Take advantage of ceramic capacitors to improve power density and conversion efficiency

From data servers in the Internet of Things (IoT) to electric vehicles (EV), the common pressure faced by power system designers is how to achieve higher power density and conversion efficiency. Although people are focusing more on semiconductor switching devices that achieve these improvements, the inherent characteristics of multilayer ceramic capacitors (MLCCs) mean that they can also play an important role in helping designers meet design requirements. These features include low loss, high voltage and ripple current handling capabilities, high voltage withstand capability, and high stability under extreme operating temperatures.

From data servers in the Internet of Things (IoT) to electric vehicles (EV), the common pressure faced by power system designers is how to achieve higher power density and conversion efficiency. Although people are focusing more on semiconductor switching devices that achieve these improvements, the inherent characteristics of multilayer ceramic capacitors (MLCCs) mean that they can also play an important role in helping designers meet design requirements. These features include low loss, high voltage and ripple current handling capabilities, high voltage withstand capability, and high stability under extreme operating temperatures.

This article describes the structure of the MLCC and how ceramic capacitors enhance the power handling capabilities of the DC and AC power rails. It also provides a supplementary explanation of fast switching mode semiconductors. This article also clarifies Type I and Type II dielectrics and how these materials enable micro MLCCs to be used in power systems such as buffers and resonant converters.

How to make MLCC

MLCC is a monolithic device composed of alternating layers of ceramic dielectric layers and metal electrodes (Figure 1). The stack in MLCC is made at high temperature to produce sintered capacitor devices with high volumetric efficiency. Next, integrate a conductive termination isolation system on the exposed end of the device to complete the connection.

Take advantage of ceramic capacitors to improve power density and conversion efficiency

Figure 1: Ceramic dielectrics classified by temperature stability and dielectric constant. (Image source: KEMET)

Ceramics are non-polar devices with higher volumetric efficiency and can achieve higher capacitance in a smaller package size. In addition, this device is more reliable in high frequency operation. This allows the MLCC to properly combine the dielectric, termination system, form factor and shielding performance.

Nevertheless, when choosing ceramic capacitors for high power density applications, designers still need to conduct rigorous evaluations on some issues. First, the operating temperature, the applied DC bias, and the time elapsed since the last heating all affect the capacitance. For example, the time elapsed since the last heating can cause changes in capacitance and cause the capacitor to age (Figure 2).

Figure 2: The aging rate in the form of “capacitance time percentage”. (Image source: KEMET)

More importantly, because each capacitor has a certain impedance and self-inductance, the ripple generated by the fast switching IGBT or MOSFET semiconductor device will affect the performance. Therefore, when devices such as inverters occasionally require large currents, capacitors must be used to limit fluctuations, which requires higher ripple current tolerance.

Then there is the effective series resistance (ESR) of the capacitor, which is critical and represents the total internal resistance specified at a given frequency and temperature. By optimizing ESR, designers can reduce power consumption caused by heat.

Next, low effective series inductance (ESL) will increase the operating frequency range and further miniaturize ceramic capacitors. Low ESR and low ESL together improve the power handling capability of the capacitor and make the device parasitic. Moreover, they help reduce losses, thereby enabling capacitors to operate at high ripple current levels.

Another key design consideration is the choice of dielectric material. This will determine the performance of the capacitance with temperature (Figure 3). Although Class I dielectric materials (such as C0G and U2J) provide higher temperature-stable dielectrics, their dielectric constant (K) is lower. On the other hand, Class II materials (such as X7R and X5R) have a mid-range stability and K value, and also have a higher capacitance value.

Figure 3: The main difference between Type I and Type II electrolyte dielectric materials is the magnitude of capacitance change at a specific temperature. (Image source: KEMET)

However, for fast switching power supply systems, the higher the operating frequency, the lower the capacitance required to deliver power. This allows ceramic capacitors with lower K values ​​to replace bulky high-capacitance film capacitors, thereby significantly increasing power density. This ceramic capacitor has a small base area, so it can be mounted closer to fast switching semiconductors, and requires less cooling in high power density applications.

Class I dielectric material MLCC

KEMET’s KC-LINK capacitors, such as CKC33C224KCGACAUTO (0.22 microfarad (μF), 500 V), CKC33C224JCGACAUTO (0.22 μF, 500 V) and CKC18C153JDGACAUTO (15 nanofarad (nF), 1000 V) are good examples. This type of capacitor uses a grade 1 calcium zirconate dielectric material, which helps to achieve extremely stable operation without loss of capacitance due to switching frequency, applied voltage, or ambient temperature. Because the capacitance does not change over time, low-loss calcium zirconate dielectric materials can also reduce the effects of aging.

KC-LINK capacitors use C0G dielectric technology to achieve very low ESR and can manage very high ripple currents, which are precisely necessary for high power density designs. The high mechanical strength allows these Class I ceramic capacitors to be installed without the use of a lead frame, which also results in extremely low ESL.

This kind of ceramic capacitor can work under very high ripple current, and the capacitance does not change compared with the DC voltage, and the capacitance change in the operating temperature range of -55°C to 150°C is negligible. Their capacitance value ranges from 4.7 nF to 220 nF, and their rated voltage ranges from 500 V to 1,700 V (Figure 4).

