20 POWER CAPACITORS www.empowersemi.com Issue 4 2023 Power Electronics Europe www.power-mag.com applications become the norm, several factors will limit MLCCs’ abilities for specific applications. There are several parasitic factors that will cause a capacitor’s impedance to change across frequency. The equivalent circuit of a capacitor includes a series resistor (ESR) and series inductance (ESL). For example, the metal electrodes and end caps contribute to an equivalent series inductance or ESL that impacts the capacitor’s resonant frequency. All things being equal, the lower the ESL, the higher the resonant frequency. Above its resonant frequency, a capacitor’s effective impedance becomes inductive in nature, i.e., increases with frequency. There is an imperative to keep ESL as low as possible for capacitors used in high performance, high frequency applications. Another critical issue is capacitor derating. Key de-rating factors that impact MLCCs are voltage, temperature and age. An average MLCC will see its capacitance value reduce as the DC bias voltage increases. Capacitance also reduces as the temperature increases, with the degree of change dictated by EIA (American Electronic Industries Alliance) code used to classify the temperature coefficient. AC or DC bias voltage has an impact on ageing characteristics that produce a decrease in capacitance over time due to changes in the dielectric’s crystal structure. Increasing the number of MLCCs to address these characteristics ensures that the necessary decoupling capacitance is provided over the product’s lifetime and across the entire anticipated range of operating conditions. Accommodating the many MLCCs, however, effects the product’s mechanical design attributes, from form factor and power density to PCB layout flexibility, reliability, and cost. In general, the more capacitors that are deployed, the further away the capacitor network is likely to be from the processor, increasing the series inductance and introducing further opportunities for parasitics, especially at high frequencies. The product’s calculated reliability metric will also be lower because the overall component count negatively influences reliability. Differences between MLCCs and silicon capacitors MLCCs are formed by alternating plates of metal (electrodes) with a dielectric material in between. The long plates connect to the terminals on either side providing contacts to the outside world. Since parasitic inductance is proportional to length of the metal path that current or charge must travel along, the long electrodes within MLCCs lead to inherently higher ESL. Silicon capacitors, like E-CAP, can improve layout density compared to standard MLCCs. Figure 2: Comparison of MLCC nominal capacitance values to achieve a desired effective capacitance goal. (Source: Empower)
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