Abstract – This article presents three topologies of active 3 Phase Power Factor Correction Circuits which satisfy the following basic requirements: (a) near unity Power Factor, (b) Input current harmonics of below 5% THD and 3% SHD, and (c) Isolated DC output from the input utility. The need to satisfy the first 2 requirements is mandatory for compliance to MIL-STD-1399 shipboard power*. The last requirement stems from the application needs. Impinging on the choice of most suitable topology to a given application is considerations of volume, weight and EMC performance. The paper is confined in focus to 3-5kW overall output power. Results presented herein are based mostly on testing of actual hardware.

I. Introduction

Power systems designers familiar with Single Phase PFC circuit are perplexed by the complexity of 3 Phase PFC circuits described in a multitude of technical publication. Moreover they usually remain skeptical of these topologies’ ability to meet the 5% THD and 3% SHD current harmonics. In addition, the isolation requirement complicates matters greatly being that most PFC schemes single or 3 Phase, provides a non isolated output.

A switching power supply driven from single Phase and equipped with PFC front-end achieves the galvanic isolation of input to output by employing a non isolated PFC circuit followed by a high frequency DC to DC converter with galvanic isolation.

However in a case of an AC output power systems such as a Frequency Converter or Uninterruptible Power Supply (UPS), isolation is more difficult to achieve, unless a low frequency transformer is placed either at the input or at the output of the unit.

Any single stage three phase PFC circuit is a quantum jump in complexity compared to a conventional single phase PFC circuit. Therefore it is understandable that a design engineer confronted with the task of 3 Phase PFC will elect to correct the PF of his given 3 Phase input by using 3 individual single phase PFC circuits (or modules), one for each Phase and OR their output into a common DC bus. This however provides no galvanic isolation from input to output.

We shall describe here 3 approaches to the desired outcome: (a) Three non isolated single phase PFC modules buffered from the Delta input by a line frequency power transformer, (b) Three PFC modules with isolated output, Each connected across phase to phase, and the three outputs ORed at the DC bus, and (c) A single stage, 3 Phase PFC circuit connected directly to the Delta input, and followed by a DC to DC converter to achieve isolation. A variation on this theme will be a line frequency transformer with Delta or Star primary, and Delta configured (galvanically isolated) secondary, which drives a single stage, 3 Phase (non isolated) PFC circuit. This combination will achieve all 3 objectives set forth.

All 3 topologies are feasible in the 3-5kW range, and all yield good results. However, each has its own tradeoffs. We constructed and tested all 3 schemes as evidenced by the photographs included herein. The first two are in current production in military systems. The results listed are mostly from actual hardware.