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Guard against capacitor corrosion for long life apps

Posted: 28 Jul 2015 ?? ?Print Version ?Bookmark and Share

Keywords:smart energy meters? power supplies? capacitance? film capacitors? EMI filtering?

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 metallisation. 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.

Metallisation layer in suppression capacitors must be extremely thin (30-50nm): during breakdown / clearing events such a thin metallisation can vaporise 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).

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