generally mounted in parallel with the mains, with the purpose of filtering electromagnetic interference, both from the mains and the equipment, and of protecting the appliance from voltage spikes. In this application, usually, the customers use capacitors with large nominal tolerance (+/-20%). In recent years high capacitance stability and low tolerance have become a key feature for X2 capacitors, due to the development of more and more applications in series to the mains, in which the capacitor itself has the role of feeding AC voltage to the circuit, via a capacitive divider. Besides the peak voltage withstanding capacitors have to be qualified, when used in series to the mains, for a high level of capacitance stability in order to feed the right AC voltage and power to the circuit through time. For these applications, besides the typical requirements for X2, the main challenge is the temperature-humidity-bias performances, considering different grades presented in the previous section.
The requirement of high capacitance stability should be considered during the design of these RFI capacitors, for example in the selection of film base, metallization resistance and all materials that give an enclosure of the internal film element. All these materials usually have special features that increase the cost of the capacitors. During the selection of suppressors, designers should require the real-application-requirement level of humidity withstanding capability, in order to find the right capacitor at the proper cost.
Metallized film capacitors, designed for harsh environmental conditions, are usually sealed with epoxy resin. The moisture ingresses to the interior of the capacitor through the sealing material, thus accelerating electrode corrosion . As the water is continuously consumed during the chemical or electrochemical reaction, the reaction stops when the water is depleted. Therefore, the speed of external moisture ingress to the capacitor will significantly affect the corrosion rate of the electrode. The electrochemical reaction reduces the internal moisture concentration of the capacitor, so the external moisture will continue to diffuse into the capacitor through the epoxy and the gap.
In general, the film base material should have good self-healing capability, but also show a good behavior under tough humidity conditions: this means that some film base materials are not suitable at all for these applications. The metallization resistance selection has an important role in the right compromise between safety aspects and capacitance stability, like shown in figure 6. As previously stated, the higher the resistance value, the higher the performance on peak voltage withstanding, but the lower the capacitance stability in humidity conditions. Increasing the resistance, without modifying the morphological structure of the metal layer, means reducing the thickness of the metal that could be oxidized during the electrochemical corrosion phenomenon.
In the latest EMI suppression and DC-link power box film technology developments, KEMET is achieving excellent protection of film elements by utilizing new materials and improving the capacitor manufacturing processes. In this way, several products (see Table 5) can 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 can be particularly challenging.
Some of the constraints of DC-link power box and EMI suppression capacitors correlate to the film quality and protection surrounding it. The amount and type of resin used, the epoxy filling the surrounding of the capacitor element, and the material and thickness of the radial box encapsulating them all play vital roles in a product’s reliability. Moreover, there is a mechanical challenge in manufacturing capacitors with smaller capacitance values; lower capacitances require less film and metallization material, making the product more susceptible to damage due to humidity.