Class I Ceramic Technology for High Power Density Applications
“Our society is increasingly reliant on electricity, whether for communication, providing secure and reliable storage for vast amounts of data, or for transportation by electric and hybrid electric vehicles. Therefore, power supply security is essential to enjoy modern life. One of the most pressing issues is energy efficiency – due in part to the rising cost of electricity and our desire to protect the natural resources used to generate electricity. Efficiency is one of the most important considerations when switching to new energy sources such as solar, wind, or considering the cooling requirements of Electronic circuits.
Our society is increasingly reliant on electricity, whether for communication, providing secure and reliable storage for vast amounts of data, or for transportation by electric and hybrid electric vehicles. Therefore, power supply security is essential to enjoy modern life. One of the most pressing issues is energy efficiency – due in part to the rising cost of electricity and our desire to protect the natural resources used to generate electricity. Efficiency is one of the most important considerations when switching to new energy sources such as solar, wind, or considering the cooling requirements of electronic circuits.
Engineers around the world are constantly seeking ways to improve device efficiency, including advanced circuit topologies such as resonant converters, smart power management, and the adoption of new materials. In power semiconductors, wide bandgap (WBG) devices are beginning to gain popularity, allowing power conversion devices to operate at higher frequencies, higher temperatures, and higher voltages. As switching speeds increase, the size of key components such as capacitors and magnetics can be reduced, enabling greater power density at higher power conversion efficiencies.
In terms of improving efficiency and power density, most attention has been paid to switching semiconductor devices as they contribute greatly to the static and dynamic losses in any power system. However, it’s getting harder and more expensive to make small improvements to what’s already available, so engineers are looking for other ways to increase efficiency.
While many engineers consider capacitors to be just supportive devices for power supply designs, more and more people recognize the need to improve their efficiency and thus power density. In power supply design, capacitors can positively impact system efficiency in three areas, each of which requires slightly different capacitors.
First, the snubber may require high dV/dT, high ripple current, high voltage, high temperature, and low inductance. Second, the DC link requires high ripple current, voltage, temperature and frequency. Third, resonant converters require high ripple current, wide operating voltage range, and capacitive stability over temperature and AC and DC voltages. Considering the combined requirements of the above-mentioned applications, capacitors should have extremely low losses, high ripple current handling capability, be able to withstand high voltages and accept higher operating temperatures, and at the same time have stable capacitance and high mechanical stability. To achieve high-density, high-efficiency power supplies using wide-bandgap semiconductors, capacitors in these packages must be thermally and mechanically stable.
Class I and Class II Multilayer Ceramic Capacitors (MLCC)
Among the many types of capacitors on the market, ceramic capacitors (especially multilayer MLCCs) can provide key characteristics required for snubber, DC link and resonant applications. MLCCs are formed by alternating metal electrode layers and ceramic dielectric layers. Each layer represents an individual capacitor, and since they are arranged in parallel, increasing the number of layers provides more capacitance. The vast majority of MLCCs produced today are fabricated with base metal electrodes (BME)—with nickel metal electrodes—and either CaZrO3 dielectrics (type I) or BaTiO3 dielectrics (type II).
Figure 1: Exploded view of a base metal electrode MLCC
Dielectrics are classified according to their capacitance stability over temperature. Class I contains dielectrics (C0G, NPO, U2J) that are the most stable to temperature, but they have the lowest permittivity (K), requiring a larger volume in order to achieve the same capacitance as more traditional types of MLCCs. Class II includes dielectric materials (X7R, X5R) with intermediate temperature stability and K values. Therefore, Class II offers higher capacitance per unit volume compared to Class I MLCCs. Although Class II MLCCs have higher overall capacitance, engineers must be aware of some key design considerations to avoid greatly affecting their use in power supply applications.
Since Type II MLCCs use BaTiO3 dielectrics, the actual capacitance will be affected by operating temperature, applied DC bias, and time elapsed since last heating (aging). The stability of capacitance over temperature is called the temperature capacitance coefficient (TCC), and it can be determined by the Electronic Industries Alliance (EIA) dielectric classification (eg “X7R”). The EIA defines X7R as an operating temperature range of -55°C to 125°C with a maximum capacitance limit of ±15%. The X5R has the same ±15% capacitance limit, but has an operating temperature range of -55°C to 85°C. Capacitance versus voltage (VCC) stability is also an important consideration, but it is not formally defined by the EIA. However, for higher capacitance Class II MLCCs, the capacitance can be reduced by as much as 80% at rated voltage, which can have a considerable impact on the application. This VCC characteristic may also vary by vendor. In addition to temperature and voltage, capacitance also decreases due to the elapsed time since the last heat. This is called burn-in and typically decreases by 2-5% every ten hours after the last heating above 130°C (usually when welding parts during manufacturing).
