8.2.2.4 Flying Capacitor

Flying capacitor selection must meet criteria related to current ripple and voltage ripple. Just as the input capacitor, the flying capacitor should also have enough ripple current rating to absorb the RMS current through it:

Equation 4.

This function becomes maximum when D = 0.5, because at that point the capacitor is in series with the inductor for a complete switching cycle, and their RMS currents are the same. The flying capacitor should be sized to handle the full charge current in the scenario where D = 0.5.

Figure 47. Flying Capacitor RMS Current vs. V_BUS with Fixed V_BAT and I_CHG

Additionally, the flying capacitor voltage ripple should be kept under 10% of V_BUS/2 to ensure loop stability. This quantity is given by the following equation:

Equation 5.

It is recommended to use at least two 16-V, 10-μF, low ESR ceramic capacitors in parallel to achieve both RMS current rating and maintain voltage ripple <10% in the flying capacitor for up-to 6-A charge current application. The following curve shows what the ripple voltage of CFLY might look like for such a configuration by taking into account voltage derating of the capacitor and plugging the effective capacitance value into equation above at different charge currents and V_BUS voltages.

Figure 48. Flying Capacitor Ripple Voltage vs. V_BUS with Fixed V_BAT