What’s the capacitance of your MLCC?
What capacitance do you get when you buy a multilayer ceramic capacitor? It might sound like an odd question but the actual answer is down to dielectric type, design and operational conditions which make for a surprising degree of variation, writes Matt Ellis
There are many factors which affect the actual capacitance value of a multilayer ceramic capacitor (MLCC), some well documented, some less so. The tolerance and temperature coefficient of capacitance is clearly defined in dielectric classification codes and part numbering systems.
However voltage coefficient of capacitance (VCC) is less clearly defined.
Dielectrics used in MLCCs generally fall into two categories; Class II (stable) and Class I (ultra-stable). Class I are typically C0G or NP0 dielectrics and are very stable with temperature and voltage, whereas Class II are more variable and the lack of VCC definition is where problems occur.
VCC is a function of the properties of the dielectric material and the voltage stress applied, typically in volts per micron. The effect is negative and non-linear becoming asymptotic toward the limit of dielectric strength.
For example, an X7R dielectric at “K” tolerance: The TCC is ±15%/°C, and the tolerance is ±10% so running at the extremities of the specification there may be a variation from nominal capacitance of 23.5%. This will most likely be negative so a nominal value of 100nF could be 76.5nF.
There is further impact when voltage is applied. The VCC is not defined so there is no onus on the manufacturer to put this in the datasheet. A 100V rated part used at 80V, in a relatively conservative design, will produce a voltage stress of 3.2v/µm and could result in a drop in capacitance of around 40%.
If the effect of temperature and the 10% tolerance is factored in, then the capacitance could end up being only 40nF – when it was specified at 100nF in the first place. This may appear to be bad, but it can get worse.
With the pressures of price and size reduction, manufactures of MLCCs are forever reducing dielectric thickness, also voltage de-rating is becoming a thing of the past for end users.
Greater than 90% loss of capacitance at rated voltage is not uncommon in the general market place; this can be avoided for some parts by specifying 2C1 (BZ) or 2X1 (BX) dielectrics rather than standard commercial X7R (2R1). These options have a more tightly controlled VCC at the expense of absolute capacitance value.
VCC is not just a problem with respect to circuit functionality.
There can be legislative implications for certain equipment even if VCC is fully understood. EN 61010-1:2010 Safety requirements for electrical equipment for measurement, control, and laboratory use advises that for voltages up to 15kV equipment is considered hazardous live if it can discharge greater than 45µC and greater than or equal to 2mA. Above 15kV the same applies for energy levels greater than 350mJ.
In the case of, say, a high voltage laboratory power supply of 8kV, with an accessible capacitive circuit, then the 45µC rule restricts us to 5.6nF at 8kV. If the circuit in question requires a minimum of 2nF to function correctly then, as it’s known there will be some instability in the capacitor, five 3640 10kV 1nF parts would be specified.
Unfortunately, the capacitors tend to lose 75% of their value under 8kV so the capacitance could only be 1.25nF.
More capacitors in parallel can’t just be added because it will push over the 45µC limit; nor can parts be matrixed in series and parallel to reduce the voltage stress and keep the nominal capacitance low, because this will take up a lot of board space and be costly. The only answer is a more stable capacitor.
Some X7R materials are more stable than others and, if requested, more suitable capacitors can be manufactured.
For example, if a 3640 1nF 10kV with less than 50% capacitance drop at 8kV was used in the previous example, it would provide a residual capacitance at operational voltage of around 2.6nF as opposed to 1.25nF. This would allow the circuit to function correctly and even allow for a reduction in component count from 5 to 4.
VCC is an important component characteristic which is often overlooked and can cause significant problems in certain applications. Standard off the shelf components will vary in their performance so it is best to seek the advice of your supplier at the initial design stage to avoid the need for corrective action.
Matt Ellis is senior applications engineer at Syfer Technology