However, class I dielectrics are more stable than class II. The capacitance drift of dielectrics such as C0G is negligible at only 30ppm/°C or 0.3% at 125°C, while U2J is 750ppm/°C or 7.5% at 125°C, but linear and predictable. Both C0G and U2J have negligible changes in capacitance with respect to DC bias and little change with respect to time (aging). These properties make Class I dielectrics ideal for resonant applications such as LLC resonant converters and wireless charging circuits where keeping capacitance within tight tolerances is important.
Equivalent series resistance
In power applications, in addition to capacitance stability, due to i2R loss, equivalent series resistance (ESR) is also an important characteristic of capacitors. Figure 2 compares the ESR of Class II X7R and Class I C0G/U2J MLCCs from 100Hz to 100MHz. Since BaTiO3 is a ferroelectric material, it creates the characteristics of domain domains within the dielectric, which also causes domain wall heating and increased ESR, compared to Type I dielectrics. Therefore, class II MLCCs typically have an ESR that is one to two orders of magnitude higher than class I.
Figure 2: ESR comparison between class II X7R and class I C0G/U2J
Due to the large AC currents in power applications, a large MLCC ESR will directly lead to overheating. Figure 3 shows the temperature versus AC current for X7R, C0G, and U2J MLCCs. The data show that the self-temperature rise of C0G and U2J is about 15°C at 10A, while the temperature rise of X7R reaches 40°C only at 5A.
Figure 3: Ripple current comparison between Class II X7R and Class I C0G/U2J
Figure 4: Comparison of key characteristics between Class II X7R and Class I C0G/U2J
Class I technology progress
Due to the high temperature stability, low loss and high ripple current capability of MLCCs using Class I BME dielectrics, they are clearly ideal for high power density applications. KEMET has created a product portfolio using patented Class I BME CaZrO3 dielectric technology, which further improves power handling capabilities for snubber, DC link and resonant applications.
Products in this category include C0G high voltage commercial and automotive grade families that offer a wide voltage range from 500 to 10,000VDC and are available in EIA case sizes from 0603 to 4540. The BME C0G CaZrO3 dielectric enables extremely low ESR, low ESL, high ripple current handling and high dV/dT.
The electronic components specialist also introduced the surface mount KC-LINK 3640 220nF 500V ceramic capacitor, which is made of CaZrO3 dielectric material, resulting in a very low loss solution – ESR values from 40kHz to 1MHz are all below 4mΩ, At around 50kHz it is as low as 2mΩ. Therefore, at 105°C ambient temperature and 0VDC bias, its typical ripple current from 50kHz to 300kHz is about 20A, as shown in Figure 5.
Figure 5: KC-LINK Impedance, ESR and Ripple Current
Even if an application is designed with high performance Class I dielectric capacitors, it is often necessary to provide a higher level of capacitance, which is achieved by increasing the board area. However, traditionally, increasing the board area reduces the power density of the solution. Therefore, KEMET has developed KONNEKT technology, a leadless multi-layer chip solution for high-efficiency, high-density power supply applications, that solves this problem. KONNEKT uses a transient liquid phase sintering (TLPS) process to assemble Type I MLCCs that can be mounted using standard reflow soldering methods. An example is provided in the appendix to illustrate how this technology can provide high power handling capabilities.
Applications such as automobiles and data centers involve large amounts of electricity, and in order to reduce their operating costs, improving energy efficiency is an important consideration in the modern world. While much of the development effort to date has focused on circuit topology and semiconductor performance, passive components such as capacitors can also have a significant impact on power supply efficiency.
Class I materials, including C0G and U2J, have excellent stability in power applications, and because MLCC performance is predictable, designers can achieve precise tolerances. New technologies such as KONNEKT technology can provide large capacitance in a smaller footprint, and therefore can significantly increase power density.