An Innovative Approach to Achieving Single Stage PFC and Step-Down Conversion for Distributive Systems

In most modern PFC circuits, to lower the input current harmonics and improve the input power factor, designers have historically used a boost topology. The boost topology can operate in the Continuous Conduction Mode (CCM), Discontinuous Conduction Mode (DCM), or Critical Conduction Mode. Most PFC applications using the boost topology are designed to operate over the universal input AC voltage range (85−265 Vac), at 50 or 60 Hz, and provide a regulated DC bus (typically 400 Vdc). In most applications, the load can not operate from the high voltage DC bus, so a DC−DC converter is used to provide isolation between the AC source and load, and provide a low voltage output. The advantages to this system configuration are low Total Harmonic Distortion (THD), a power factor close to unity, excellent voltage regulation, and fast transient response on the isolated DC output. The major disadvantage of the boost topology is that two power stages are required which lowers the systems efficiency, increases component count, cost, and increases the size of the power supply. ON Semiconductor’s NCP1651 (www.onsemi.com) offers a unique alternative for Power Factor Correction designs, where the NCP1651 has been designed to control a PFC circuit operating in a flyback topology. There are several major advantages to using the flyback topology. First, the user can create a low voltage isolated secondary output, with a single power stage, and still achieve a low input current distortion, and a power factor close to unity. A second advantage, compared to the boost topology with a DC−DC converter, is a lower component count which reduces the size and the cost of the power supply. Traditionally, the flyback approach has been ignored for PFC applications because of the perceived limitations such as high peak currents and high switch voltage ratings. This paper will demonstrate the novel control approach incorporated in the NCP1651 design, coupled with advances in discrete semiconductor technology that have made the flyback approach very feasible for a range of applications. Controller Analysis The NCP1651 can operate in either the Continuous or Discontinuous mode of operation. The following analysis will help to highlight the advantages of Continuous versus Discontinuous mode of operation. The table below defines a set of conditions from which the comparison will be made between the two modes of operation.