Figure 4: At 150°C operating temperature, in high power density applications that require cooling, KC-LINK ceramic capacitors can be placed close to fast switching mode semiconductors. (Image source: KEMET)

It is worth noting here that KC-LINK capacitors based on Class 1 dielectric materials provide lower on-chip capacitance than Class 2 capacitors of the same size. Therefore, if more capacitance is needed, multiple KC-LINK capacitors can be combined to form an overall structure to form a higher density package.

The result of capacitor merging is a low-noise solution similar to KC-LINK, but the capacitance increases by as much as 125%. KEMET’s KONNEKT surface mount capacitors are also based on Class I dielectric materials and can provide higher capacitances from 100 picofarads (pF) to 0.47 ?F. This kind of capacitor can still maintain more than 99% of its nominal capacitance under the rated voltage, and is very suitable for applications with strict timing requirements, applications that are restricted by temperature cycling and circuit board bending.

Get more capacitance by stacking MLCC

KONNEKT ceramic capacitors (including C1812C145J5JLC7805, C1812C944J5JLC7800 and C1812C944J5JLC7805) are made by vertically or horizontally stacking two to four ceramic capacitors, while maintaining the integrity of each device. The C1812C944J5JLC7800 ceramic capacitor can provide 0.94 ?F by stacking two devices, while the C1812C145J5JLC7805 ceramic capacitor can increase the capacitance to 1.4 ?F by stacking three devices together.

These MLCCs use transient liquid phase sintering (TLPS) materials to bond the component terminations together to create a lead-free multi-chip solution. The lead-free multi-chip solution makes the capacitor compatible with existing reflow processes. TLPS is a metal matrix composite adhesive made of copper-tin materials, used to replace solder. This material forms a metallurgical bond between the two surfaces (here, the U2J layer).

In view of the fact that capacitors can be integrated in two directions, designers can limit the reduction of the component footprint and limit the increase in the total capacitance of stacked MLCC devices (Figure 5), so that KONNEKT ceramic capacitors can achieve the previous only type II dielectric materials Time (such as X5R and X7R).

Figure 5: The capacitance can be increased by stacking MLCCs and placed in the direction of low loss to reduce ESR and ESL. (Image source: KEMET)

In the direction of low loss, only less electrical energy is converted into heat, which improves energy efficiency and further enhances the power handling capability of the capacitor. In the direction of low loss, ESR and ESL are also reduced, thereby improving the ability of ceramic capacitors to handle ripple currents.

The combination of TLPS materials and ultra-stable dielectrics enables ceramic capacitors to handle extremely high ripple currents in the hundreds of kilohertz range. For example, for the 1812C145J5JLC7805 U2J 1.4μFKONNEKT capacitor, the ESL is 1.6 nanohenries (nH) when installed in the standard direction, but it is reduced to 0.4 nH when installed in the low-loss direction. Similarly, in the direction of low loss, ESR is reduced from 1.3 milliohms (mΩ) to 0.35 mΩ, thereby reducing system losses and limiting temperature rise.

KEMET’s U2J KONNEKT surface mount capacitors limit the change in capacitance from C55°C to +125°C to C750±120 parts per million (ppm)/°C. This makes the U2J ceramic capacitor’s capacitance relative to the DC voltage change negligible, and the linear change of the capacitance relative to the ambient temperature is predictable.

AC line ceramic capacitors

The ceramic capacitors mentioned in the above sections can stabilize and smooth the voltage and current on the DC power rail, thereby preventing decoupling spikes caused by fast switching operations. However, ceramic capacitors are also used in AC line filtering, AC/DC converters, and power factor correction (PFC) circuits.

Here, it is important to note that there are safety and non-safety grade formats for AC line ceramic capacitors. Although safety-level capacitors can suppress electrical noise and protect the design from overvoltage and transients, these safe MLCCs cannot provide higher capacitance/voltage (CV) levels.

Non-safety grade AC ceramic capacitors with various sizes and CV values ​​can be used continuously under AC line conditions. KEMET’s CAN series ceramic capacitors meet the requirements of 50/60 Hz line frequency, 250 VAC AC line conditions and other non-safety applications.

Figure 6: CAN series AC line capacitors have low leakage current and low ESR at higher frequencies. (Image source: KEMET)

AC line capacitors have low leakage current and low ESR at higher frequencies (Figure 6). This series is suitable for both line-to-line (X-type) and line-to-ground (Y-type) applications, and complies with the pulse regulations in the IEC 60384 standard.

CAN series ceramic capacitors all use X7R and C0G dielectric materials. As shown by the DC link capacitor, the C0G dielectric has no change in capacitance with respect to time and voltage, and the change in capacitance is negligible with respect to the ambient temperature. On the other hand, in ceramic capacitors such as CAN12X153KARAC7800 and CAN12X223KARAC7800, X7R has a predictable change in capacitance with respect to time and voltage, and the change in capacitance due to ambient temperature.

The capacitance of the CAN12X153KARAC7800 ceramic capacitor is 0.015 ?F, while the capacitance of the CAN12X223KARAC7800 device is 0.022 ?F. The tolerances of these two MLCC devices are both 10%.

Summarize

As the size of power transmission systems continues to shrink and more power devices are packaged in smaller sizes, MLCC plays a vital role in the design of everything from server power supplies to wireless chargers to inverters. They can smooth DC and AC voltage, stabilize current ripple, and ensure thermal management performance for power supply designs seeking to improve conversion efficiency. As shown here, by selecting Type I or Type II dielectric materials, designers can adjust the capacitance and other key parameters of the MLCC (such as ESR and ESL) according to specific application requirements.

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Author: Fymicohuang

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