In many cases, engineers now have a choice between ceramic and tantalum when it comes to selecting capacitors. Let's look at some trade-offs between the two to help select the proper device for a specific application.
Over the past few years, drastic improvements in multilayer ceramic (MLC) and tantalum capacitor technologies have resulted in smaller case sizes, better performances, and lower costs. Although construction techniques and materials used to manufacture these capacitors are completely different, the basic applications still remain the same.
Capacitors in the 0.1µF to 100µF range are used mainly for digital circuit decoupling and filtering. Acting as local supplies of charge, capacitors assist power supplies in remaining at a constant dc voltage, despite the continuous switching of digital signal circuitry. Capacitors also function as simple, single-pole filters and can be used in conjunction with other devices (resistors and inductors) to create higher order filter circuits. Limiting our focus to only the surface mount versions of ceramics and tantalums, we must keep in mind that the only real difference would be the added inductance from the leads themselves for the through-hole versions.
Improvements in MLC technology in the last few years have changed the dependency on parasitic inductance (ESL or equivalent series inductance), equivalent series resistance (ESR) and cost for the designer. Thus, the result is higher capacitance values in smaller case sizes without increases in cost. Today, MLCs are less dependent on high-cost metals due to less expensive electrode materials. Finer ceramic material allows for thin layers and higher capacitance values. In addition, finer materials allow for downsizing, making available case sizes as low as 0201.
Material changes within tantalum technology have also shown improvements in the electrical characteristics of the components. By changing the counter electrode from MnO2 to a conductive polymer, the ESR of the device has been reduced by approximately one-quarter.
Table 1 lists the range of devices to be addressed. This list includes the extended ranges of both the tantalum and ceramic technologies. Keep in mind that there is a recommended 50% derating of the voltage for tantalum capacitors. The X7R and X5R dielectrics were chosen because they are widely available and can achieve the capacitance ranges required. Also, tantalums are polar devices, so be careful how much reverse voltage gets applied — as in a dc blocking application.
The impedance curve of a capacitor can tell a lot about its performance in an actual circuit. Every capacitor has parasitic ESL and ESR, simply because of the physics involved in manufacturing the devices. Impedance implies real and imaginary parts, and today's impedance analyzers (such as an HP4194) measure both magnitude and phase and from this one can calculate ESL and ESR. Fig. 1, on page 18, shows the magnitude of the impedance as well as ESR for 22µF, X5R, 1210, 10V ceramic and a 22µF, 10V, B case tantalum.
Fig. 1 reveals some interesting results. First, you can tell they are the same capacitance value because the impedance curves are the same at low frequencies, i.e. at 1kHz. The ESR of the ceramics is also much lower over most of the frequency spectrum. Lastly, by looking at the upper end of the frequency spectrum, the ESL of the ceramic package is also much lower. Mainly the lead frames used in the packaging of surface mount tantalum capacitors cause this effect.
Table 2 lists the measured parasitic inductance for a variety of tantalum and ceramic case sizes. The capacitance value has almost no effect on the change in inductance. The governing properties are the path length and aspect ratio that the current “sees” flowing through the capacitor.
While ESL remains fairly constant with frequency, ESR is very frequency dependent. Both tantalums and ceramics have a dissipation factor (DF = ESR/2pfC) that must be met at 120Hz and 1kHz, respectively. However, most electronic circuits do not operate at 120Hz or 1kHz. Maximum ESR for tantalum capacitors is specified at 100kHz — fairly close to the switching frequency of most power supplies, while ceramics are typically not specified or are given for resonant frequency only. Table 3 shows the typical ESR at 100kHz and 1MHz for some comparable ceramic and tantalum capacitors.
Enhancements in Construction
Construction modification to the MLC have shown improvements to the ESL part of the device. By terminating the long ends of the chip, the ESL is reduced to about one-half of the standard MLC. These reverse geometry parts are growing in popularity as application frequencies and the need for lower inductance increases. To further reduce the ESL, designers have the option of the Inter-Digitated Capacitor (IDC). The IDC is manufactured with eight terminals and can reduce the ESL to about one-third of the reverse geometry parts.
Improvements to the construction of the capacitors have allowed for smaller case sizes and improved electrical properties for a variety of applications. For instance, by comparison, AVX Corp.'s TAC tantalum components offer lower ESR, ESL and direct current leakage (DCL) than tantalums using conventional construction. This new construction has made 0402 case sizes available to the tantalum world, and further development could bring 0201 case sizes to the market. A multi-anode configuration within the tantalum capacitor is another example of construction enhancements that improves the ESR of the component. This can provide ESR levels in the 10mΩ to 15mΩ range at 100KHz.
MLCs Versus Tantalums
Innovations in new materials, advanced construction and enhanced specifications have led to the availability of ceramic and tantalum capacitors that provide superior performance at lower costs every year. Table 4 compares old and new specifications of MLCs and tantalum capacitors.
Ceramic capacitors are made with high K (permittivity) materials, which exhibit a change in dielectric constant with an applied dc voltage. Tantalum capacitors do not change capacitance with applied dc bias. Because almost all capacitors are operated with a dc voltage involved, this is a very important feature to keep in mind when designing a circuit. A good rule of thumb is a 15% to 20% loss for X7R at rated voltage and 25% to 30% for X5R, regardless of the rated voltage. A linear fit in between works quite nicely for a first order approximation.
Power supply designers are often concerned about the ripple current capabilities of capacitors on the input and output sides of converters. The biggest concern is the internal temperature rise caused by the I2R power consumption of the capacitor. Because tantalums are a polar device, this ripple should always be accompanied by a dc bias. Because ESR is so dependent on frequency and temperature, the power ratings listed may not always be accurate. The tantalum capacitors have a published data set for a 10°C rise above the ambient. The experimental setup used to arrive at this number was emulated and done for a series of ceramic capacitor chips. It should be noted that different mounting techniques could alter the thermal conductivity greatly. Table 5 lists the empirical data from this experiment. From this table, the power handling of the ceramics is typically much better than the tantalums. Keep in mind that the ESR of ceramics is also typically lower (see Table 3, on page 21), so more current can be driven through the capacitor (P=I2R).
Of much less concern, yet still important especially in audio applications, is the microphonic or piezoelectric effect. Barium titanate, which is the base ceramic material for most dielectric systems, will exhibit microphonic effects, while tantalum capacitors exhibit no microphonic effects. An internal experiment involved the opposite phenomena, whereby the part was shaken while under bias and the resulting generated voltage was measured. It was performed on a series of 1µF devices. While this experiment does not give an empirical number, the resulting relative voltage generation tells the story. This experiment confirms that the higher the K of the ceramic capacitor, the worse the microphonic effect becomes.
These recent advances in ceramic and tantalum technologies have increased volumetric and cost efficiencies such that high value, small physical size capacitors are now practical in both technologies. MLCs are nonpolar, have lower ESR and ESL, and higher ripple current handling. Tantalums have no dc bias or microphonic effects. However, tantalums still have higher maximum capacitance values (10:1) and have closed the gap in regards to ESR with the latest improvements.
There's no simple answer to the question of what's the best time to replace a tantalum with a ceramic, or vice versa. You must thoroughly examine the parameters for the capacitor in its lifetime within its application. You can see the benefits and highlights of each listed in Table 6.
It isn't feasible to blindly replace one type of capacitor technology with another and expect equal performance over all conditions. It's important that you always take into account the general knowledge of what the circuit will see.
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