Capacitors are one of the three quintessential passive components found in just about any PCB. Simpler hobby projects might use simple electrolytic capacitors without much forethought, but precision systems with high-speed or high-frequency components require ceramics and/or tantalum capacitors. These two capacitor materials offer important advantages at lower voltages, lower capacitances, and tighter tolerance requirements that you might not find with bulk electrolytic capacitors.
If you start to look in your schematic design for opportunities to improve capacitor stability and performance, capacitors are a simple place to start. Depending on where you look, you may find that a switch from ceramics to tantalums (or vice versa) gives you a favorable tradeoff in terms of performance and/or cost. We’ll examine some of these tradeoffs in this article, specifically regarding SMD capacitors with small case codes.
Are Tantalums a Replacement For Ceramics?
The answer to this question is multi-faceted and tantalums will not be suitable replacements for ceramics in all cases. Ceramics and tantalums do share common case sizes for SMD components (0603, etc.), so they can be drop-in replacements for each other in certain situations. However, the behavior of a ceramic’s capacitance value at its DC bias voltage, as well as the temperature coefficient of capacitance (TCC) value for these parts, might limit the allowed replacements between ceramics and tantalums.
Voltage Rating vs. Capacitance
Compared to ceramics, tantalum capacitors can have higher capacitance for the same voltage rating and case size. The tradeoff here is cost; the more specialized higher capacitance tantalums can have higher cost than a ceramic with the same voltage rating. This is especially important for small embedded systems that have high compute requirements and thus need a lot of power at standard voltages.
There is a simple strategy for using these various types of capacitors:
- Use a small number of tantalums on power supplies with higher standard voltages (12 V, 24 V, etc.)
- Use a large number of ceramics on the PDN, analog circuits, configuration on ASICs, and lower voltage nets
Mixing and matching the two types of small-case capacitors in targeted areas is a great way to ensure performance targets are hit, but without excessive costs. This is becoming more important as standardized voltages at higher values (24 V and higher) are becoming present in smaller embedded systems. In the past, you would need larger case radial capacitors that were very tall, but low profile can be ensured with small-case tantalums in parallel.
Usage in Power Supplies
Tantalum capacitors are typically associated with larger cases, such as the radial SMD capacitor packages shown below. It is true that these capacitors are often used due to their high capacitance values and high voltage ratings. This makes these capacitors very useful for bulk capacitance on switching power supplies, both on the input and output stages. They can also have some ESR on the order of ~0.1 to 1 Ohm, which is beneficial for damping transient ringing on the output from a switching regulator.
The above case configuration is very common in power supplies, especially when the required capacitance is very large. The capacitance that can be accessed in the above case size can reach hundreds of uF; this would not be found in SMD ceramics due to the internal structure of these capacitors (especially in multilayer ceramic capacitors (MLCCs)).
Polarization and RF Usage
Tantalum capacitors are polarized, meaning they require a specific application of voltage to their positive terminal, just like most electrolytic capacitors. In contrast, ceramic capacitors are unpolarized, so they can be run with AC voltages. Because ceramic capacitors can be designed with low ESR and low ESL, and they can operate with AC, ceramics are very often used in RF systems, such as wireless systems. The same advantages in RF systems motivate the use of ceramics as decoupling/bypass capacitors in PDN impedance engineering.
While RF systems may use ceramics to great effect, these capacitors start to behave inductive well above WiFi frequencies. In these more specialized areas, if discrete capacitors are needed they are built from more specialized materials, which we will discuss in a different article. The other possibility is that these discrete capacitors are eliminated in very high frequency systems by incorporating them directly on the Si die in integrated circuits. This eliminates the PCB-level parasitics and simplifies RF design far above WiFi frequencies.
