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Classifield MLCCs for confidence in AC circuits

The result is the availability of a range of capacitors for AC applications, with a consistent and predictable response, allowing designers to make a more informed selection decision.
Traditionally, for AC use, surface mount surge and safety capacitors with an AC rating of 250Vrms have been available but the capacitance range is limited. The typical maximum capacitance value available for a Y2 or Y2/X1 safety certified MLCC is 4.7nF. This is a result of the strict impulse and voltage proof requirements of the international (IEC/EN) standards. When it comes to usage under AC conditions, the characterisation of capacitor dielectrics has simply never been required, with no subsequent onus on the dielectric or capacitor manufacturers to define the behaviour of their materials and components. As a result, circuit designers have no real reference for assessing what performance they can expect from these parts when applied in real world AC applications.
Yet, in non-safety critical AC applications, extended capacitance ranges are often required. Applications include power supplies, AC/DC converters and other power circuits where there is no danger of shock, surges or lightning strikes, or in designs that are intentionally overrated and will rarely operate at AC voltage levels. To date, the solution has been the largely intuitive use of de-rated DC capacitors. Syfer Technology has now developed a range of MLCCs which can be used confidently at up to 250VAC 60Hz continuous use, with extended capacitance ranges up to 120nF.
It is important that engineers understand the behaviour of these components when in use under operational conditions. The way that a ceramic capacitor reacts to applied voltage is dependent not only on the dielectric material but also on how that dielectric material is used. Factors such as self heating, capacitance change and the piezoelectric effect, should all be considered.
Standard multilayer ceramic capacitors are normally supplied with a DC rating only. The behaviour of dielectrics for use under DC bias is well defined and understood, and capacitor manufacturers are experienced in controlling it to best effect. This was Syfer’s starting point.
The common set of specifications used by all manufacturers are detailed below:
C0G or Ultra Stable Class 1: the most stable of commercial ceramic dielectrics. These materials have little or no variation with applied DC voltage or temperature. They also have the lowest losses and therefore the lowest self heating. Conversely, they have the lowest dielectric constant, typically 20 to 100, which means only relatively low capacitance values are achievable.
X7R or Stable Class 2 - several subcategories exist: the EIA dielectric type X7R (CECC 2R1) is the most commonly used for higher capacitance parts as it has a dielectric constant in the region of 2000-4000.The stability with temperature is ±15% from -55°C to +125°C. However, it is not uncommon for significant capacitance loss to occur at rated voltage. There are more voltage-stable X7R versions: CECC 2C1/MIL BZ at: +20%-30%; and CECC 2X1/MIL BX at: +15%-25%, with the full rated DC voltage applied across the full temperature range.
Syfer has undertaken extensive testing and analysis of both its C0G and X7R dielectric materials, to define the behaviour of MLCCs under AC conditions. Under test conditions, the effects on capacitance, current flow and temperature change have been measured in order to provide the circuit designer with the data required to simulate the behaviour of the component in normal operation. Devices are mounted on a typical PCB, which provides no heating to the system under operational conditions. Temperature rise at room temperature is restricted to a maximum of 25°C. Accelerated life testing has also been carried out at maximum rated voltage and frequency, and at elevated temperatures, to ensure reliability.
As expected, all X7R types have higher capacitance variation than C0G materials. Furthermore, they can be subject to piezoelectric effects under AC conditions. This only tends to be an issue for large components and high capacitance values at 50/60Hz or more generally at higher audible frequencies.
The graphs below show the change in capacitance and the current flow through the capacitor per nF of rated capacitance at 50Hz, for C0G dielectric and for three types of X7R design. As anticipated, the C0G is as stable with AC applied as it is with DC. However, maximum capacitance is limited to 10nF.
Meanwhile, The X7R can be used to provide AC voltage coefficient characteristics of ±30%, +30%/-50%, and +30%/-80% depending on requirement. As a result of this characterisation work, using both temperature rise and reliability as limiting factors, Syfer is able to offer a range of devices for AC operation, with capacitances of up to 120nF in a 2220 case size with an AC voltage coefficient of ±30%.

The graphs attached.

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