SLUSBL5A February 2015 – June 2019 UCC28730
Bulk capacitance may consist of one or more capacitors connected in parallel, often with some inductance between them to suppress differential-mode conducted noise. EMI filter design is beyond the scope of this procedure.
Determine the minimum voltage on the input capacitance, CB1 and CB2 total, in order to determine the maximum NP to NS turns ratio of the transformer. The input power of the converter based on target full-load efficiency, minimum input RMS voltage, and minimum AC input frequency are used to determine the input capacitance value.
Maximum input power is used in the CBULK calculation and is determined by the VOCV, IOCC, and full-load efficiency targets.
The below equation provides an accurate solution for input capacitance needed to achieve a minimum bulk valley voltage target VBULK(min), accounting for hold-up during any loss of AC power for a certain number of half-cycles, NHC, by an AC-line drop-out condition. Alternatively, if a given input capacitance value is prescribed, iterate the VBULK(min) value until that target capacitance is obtained, which determines the VBULK(min) expected for that capacitance.