Solution
The ripple voltage across the capacitor (neglecting parasitic inductance) is a vector sum of voltage components across the capacitor's resistance and capacitance. The capacitance selected for overshoot is almost always enough so the output ripple is primarily due to the ESR (Equivalent Series Resistance) of the capacitor. This then provides a starting point for optimizing the filter design, taking in all the other design criteria that has to be met (i.e. overshoot, ripple, attenuation of input noise, bandwidth, component stress, discontinuous current boundary, stability, Middlebrook criteria, and interaction with other reflected filters). Notice this is much different then the algorithm used in most text books and application notes that use break frequency and output ripple to determine output filter values. The main thing to remember is that either algorithm is just a starting point to get to an optimum filter (one that meets performance criteria in the smallest size, weight, or cost, or acceptable combination). The algorithm I mostly use is more complex than either of these and starts with the converter power level, conducted EMI specification, and a mockup of physical components. These are used to design the input filter, and then design the output filter to "fit" the reflected input filter with acceptable overshoot. Mockups of the package and components are continually used. Then the optimization starts with this output filter, which may affect the design of the input filter. By using impedance paper to do the preliminary design, this goes very rapidly. By using physical mockups, there are no packaging or manufacturing surprises, important because filters are a major contributor to the size and weight of a switching-mode power supply. Do not use this information for design without independent verification of the information. Editor: Jerrold Foutz |
