240V AC to ~380V DC converter at 3500 continuous watts
The formula for a capacitor is I = C dU/dt . If you want the ripple to be less than say 10V all the symbols are known allowing you to calculate the capacitor. Because I don't have a calculator here, 3500W/about 300V is about 10 A. DT is 10ms, so you get: C = I dt/dU = 10A.10ms/10V = 10mF = 10000uF. Quite a lot for such a large voltage.....