Combating capacitor corrosion for long life applications
As energy policies worldwide move to adopt more renewable sources, such as solar, and implement smart-grid infrastructures to maintain stability, there is an increasing demand for smart energy meters. In order to assure a long life expectancy it is necessary that their components, like film capacitors, are able to withstand harsh environmental conditions.
The EU alone is aiming to install 200 million smart meters to achieve its 2020 energy goals. Cost is a major factor that will determine uptake, and is guiding designers to implement economical circuitry, particularly for the power supply.
Low-cost capacitive power supplies are typically used in preference to a switched-mode circuit. A capacitive power supply takes advantage of the impedance of the capacitor inserted in series with the AC line, as shown in figure 1determining the maximum current that can be supplied to the load.
In capacitive power supply the capacitance (C1) must be stable during life of the device, as its value determines the impedance of the circuit, thus the voltage fed to the rest of the circuit.
Figure 1. Voltage-dropping capacitor in series with AC line in a low-cost capacitive power supply.
Metallised film capacitors are the preferred technology for connection “in series with the mains” as in a capacitive power supply (C1 in Figure 1) or “across the line” applications such as in EMI filtering (C1, C2 in Figure 2)
Figure 2: EMI filtering.
In particular, the self-healing capability of X2 safety-rated film capacitors enables the devices to operate safely and reliably even when exposed to voltage spikes, possibly present on the AC line.
The self-healing mechanism allow the X2 EMI film capacitor to withstand voltage spikes (up to 2.5 KV) without causing catastrophic breakdown. Successful self-healing is determined mainly by two factors: dielectric material and metallization. The dielectric must be able to withstand the extreme conditions of the clearing without forming conductive products so to maintain the high insulation resistance typical of film capacitors: in this regard polypropylene is the best dielectric due to its molecular structure.
Metallization layer in suppression capacitors must be extremely thin (30-50nm): during breakdown / clearing events such a thin metallization can vaporize and insulate the affected area, restoring insulation resistance between the armatures.
In standard X2 EMI film capacitors the presence of moisture combined with high temperature and AC voltage applied can result in corrosion of the thin metallic armatures. This phenomenon will affect the electrical characteristics of the capacitor, generally resulting in the reduction of capacitance (due to reduction of active area) and increase of dissipation factor (due to reduction of conductivity of the armatures).
In a capacitive power supply, capacitance loss can prevent the circuit delivering adequate current to the load. In an X or Y capacitor circuit, loss of capacitance impairs EMI attenuation thereby exposing other components to potentially damaging interference signals.
In an X or Y circuit, an increase in the capacitor’s dissipation factor may lead to overheating that can cause further damage to the capacitor and hasten system failure.
Therefore, the constructive solutions adopted on X2 capacitors have a significant influence over the ability to operate for extended periods in critical condition.
As demand continues to grow for X, Y and series-connected capacitors that are capable of withstanding extreme environmental conditions for extended periods of more than a decade in the field, there is demand for a more robust capacitor technology effective for use in equipment such as smart meters and solar generators
KEMET sought a solution to this challenge focussing on materials, in collaboration with academic research groups, and developed the new F862 series based on high-quality polypropylene film as dielectric and specific production process.
KEMET has introduced these new materials and processes to its proven X2 capacitor platforms to create new ultra-robust film capacitors such as the F862 series. These devices are able to deliver significantly greater lifetime compared to standard polypropylene or polyester film capacitors in conditions of high humidity. Whereas standard devices may have had lifetimes of only 2-3 years, lifetime ratings over 15 years are now achievable.
THB (Temperature-Humidity-Bias) testing exposes the capacitor to the three main factors that cause loss of capacitance and increased dissipation factor, and measures the effects on capacitance, dissipation factor and insulation resistance over time. THB test conditions of 85°C, 85% Relative Humidity and 240Vac operating voltage have confirmed a significant improvement in the stability of capacitance and dissipation factor, as shown in figure 3 and figure 4.
Figure 3. Improved capacitance stability with latest-generation materials and processes.
Figure 4. Slower increase in dissipation factor due to temperature, humidity and bias.
The use of polypropylene not only assures optimum self-healing, but also, due to the low and stable dissipation factor, significantly enhances tolerance of mains-frequency ripple and of the high-frequency ripple currents present when the capacitor is connected across the line at the input to a switching circuit such as an inverter.
The enhanced environmental performance and self-healing capacity of these devices also meet the requirements of the automotive industry for control of battery charging in plug-in hybrids and electric vehicles (PHEVs/EVs). The F862 series has been qualified to the component-level automotive quality specification AEC-Q200, in addition to ENEC, UL, cUL, and CQC safety-agency standards.
Advanced technologies such as the smart grid, renewable energy, industrial inverters and hybrid/electric vehicles hold the key to meeting important targets such as reducing energy consumption and greenhouse gas emissions. Low-cost solutions are needed, to encourage widespread adoption. Equally important, however, are demands for reliability and extended lifetime even when operating in harsh environmental conditions.
The latest metallised polypropylene-film F862 series capacitors incorporating advanced materials and process technologies could enable equipment such as smart meters, solar generators and automotive electronics to meet these conflicting requirements.
About the author:
Stefano Sartini is Director – Product Management Film Capacitors at KEMET Corporation – www.kemet.com