4 Comparison Between DCM and CCM Output Voltage Ripple in D-CAP Buck Converter

The comparison between DCM ripple and CCM ripple is often mentioned. From bench test results, DCM ripple is higher than CCM ripple but few materials give fundamental analysis on it. This section will analyze why DCM ripple is higher than CCM ripple in D-CAP buck converter.

Figure 4-2 illustrates inductor current in DCM operation and CCM operation. Based on above analysis of DCM ripple calculation, excessive charge flowing into output capacitor, which is area of shadow marked triangle, determines the value of output voltage ripple.

Figure 4-1 Inductor Current Illustration for DCM Operation

Figure 4-2 Inductor Current Illustration for CCM Operation

Time period for one pulse (T1+T2+T3 and T11+T12+T13, shown in Figure 4-2 and Figure 4-2) is same in CCM and DCM operation, which is show in Equation 10.

Meanwhile, inductor rising slew rate and falling slew rate are same in CCM and DCM operation, meaning the ΔIL is same as well. For CCM operation, excessive charge is ΔQ2, which is achieved by using Equation 12,and its loading ILoad2 equals to 0.5ΔIL. While for DCM operation, excessive charge is ΔQ1 and it could be calculated by Equation 11.

For DCAP control, converter enters DCM operation once loading is lower than 0.5ΔIL, so ΔIL-ILoad1 > 0.5 ΔIL, leading to ΔIL-ILoad1 > ILoad2.

Based on calculations mentioned in previous section, since ILoad2 is higher than ILoad1, the rising time and falling time of ILoad2 is higher than that of ILoad1, leading to following comparison results:

From previous analysis, it is clear that ΔQ1 > ΔQ2, meaning more energy is charged to Cout in DCM operation. So DCM output voltage ripple is higher that in CCM operation in D-CAP buck converter.