Dielectrics are classified according to their capacitance stability at temperature. Class I contains dielectrics (C0G, NPO, U2J) that are the most stable for temperature, but they have the lowest permittivity (K) and require larger volumes in order to achieve the same capacitance as more traditional types of MLCCS. Class II includes dielectric materials (X7R, X5R) with moderate temperature stability and K values. As a result, Class II provides 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 having a significant impact when using them for power applications.

 

Because Class II MLCCS use BaTiO3 dielectrics, the actual capacitance is affected by the operating temperature, the DC bias applied, and the time that has elapsed since the last heating (aging). The stability of a capacitor to temperature is called the temperature capacitance coefficient (TCC), and it can be determined by the Electronic Industry Alliance (EIA) dielectric classification (e.g. "X7R"). The EIA defines X7R as operating in a temperature range of -55 ° C to 125 ° C with a maximum capacitance limit of ±15%. The X5R has the same ±15% capacitance limit but operates in a temperature range of -55C to 85C. The stability of capacitance to voltage (VCC) is also an important consideration, but it is not formally defined by EIA. However, for Class II MLCCS with higher capacitance, the capacitance can be reduced by up to 80% at rated voltage, which can have a considerable impact on applications. This VCC feature may also vary from vendor to vendor. In addition to temperature and voltage, capacitance is also reduced by the amount of time that has elapsed since it was last heated. This is called aging and typically decreases by 2-5% every ten hours after the last heat above 130℃ (usually when welding parts during manufacturing).

 

However, Class I dielectrics are more stable than Class II dielectrics. Dielectric such as C0G has negligible capacitance drift of only 30ppm/℃ or 0.3% at 125℃, whereas U2J is 750ppm/℃ or 7.5% at 125℃, but is linear and predictable. Both C0G and U2J have negligible capacitance changes 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 it is important to keep capacitors within narrow tolerances.

 

 

In power applications, in addition to capacitance stability, the equivalent series resistance (ESR) is an important feature of capacitors due to i2R losses. Figure 2 compares the ESR of Class II X7R and Class I C0G/U2J MLCC at 100Hz to 100MHz. Because BaTiO3 is a ferroelectric material, it generates domain properties within the dielectric, as well as domain wall heating and ESR increase compared to Class I dielectric. As a result, Class II MLCCS typically have an ESR one or two orders of magnitude higher than Class I MLCCS.

 

                                                                        

                                                                                      Figure 2: Comparison of ESR between Class II X7R and Class I C0G/U2J

 

Because of the larger AC current in power applications, MLCC ESRs are larger and can directly cause overheating. FIG. 3 shows the relationship between temperature and AC current of X7R, C0G and U2J MLCC. Data show that the self-temperature rise of C0G and U2J at 10A is about 15℃, while the temperature rise of X7R reaches 40℃ only at 5A.

                                                                  

                                                                                    

                                                                               Figure 3: Comparison of ripple currents between Class II X7R and Class I C0G/U2J

 

                                                                           

                                                                                Figure 4: Comparison of key features between Class II X7R and Class I C0G/U2J