One of the most undesirable phenomena experienced by utility companies is the high harmonic content of the line current. The harmonic content of this line current tends to cause transformer overheating at the substations, which are responsible for providing power to all sectors of a given area. For three-phase distribution, neutral currents will flow in the presence of these harmonics. For single- phase distribution, the narrow conduction angle demanded by capacitive input filters in switching power converters causes high harmonic content in the current waveform, resulting in lower operating efficiency.
A viable solution to this problem is the inclusion of a power factor correction (PFC) stage to facilitate more efficient power usage as well as lowering the harmonic content of the line current. The most popular topology for this task is the switched-mode boost converter. Here the boost converter stage is inserted between the input rectifier and the bulk storage capacitor. This forces the input current to be in phase with the input voltage and provides a boosted D.C. voltage reservoir for the
following power stage.
Micro Linear‘s ML4824 Combo Controller IC is an integrated solution for systems benefitting from the advantages made possible by synchronizing the 2 cascaded power stages. In this application note the differences between the traditional combo modulation scheme (unsynchronized or trailing edge modulation) and the ML4824’s leading/trailing edge modulation scheme are explained. A typical application is shown and test results are compared with the traditional approach. Then, a detailed look is taken inside the ML4824 and key design formulas are reviewed which will enable users to begin their own design.
For further information, view application note online: ML4824 (pdf | 12 pages)
