Ceramic Capacitors - Why Voltage and Capacitance Ratings Aren't the Full Picture
The true capacitance of ceramic capacitors rarely matches what is stated on the specifications once they’re installed in end applications. Depending on what frequency, voltage, and dielectric you’re using, true capacitance can be vastly different from what you’d expect. This guide walks through the common misconceptions and demonstrates how to select the right capacitor for your application when it’s time to pick real-world parts. The DC bias that a ceramic capacitor is operating at can have a large impact on its actual capacitance.
Here are the main rules to consider, which we’ll expand on:
For ceramic capacitors in general :
The higher the voltage the capacitor normally sees, the less capacitance you will get from the part
A larger package is going to provide better capacitance stability across voltage
Voltage coefficient changes with dielectric and is related to temperature coefficient (but not guaranteed!)
Capacitor manufacturers can use any material that meets the requirements of the temperature rating, but this doesn't mean that all X7R capacitors have the same voltage coefficient or are made from the same dielectric material. The capacitance tolerance stated is usually based on temperature not bias voltage! This is especially important to keep in mind when you’re choosing alternatives for approved capacitors. Its critical to check that the voltage coefficient is the same for your parts if the capacitance is important to maintain. Unfortunately, this information is often hard to find and is usually not even in the datasheet. If true capacitance is critical in your application, it’s important to test in your application. AC voltage and frequency will also affect true capacitance. Talking to an FAE from your capacitor manufacturer is a great place to start when refining your capacitor selection.
Murata includes an example in their datasheet of what a plot looks like, but doesn't have the actual parameters available. From their datasheet Chip Monolithic Ceramic Capacitor for General GRM0335C1H101JA01_ (0201, C0G:EIA, 100pF, DC50V)
DC Voltage Characteristics with Capacitance Change on the vertical axis and DC Voltage on the horizontal axis.
For power supply applications where the true decoupling bulk capacitance can have a relatively large tolerance, make sure that the capacitor voltage rating is several times higher than the operating point if you’re using economical temperature coefficient dielectrics like Y5V. This will ensure that your application is using the capacitor in its operating region where most of the capacitance is still available.
In filter, timing, or high precision applications where the voltage will fluctuate and capacitance needs to remain stable, it’s important to use a higher grade capacitor. Metal film capacitors are great for this but are typically much larger. If you have to use a ceramic capacitor for reasons of size, Class I C0G or NP0 dielectrics are the way to go. These dielectrics are not only more stable across temperature, but also across voltage. They also wont exhibit piezoelectric “buzzing” effects like Class II or III components.
If capacitance is important, you need to test your chosen capacitor in the conditions where it will operate. This data is not usually in the datasheet.
An FAE from your capacitor manufacturer should help you with this if you ask the right questions. For example, “What capacitor do I need to get X capacitance at X DC bias at X frequency over X temperatures and what packages are available?”
Temperature coefficient is related to DC Voltage coefficient, but not by design. It’s an okay place to start, but no guarantee.
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