Mark Laps, KEMET
Society increasingly relies on electrical power, whether it’s to communicate, provide safe and secure storage for the vast amount of data generated, or for transportation in electric, and hybrid-electric vehicles. As a result, the availability of electrical power is crucial to the enjoyment of modern lives. One of the most pressing topics is energy efficiency – driven in part by the cost of electricity which is on the rise, as well as the desire to preserve the natural resources from which electricity is generated. Efficiency is one of the most important considerations when moving to new energy sources such as solar, wind, or when considering cooling requirements of electronic circuits.
Engineers worldwide are constantly seeking ways to make devices more efficient. Approaches include advanced circuit topologies such as resonant converters, intelligent power management and the adoption of new materials. In the world of power semiconductors, wide bandgap (WBG) devices are starting to gain traction, allowing power conversion devices to operate at higher frequencies, higher temperatures and higher voltages. As switching speeds increase, so the size of key components such as capacitors and magnetic devices can decrease, delivering greater power density at higher power conversion efficiency.
Much of the focus on increasing efficiency and power density has been directed towards the switching semiconductor devices as these contribute significantly towards the static and dynamic losses in any power system. However, the small incremental improvements become ever more challenging and expensive to achieve, so engineers are looking elsewhere for efficiency gains.
Many engineers see capacitors as simply supporting devices in power designs, but an increasing number are understanding the potential that they have to improve efficiency and, as a consequence, power density. There are three areas in power design where capacitors can positively impact the efficiency of the system, each with slightly different requirements for the capacitor.
First, snubbers can require high dV/dTs, high ripple currents, high voltages, and high temperatures as well as low inductance. Second, DC-LINK requires high ripple current, voltages, temperatures and frequencies. Third, resonant converters need high ripple currents, a wide operating voltage range, and capacitance stability over temperature, DC and AC voltage. Considering the combined requirements of these applications defines a capacitor with very low loss, high ripple current handling capabilities, the ability to withstand high voltages and accept higher operating temperatures while exhibiting stable capacitance and high mechanical stability. To achieve the high density and efficiency power supplies using WGB semiconductors it is important that the capacitors in these packages have high temperature and mechanical stability.
Class I vs Class II Multilayer Ceramic Capacitors (MLCCs)
Among the many types of capacitors available, ceramic capacitors – particularly multilayer MLCCs – can exhibit key properties that are ideal for snubber, DC-LINK, and resonant applications. MLCCs are formed by alternating layers of metal electrodes and ceramic dielectric. Each layer represents an individual capacitor and adding layers provides additional capacitance since they are in parallel. The vast majority of MLCCs produced today are Base Metal Electrode (BME) which have Nickel metal electrodes and a CaZrO3 dielectric for Class I or BaTiO3 dielectric for Class II.
Dielectrics are classified by their capacitance stability over temperature. Class I contains the most temperature stable dielectrics (C0G, NPO, U2J) although these exhibit the lowest dielectric constant (K), requiring a greater volume to achieve the same capacitance as more traditional MLCC types. Class II includes dielectric materials (X7R, X5R) that have mid-range temperature stability and values of K. So, Class II MLCCs will provide a much higher capacitance per unit volume as compared to Class I. Although Class II MLCCs have much higher bulk capacitance, there are some key design considerations that engineers must understand that can drastically influence their use in power applications.
Since Class II MLCCs utilize a BaTiO3 dielectric, the actual capacitance can be affected by the operating temperature, applied DC bias, and time after last heat (aging). The stability of capacitance versus temperature is called Temperature Coefficient of Capacitance (TCC) and can be determined by the Electronics Industries Alliance (EIA) dielectric classification such as “X7R”. The EIA definition of X7R is an operating temperature range of -55oC to 125oC with maximum capacitance limits of ±15%. X5R has the same ±15% capacitance limits but with an operating temperature range of -55oC to 85oC. Stability of capacitance versus voltage (VCC) is also an important consideration but has no formal EIA definition. However, for higher capacitance Class II MLCCs, users can see a decrease in capacitance as much as 80% at rated voltage which can have a considerable impact on the application. This VCC characteristic can also vary widely from vendor to vendor. In addition to temperature and voltage, capacitance can also decrease due to time after last heat. This is called aging and is usually in the range of 2-5% per decade hour after the last heat above 130oC – typically when soldering the parts during the manufacturing process.