The most important area where tantalum capacitors have an advantage over ceramic capacitors is in their stability under DC bias. Ceramic capacitors and tantalum capacitors can have the same (or similar) voltage ratings, but they cannot be run with the same voltage derating. In general:
- Tantalum capacitors can be run near their voltage rating without degradation of capacitance over time
- Ceramic capacitors generally require at minimum 50% derating to ensure stability and operation near rated capacitance
We can see the effects of stability by looking at a capacitance vs. temperature curve for a fixed voltage rating, which allows the two types of capacitors to be compared. There is significant drop in the capacitance of ceramics as the applied DC bias increases and approaches the voltage rating of the capacitor. The only exception is the C0G/NP0 dielectric used in ceramic capacitors, which overlaps with tantalum on the graph shown below.
[Source: RCD Components]
From the above graph, we see very clearly that capacitors require significant derating. If you plan to use a ceramic capacitor at low logic levels, its derating should probably reach up to 80%-90%. Contrast this with tantalums, which require little to no derating and instead can be chosen entirely based on the breakdown voltage rating.
Temperature and Operation Stability
In terms of the temperature stability of these components, tantalum and ceramics are comparable. Although their slopes point in opposite directions, the expected capacitance change over typical operating temperature ranges is not expected to be greater than +/- 10%.
Another issue that specifically relates to ceramic capacitors is their capacitance drop over lifetime. The graph below is provided by LCSC and shows how capacitance for various ceramics degrades over operational lifetime. These curves will be different for different deratings, applied voltages, and operating temperatures, but the graph does illustrate the correct broader trend regarding the stability of different ceramic capacitor dielectrics (see below for a list).
Change in capacitance [Source: LCSC]
Ceramics Have Multiple Options
A final point to remember is that there is only one style of tantalum capacitor, but ceramics come in different compositions that offer different properties. The most common dielectrics that are used in commercially available ceramic capacitors are:
- C0G/NP0: C0G (also known as NP0) offers a high capacitance stability over a very broad temperature range (-55°C to +125°C) with low dielectric losses and leakage. C0G capacitors have among the lowest TCC values, making them suitable for applications where stability and accuracy are most important.
- X7R: The X7R dielectric exhibits moderate capacitance stability over a very broad temperature range (-55°C to +125°C) with a moderate TCC. X7R capacitors have higher capacitance values compared to C0G, making them suitable for general-purpose applications. They offer a good balance between cost, size, and performance.
- X5R: X5R dielectric is very similar to X7R in terms of capacitance stability but has a larger capacitance change with temperature. X5R capacitors are commonly used in applications where cost and space constraints are a concern, and a moderate level of capacitance stability is sufficient. Typically, X5R can be a direct substitute for X7R as long as temperature changes are not extreme.
- Y5V: Y5V dielectric has a high volumetric efficiency, allowing for higher capacitance values in smaller sizes. However, Y5V capacitors have a large capacitance change with temperature and are less stable compared to C0G, X7R, or X5R. This dielectric is often used for capacitors in power systems as safety capacitors (bridge between isolated grounds) for EMI control. However, their leakage current could be too high for some systems and could cause EMI failure.
- Z5U: The Z5U dielectric is similar to Y5V in terms of its capacitance change with changes in temperature. This dielectric offers a high capacitance density but is less stable compared to C0G, X7R, or X5R. Z5U capacitors are commonly used in systems where the expected temperature range spans from +10°C to +85°C.
X7R and X5R are probably the most commonly used dielectrics in ceramic capacitors. They appear in MLCCs and are lower cost options compared to the other dielectrics in the above list.
For precision measurement applications, C0G/NP0 is the best dielectric option for ceramic capacitors. This is because the TCC curve is very flat over broad temperatures, so you can expect highly stable performance in a measurement system even as the system warms up to its rated operating temperature. If ceramics are needed in these systems, such as in a measurement or filtering circuit, use the C0G/NP0 ceramic capacitors on the analog front-end as these parts can be more expensive.
To summarize, there are some important instances where tantalum capacitors are preferred over ceramic capacitors. These instances can be found in power systems or decoupling on power rails, including at the higher end of the frequency range where a system would operate. In other aspects, such as temperature stability, commonly used ceramics (X7R and X5R) are comparable with tantalums, so a tantalum capacitor can act as a drop-in replacement on a DC net.
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