The demand for multilayer ceramic capacitors (MLCCs) has increased due to the growth of their use in smartphones, automotive and industrial electronic products. As a result, the supply of MLCCs required for many general purpose applications is under stress, and lead-times that are already long are expected to be stretched further. To deal with these conditions, hardware designers and OEMs need to evaluate alternative technologies that can replace MLCC parts in their designs. This article gives insight into how an MLCC can be replaced with a polymer capacitor, including an application example.
In brief, MLCC capacitors are nonpolarized and hence can be used in both AC and DC applications, while polymer capacitors are polarized and can be used only in DC applications. Yet, polymer electrolytic capacitors with some unique characteristics can be considered as an alternative for MLCCs by tuning specific parameters.
Based on the dielectric used, MLCCs are classified as Class I, II, or III. Class I MLCCs use Calcium Zirconate, a para-electric material, which is reasonably stable. Class II and III dielectrics use Barium Titanate, a ferroelectric material which causes capacitance instability.
Polymer uses PEDT (Poly Ethylene Dioxy Thiophene) as a cathode counter electrode material and is the successor of Tantalum MnO2 Capacitors. It has a low oxygen index and hence no ignition failure. It also has an acceptable single digit ESR though higher than that for an MLCC. Let’s explore how the characteristics like capacitance, DC bias, aging, temperature effect, piezoelectric noise, and equivalent series resistance (ESR) of a polymer capacitor compares with an MLCC.
Capacitance change with applied voltage
MLCCs lose more than 60% of their rated capacitance with the applied rated voltage due to their dielectric preparation. Reducing the dielectric thickness to achieve smaller chip sizes keeps the same level of capacitance causes higher voltage stress which results in more capacitance loss. In polymer capacitors, the dielectric constant does not vary when the applied voltage changes, so there isn't much deviation in capacitance when a voltage is applied. For practical purposes, derating about 10 to 20% should be considered. In the case of tantalum polymer capacitor of the same rating, the capacitance is nearly the same.
Capacitance change with temperature
The capacitance of ceramic capacitors fluctuates with temperature and varies with different dielectrics used. Whatever class the MLCC may belong to, there is a derating due to temperature effects on the material. This impact may lower the capacitance by as much as 30%. However, in the case of tantalum polymer capacitor of the same rating, it has around same capacitance. The tantalum polymer electrolyte capacitor also provides proper temperature and humidity stability.
Capacitance change with frequency
Most MLCC types have increased capacitance changes with increasing frequencies. The dielectric strength of Class II and Class III MLCCs reduces with an increase in frequency resulting in a decrease of capacitance. For a tantalum polymer capacitor, we get a small deviation in the capacitance with a change in frequency.
Aging Effect
All capacitors exhibit aging characteristics, and the working life of electronic products is severely affected by this phenomenon. Barium Titanate devices have a Curie temperature in the range of 130°C to 150° C. Class II and Class III dielectrics are exposed to high temperatures of 1000°C during their manufacturing process, which introduces structural changes to the material; the dielectric constant of the material also gets changed thereby reducing the capacitance as it ages. In comparison, the tantalum polymer capacitors have a relatively slow wear-out mechanism that gives them extremely long service life.
Size
MLCCs are bulkier than tantalum polymer capacitors and pack lower capacitance in the same package making them heavy and leading to PCB flex failures. They tend to occupy more PCB space which indirectly increases costs. Again, in comparison, tantalum polymer capacitors can pack higher capacitance values in smaller packages, and they are lighter.
Comparison of a Polymer Capacitor and MLCC 1210
So let's compare the above characteristics of a polymer capacitor of the same rating to an MLCC (1210, 100uF, 6V). Figure 1 shows a comparison of these capacitors according to parameters such as voltage, temperature, frequency, and aging. This will begin to show us how to replace an MLCC with a tantalum polymer capacitor.
Figure 1: Comparison of an MLCC to a Polymer Capacitor
Piezoelectric Effect
Class II and Class II MLCCs use ferroelectric material which has a piezoelectric effect, called microphonics. When it comes to the application of external stresses, the Titanium molecule oscillates back and forth, and the dielectric gets mechanically distorted. This distortion creates a “buzzing” noise in the device. The polymer capacitor does not demonstrate this effect and hence is the ideal choice for applications that are sensitive to acoustic noise.
Equivalent Series Resistance
MLCCs have very low ESR and hence are very popular for reduced ripple currents. Polymer Capacitors have higher ESR compared to MLCCs. However, as they have evolved, they have reduced their ESR to acceptable limits. The ESR of polymer capacitors is nearly constant within its operating temperature range.
Soldering
Due to soldering stress, there may be a change in the electrical parameter of the ceramic capacitor. For SMD styles, the heat of the solder bath can cause changes in contact resistance between terminals and electrodes. For Class II ceramic capacitors, the soldering temperature is above the Curie point. As a result, a recovery time of approximately 24 hours is required after the soldering process. Some electrical parameters like capacitance value, ESR, and leakage currents are changed irreversibly after recovery.
Design considerations in the replacement of an MLCC
If we map the capacitance and voltage for different technologies like ceramic, film, tantalum, tantalum polymer and aluminum electrolyte, we can find that a tantalum polymer capacitor is a better option for voltages up to 70V and also in the range between 1uF to 1mF.
One example would be the Kemet KO-CAP series of tantalum polymer capacitors, which are suitable for replacing MLCCs. It makes sense to consider polymers for the net capacitance greater than 10 µF for application voltages up to 14.4V, as well as 0.68 µF to 10 µF at application voltages of 45V and higher. The maximum application voltages are up to 67.5V (60V for Harsh Conditions). Regarding frequency response, we need to restrict to applications below the switching frequency of 1Mhz.
The process of converting from an MLCC to a Polymer SMD component is not a one-to-one method and needs to follow several considerations. The first step is to make a mechanical and dimensional match, which Table 1 illustrates. MLCC EIA codes 0805 and 1206 have a direct dimensional footprint alternative with metrics P-2012 and A-3216 footprints. The larger MLCC EIA codes 1210 and 2220 have a potential option with metrics B3528 and D7343 footprints, although the replacement is not direct.
Table 1: Dimensional Matching: MLCC vs. Polymer SMD Capacitor
Application Example: Buck Converter in an Automotive Application
Figure 2: Buck Converter
Input Side
In the above circuit, C1, C2, C3, and C10 are the input side capacitors, and they have a same capacitance value of 2.2uF with rated voltage 50V in a 1206 X7R package. There is not a drop-in replacement for each capacitor, but we can take the total 8.8uF capacitance and replace the four ceramics with one 10uF 35V KO-CAP if ESR, Leakage, and frequency are not of a concern on the input side. It is more than the original capacitance we need, but it is still within the required range for this regulator.
Output Side
On the output side, there are four capacitors C6, C7, C9, and C11 that have the same capacitance value of 22uF with 10V rated voltage in X7R 1206 package. In this case, there is a drop-in replacement in T520 series of KO-CAP with a 6.3V rated voltage which is more than the output voltage range. In this replacement, the KO-CAP ESR is higher than the ceramic equivalents, but that comes within the design specification. This circuit operates on the 300kHz frequency, and series resonant frequency of the replacement is around 1Mhz making it a better solution.
Capacitor C4, C5, and C8 are used to support the functionality of the device, and there is a suitable replacement in KO-CAP by both their size and capacitance value. We are not considering the leakage current because it is a concern of circuity where we use fixed non-rechargeable batteries.
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