How does MLCC DC bias characteristic affect a power system?
The Multilayer Ceramic Capacitor (MLCC) DC bias characteristic refers to the capacitance change of the ceramic capacitor with the applied DC voltage.
As the applied DC voltage increases, the ceramic capacitor’s effective capacitance decreases. Figure 1 shows the relationship between the rate of change in capacitance and a range of DC bias voltages for 22-µF ceramic capacitors of different sizes with voltage rating of 6.3 V.
Figure 1: Relationship between the rate of change in capacitance and a range of DC bias voltages
DC bias characteristic of MLCCs of different sizes
As applied voltage increases to 3 V, the effective capacitance of a 0603-sized capacitor decreases to 40% of its nominal value while a 1206-sized capacitor’s effective capacitance is roughly 80% of its nominal value. It is clear that a larger ceramic capacitor has less capacitance change under the same DC bias voltage comparing to smaller ones.
Figure 2 illustrates MLCC (22 µF) DC bias characteristics with the same 1206 size but different rated voltages. It is obvious that rated voltage does not affect MLCC’s DC bias characteristic. Generally, using an MLCC with higher rated voltage does not necessarily improve its DC bias characteristic.
Figure 2: MLCC (22 µF) DC bias characteristics
DC bias characteristic of MLCCs with different rate voltages
For MLCCs used in power systems, its effective capacitance might decrease significantly based on the input and output voltage. This might cause effective input and output capacitance to fall short of the required value, which leads to large switching voltage ripple and low phase margin. Therefore, when designing power systems, it is necessary to confirm MLCC’ effective capacitance based on DC bias characteristic provided by vendors to fulfill the design requirements.
(All figures are taken from SimSurfing by muRata Corporation.)