One of the most ubiquitous components we use in electronics is the Multi-Layer Chip Capacitor (MLCC). These are brown or yellow-brown jelly-bean ceramic SMT capacitors you will probably have used hundreds of times without much of thought. There are, however, a few things you really need to consider when using them. You may not be getting what you expect!
When you are selecting these capacitors, you will see that they are specified by a variety of cryptic names like C0G, X7R or Y5U. These names are specified by the EIA-198 standard and so are consistent across manufacturers. But what exactly do they mean?
These numbers describe the temperature coefficient of capacitance (TCC) of the capacitor, not the dielectric material out of which it is made, although the two are obviously related. The standard specifies several classes of capacitor – the first three of which are pretty common and will be described here.
Class I capacitors are those considered to be very stable and are generally made from ceramics with good temperature characteristics but relatively low relative permittivity. This means they will generally be larger than Class II or III capacitors for a given capacitance and voltage, but have better temperature stability.
Table 1 shows how the codes associated with Class I capacitors work. The most common Class I designation you will encounter is C0G which has a zero tempco (±30ppm). The capacitance of these devices is pretty much independent of temperature, so they are a good choice for critical timing and filtering applications.
This table describes the interpretation of Temperature Coefficient of Capacitance (TCC) codes for Class I MLCCs. The common C0G capacitors have zero (±30 ppm) TCC. These capacitors also exhibit little or no aging and so are good choices for critical timing or filter applications.
Why would we ever consider using a capacitor which does have a temperature-dependent value? Well, the smallest 100 nF 10 V Class I cap I could find was 1206 size and cost USD 0.16 in 100-off quantities. Compare this with a similarly rated Class II capacitor which is available in 0402 packages and costs USD 0.019 in 100-off quantities.
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This is because Class II capacitors are made from ceramics with much higher relative permittivity, but some level of temperature dependence. Table 2 shows how these are specified. The very common X7R type chip capacitor therefore has a maximum variation of capacitance of ±15% over the temperature range of -55 to +125 °C. This will be adequate for general bypass and non-critical filtering applications, especially if cost and size are important.
Class II capacitors, described in this table, exhibit moderate TCC. The very common X7R capacitors may have a capacitance change of ±15% over the temperature range of -55 to +125 °C. These capacitors also exhibit a capacitance change with applied DC voltage and aging effects over time. You should take this into account in your designs.
Class III capacitors are made from dielectric materials with even higher permittivity and lower temperature stability than Class II devices as shown in Table 3. The typical common Y5V capacitors for example have a capacitance that can vary by as much as 15% above their nominal rating to 85% below over the temperature range of -30 to +85 °C. Let us just pause to consider this. At high temperatures a nominally 1 µF capacitor may only provide 150 nF of capacitance. This may not be an issue if your device always operates over a limited temperature range.
The dielectric materials used in Class III capacitors have an even higher relative permittivity than those used in Class I or II devices. This means they have higher capacitance per unit volume but it comes at the expense of even higher TCC, faster aging and more pronounced voltage-dependence.
But temperature dependence is not the whole story. One of the other quirks of MLCCs is that Class II and III capacitors also exhibit aging and will lose capacitance over time. For example, a typical X7R capacitor will lose about 2% of its capacitance every decade hours of operation. This means after 10,000 hours (a little over 1 year) such a capacitor will have lost 8% of its capacitance value. Figure 1 shows a chart extracted from a manufacturer’s data sheet that illustrates this nicely. Note that this effect can be up to about 5% per decade hours for some devices.
This figure shows that the aging characteristics for a typical MLCCs. Class II and III capacitors age linearly with the log of time. This is usually specified as percentage capacitance change per decade hours. Class II capacitors typically lose around 2% of their value every decade hours and Class II capacitors up to 5% per decade hours.
And just when you thought things could not get any worse, the capacitance of many MLCCs also reduces with applied DC voltage. Class II and III capacitors exhibit this phenomenon due the DC bias limiting the mobility of Titanium ions in the dielectric. This effect varies but can mean that a capacitor may lose between 20% and 85% of its capacitance if operated at rated voltage. Not a trivial amount I think you will agree.
Figure 2 shows a graph provided by one capacitor vendor showing a typical example, in this case a 4.7µF 50V X7R capacitor. At 40V, the capacitance has reduced by half, and at the full rated voltage (where you should never operate) it has reduced by 60%. This is something to be aware of when using these capacitors in bypass operations where a DC bias is the norm.
The capacitance of a typical 4.7 µF, 50 V X7R capacitor will fall off with applied DC voltage. At 40V the capacitance will drop to about 50% of the nominal value. It is important to be aware of this and to choose the voltage and capacitance rating carefully, so you don’t get unexpected results.
So, what do we make of all this? Should we stop using Class II and III MLCCs since their capacitance is highly dependent on temperature, time and applied voltage?
The answer of course is “it depends”. You must choose the right capacitor for the right role. If you have a critical timing requirement or filter where capacitor value stability is important, choose a Class I MLCC or another type of capacitor that has the right capacitance temperature and aging characteristics.
For run-of-the-mill decoupling applications, Class II or III capacitors are fine, but you need to be aware that the capacitance will likely decrease markedly with elevated temperature, DC bias and over time and design accordingly. As usual in electronics, a little knowledge goes a long way.
References
“Temperature and Voltage Variation of Ceramic Capacitors, or Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor | Analog Devices.” Accessed March 17, 2023. https://www.analog.com/en/technical-articles/temperature-and-voltage-variation-ceramic-capacitor.html.
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Murata Manufacturing Co., Ltd. “Does the Capacitance Change When a DC Voltage Is Applied to Ceramic Capacitors? Are There Any Points to Be Aware of Regarding Changes in the Capacitance? | Capacitors FAQ.” Accessed March 17, 2023. https://www.murata.com/en-us/support/faqs/capacitor/ceramiccapacitor/char/0005.
“Here’s What Makes MLCC Dielectrics Different.” Accessed March 17, 2023. https://www.kemet.com/en/us/technical-resources/heres-what-makes-mlcc-dielectrics-different.html.
“CL10A105KO8NNNC_Spec.Pdf.” Accessed March 17, 2023. https://media.digikey.com/pdf/Data%20Sheets/Samsung%20PDFs/CL10A105KO8NNNC_Spec.pdf.
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Andrew Levido (andrew.levido@gmail.com) earned a bachelor’s degree in Electrical Engineering in Sydney, Australia, in 1986. He worked for several years in R&D for power electronics and telecommunication companies before moving into management roles. Andrew has maintained a hands-on interest in electronics, particularly embedded systems, power electronics, and control theory in his free time. Over the years he has written a number of articles for various electronics publications and occasionally provides consulting services as time allows.
