I read TI's AN SLVA059 & SLVA061. They discussed the ripple voltage during CCM and DCM. I found something is not right. In the application note, they only calcluate the Q during either D1 or D1+D3. They omit the capacitor discarging during D2.  In my opinion, C>=Io*(1+D)^2/(4*f*u) for CCM and C>=Io*(1-sqrt(L/(2*R*Ts)))^2/(f*u) for DCM. Not like what they said, C>=Io*D/(f*u) for CCM and C>=Io*(1-sqrt(2*L/(R*Ts)))/(f*u) for DCM.