Ceramic capacitor selection: three "hidden" factors that are easily overlooked | Part 2

In actual applications, "hidden" factors such as temperature, voltage, aging (storage time) will affect the actual capacitance value of ceramic capacitors. Through a series of articles, we will analyze in detail the relationship between these factors and capacitance value from three aspects: temperature, voltage, and aging. In the last issue, we introduced the temperature factor. In this issue, we will talk about the voltage factor.

If you want to understand the effect of voltage on the capacitance of ceramic capacitors, in addition to actual testing and viewing data sheets, the free online tool KEMET K-SIM can directly query various parameters and curves based on KEMET models, including the relationship curve between DC voltage and nominal capacitance value.

Example: Compare the effect of DC voltage on the nominal capacitance change of the following three capacitors (C0G/X5R/X7R)

• C0G (KEMET C1206C104J4GACTU , 0.1 µF 16V C0G 1206 ±5% )

• X5R (KEMET C0402C104K4PACTU , 0.1 µF 16V X5R 0402 ±10% )

• X7R (KEMET C0402C104K4RACTU , 0.1 µF 16V X7R 0402 ±10% )

In the Design Resource column on the product page on the Digi-Key website , you can directly link to the KEMET K-SIM online tool. No need to enter the model again, just open KEMET K-SIM with one click. KEMET ceramic capacitors on the Digi-Key website can basically be linked to K-SIM using this method.

Figure 1. Link to the free online tool KEMET K-SIM from the KEMET product page on the Digi-Key website

As shown in the figure below, when the voltage rises from 0 to 12VDC:

• C0G (C1206C104J4GACTU) capacitance value drops by 0%

• X7R (C0402C104K4RACTU) capacitance value decreased by 40%

• X5R (C0402C104K4PACTU) capacitance value decreased by 53.33%

We can see that for Class I C0G, voltage has little effect on capacitance value. For Class II X5R/ X7R, voltage has a greater effect on capacitance value.

Figure 2. Relationship between DC voltage and nominal capacitance value change (Image source: KEMET K-SIM )

The following figure is a typical curve chart of the effect of AC voltage on Class-II capacitors. Different AC voltages will result in different capacitance errors, which may even be higher than the nominal capacitance .

Figure 3. Effect of typical AC voltage on Class-II capacitors (Image source: KEMET)

Sometimes ceramic capacitor manufacturers selectively reduce the thickness of the dielectric or adjust the dielectric formula in order to maintain the same level of capacitance in a smaller package. This design change may result in higher voltage stress and greater capacitance loss.

The following figure compares the effect of applied voltage on the actual capacitance of 47 µF /6.3V ceramic capacitors in 0805/1206/1210 packages : the smaller the package, the faster the actual capacitance drops at 5V .

Figure 4. Effect of voltage on actual capacitance of 47 µF /6.3V ceramic capacitors in different packages (Image source: KEMET)

The size of the package affects the thickness of the dielectric. When the same voltage is applied, especially for ceramic capacitors with ferroelectric materials as the dielectric material, the smaller the thickness of the dielectric, the greater the internal electric field stress .

In practical applications, the voltage, dielectric material type, dielectric thickness, etc. will all affect the actual capacitance of ceramic capacitors. Among them, ferroelectric materials as dielectric materials are often the culprit. With the help of Digi-Key's online tools and KEMET K-SIM to assist in the selection and design of ceramic capacitors, you can understand the relationship between voltage and capacitance value and achieve twice the result with half the effort .