Class I dielectrics however are much more stable compared to Class II. Dielectrics such as C0G have a negligible capacitance shift of 30ppm/°C, or 0.3% at 125oC, while U2J has 750ppm/°C or 7.5% at 125oC but is linear and predictable. Both C0G and U2J have negligible capacitance change vs DC bias and almost no change vs time (aging). These properties make Class I dielectrics ideal for resonant applications such as LLC resonant converters and wireless charging circuits where it is important to retain capacitance within narrow tolerances.
Equivalent Series Resistance
In addition to capacitance stability, Equivalent Series Resistance (ESR) is also an important characteristic for capacitors in power applications due to i2R losses. Figure 2 shows an example of ESR for a Class II X7R versus Class I C0G/U2J MLCCs from 100Hz to 100MHz. Since BaTiO3 is a ferroelectric material, its ability to create domain regions within the dielectric also causes domain wall heating and higher ESR compared to Class I dielectrics. Therefore, it’s common to see between one and two orders of magnitude higher ESR for Class II versus Class I MLCCs.
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Figure 2: ESR Comparison Between Class II X7R and Class I C0G/U2J
The direct result of high ESR in MLCCs is excessive heating due to high AC current in power applications. Figure 3 shows temperature vs AC current for X7R, C0G, and U2J MLCCs. The data shows that both the C0G and U2J see a self-temperature rise of approximately 15oC at 10Arms whereas the X7R increases by 40oC with just 5Arms.
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Figure 3: Ripple Current Comparison Between Class II X7R and Class I C0G/U2J
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Figure 4: Key Characteristics Comparison Between Class II X7R and Class I C0G/U2J
Class I technologies in action
Since Class I BME dielectric based MLCCs have high temperature stability, low loss and high ripple current capability, they clearly stand out as the ideal choice for high power density applications. KEMET has created product sets using patented Class I BME CaZrO3 dielectric technology to further increase power handling capability that is targeted towards snubber, DC-Link, and resonant applications.
Such products include a C0G High Voltage Commercial and Automotive Grades series which offers a wide voltage range from 500 – 10,000VDC with EIA case sizes from 0603 to 4540. The BME C0G CaZrO3 dielectric enables extremely low ESR, low ESL, high ripple current handling capability and high dV/dT.
The electronic components specialist has also introduced the surface mount KC-LINK 3640 220nF 500V ceramic capacitor utilizing a CaZrO3 dielectric material which creates a very low-loss solution with ESR values below 4mWfrom 40kHz to 1MHz and as low as 2mWat around 50kHz. This allows for typical ripple currents of approximately 20Arms from 50kHz to 300kHz at 0VDC bias at 105°C ambient, as shown in Figure 5.
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Figure 5: KC-LINK Impedance, ESR, and Ripple Current
Even with high performance Class I dielectrics, applications often require higher levels of capacitance, which requires increased board area to achieve. However, increasing board area traditionally reduces the power density of the solution. KEMET has developed KONNEKT technology – a leadless multi-chip solution design for high efficiency and high-density power applications – to address this problem. KONNEKT uses transient liquid phase sintering (TLPS) process to combine Class I MLCCs which can be mounted using standard reflow practices. An example of how this technology is providing high power handling capability is provided in the box out.
Energy efficiency is an important consideration in the modern world as it reduces operating costs for significant power usage including automobiles and data center applications. While most of the development effort to date has focused on circuit topologies and semiconductor performance, passive components such as capacitors can have a significant impact on power efficiency.
Class I materials, including C0G and U2J, show excellent stability in power applications, and since MLCC performance is predictable, designers can achieve fine tolerances. Novel techniques such as KONNEKT technology can deliver large capacitances in small footprints which contributes significantly to improving power density.