Selecting a Ceramic Capacitor (MLCC)
- Daniel Puratich
- Nolan Haines
What is an MLCC?
You may be wondering why the term MLCC is being used, and what an MLCC. MLCC stands for Multi-Layer Ceramic Capacitor. MLCCs consist of multiple dielectric layers, made of ceramic, stacked together with metal conducting layers called electrodes. This is in contrast to other types of capacitors, such as electrolytic and tantalum capacitors. This article will only be discussing ceramic capacitors; the information here does not apply to the other types.
MLCCs are the most common type of capacitor, and it is more than likely that you are using them in your designs. They're the common SMT chip capacitor that are often used for decoupling and filtering on PCBs.
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How do I choose the right MLCCs for my design?
There are many things that you have to consider to make sure that the capacitor you’re choosing is going to work well for your design. This section will walk through the parameters that you should take into account. Note that this document will not explain how to choose the right capacitance value for your MLCC, as doing so is highly context dependent, but rather will focus on how to choose everything else about the capacitor in order to get the desired performance.
Temperature Coefficient
The temperature coefficient of a capacitor defines how its capacitance changes with temperature. X7R, X5R, NP0, C0G, etc., which you’ve probably seen when looking at datasheets/DigiKey pages for capacitors, are temperature coefficients. Temperture coefficient depends on the material(s) used for the dielectric. A breakdown of how these ratings are defined and what each of the characters means has been omitted from this document for brevity, but can be found here: https://www.kemet.com/en/us/technical-resources/heres-what-makes-mlcc-dielectrics-different.html.
In general, WARG uses NP0 (also sometimes referred to as C0G; if you see C0G know that it’s interchangeable with NP0, they are the same) and X7R capacitors for 99% of applications. NP0 capacitors, which are ‘Class I' capacitors, are extremely stable across temperature. However, because of their construction, they have much less capacitance than the ‘Class II’ capacitors, such as X7R. These Class II caps have a much wider range of capacitances at the tradeoff of not being as stable over temperature. Class III capacitors do exist. They follow the trend established by the first two classes; Class III capacitors have a larger capacitance at the cost of much poorer temperature stability. NP0 and X7R capacitors are so cheap and widely available as of the time of writing that there isn’t much incentive to use any other temperature coefficient for the vast majority of applications.
Temperature coefficient also has a significant impact on the DC bias characteristics of the capacitor, which will be discussed in further detail below.
Voltage Rating
The voltage rating of a capacitor is how much voltage you can put across it before it starts degrading in performance or becoming damaged. A general rule of thumb is to pick a voltage rating of 1.5 to 2 times the maximum voltage you’re expecting to put through the capacitor in your circuit. You want a bit of headroom so if there’s an unexpected voltage spike your caps can handle it.
DC Bias
Something else that must be taken into account here is DC bias, as mentioned earlier. The DC bias characteristics are essentially the voltage coefficient of the capacitor, as in it is how the capacitance changes with respect to an applied DC voltage. You may notice that the word ‘coefficient’ is used to describe the relationship between effective capacitance and another paramater (voltage, temperature…).
When a DC voltage is applied across a capacitor, the electric field created has an effect on the dielectric material, which lowers the effective capacitance. This depends significantly upon the temperature coefficient of the capacitor. For example, X7R capacitors can have their capacitance decrease to less than 30% (!!) of their nominal rated capacitance at a higher applied voltage, due to the DC bias effect. For NP0 capacitors, the DC bias effect is a lot less significant since the dielectric material used for those capacitors is more resistant to DC bias.
The datasheets provided for capacitors will give you an Applied DC Voltage vs Capacitance graph. In this graph, you need to find the voltage you expect to be putting across your cap and see how much the effective capacitance will decrease. From there, you need to consider if it’s necessary to choose a different capacitor or add multiple capacitors in parallel in order to add capacitance.
ESR & ESL
ESR stands for Equivalent Series Resistance. It is a measure of the total resistance caused by the capacitor itself, as in the internal resistance of the electrodes, resistive losses in the dielectric material, etc. It is ideal to keep ESR as low as possible, because higher ESR leads to increased power dissipation, and poorer frequency response and stability.
ESL represents Equivalent Series Inductance (L is the letter used to represent inductance). It exists because of the parasitic inductances of the physical connections of the capacitor. A low ESL is important for decoupling capacitors that are filtering high frequency noise (eg. small capacitors decoupling a power supply).
Basically your goal with these two parameters is to get them to be as low as possible. Temperature coefficient has an impact here, an NP0 cap will have less ESR than an X7R cap, but will probably offer less capacitance for the same package size. You also have to make a tradeoff with the price, lower ESR/ESL devices will be more expensive in general because of the more advanced manufacturing required to make them; at some point you’ll hit diminishing returns.
Package Size
Package sizes are labeled (in inches) as the length by the width in hundredths of an inch. For example, an 0603 package is 0.06 inches long and 0.03 inches wide. At WARG, we’re transitioning to using mostly the 0402 package because they save space and weight, at the cost of being slightly more challenging to solder due to their smaller size. However, there are some contexts when the 0603 package makes sense to use, eg when you aren’t worried about board space and weight and want to make it easier to bringup. Sometimes if you have a particularily large capacitance you may only be able to find that capacitance in a larger package size.
Tolerance
For capacitors used for decoupling and bypass, ±10% or even ±20% tolerance will most likely be fine. For more precise applications like filters and timers you might have to consider going down to ±5% or even ±2% tolerance.
Price & Manufacturer
We typically use Murata caps. You can use any reputable supplier that makes sense for the specific capacitor you’re ordering. Avoid ordering from sketchy looking manufacturers if possible, we want a good quality component. Obviously, try to maximize the quality:price ratio so we’re making good use of our budget.
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References/Further Reading/Related Resources: