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II. Transformer – Non Isolated PFC Modules

In this topology as shown in Fig (1), three non isolated PFC modules are connected with their output ORed together to form a DC bus. These modules are buffered from the input Delta utility by a power transformer which has a Delta configured primary, and 3 separate galvanically isolated secondaries. This topology offers excellent advantages in terms of simplicity, flexibility and efficiency. The topology is also not limited in power and may be employed in any desired power level.
* MIL-STD-1399 requires only 0.8PF lagging to 0.95 leading for 60Hz input, and 0.8 lagging to utility for
400Hz systems. However it is easy and highly desirable to achieve near unity PF.




Fig.(1) Topology (a). Power transformer feeds 3 non isolated PFC modules with ORed output.

 

Consider the following:

(a) Existing Single Phase PFC circuits or modules may be already available in the company and can readily be used to implement a 3 Phase PFC function. This simplifies the task greatly.

(b) As per Fig. (1) the secondary voltage of the transformer (Vs) is left to the designer to determine, and thus it may be selected as high as Vo/1.4 in order to minimize the boosting action of the PFC, and thus achieve higher efficiency. For instance, if Vo (the DC bus voltage) is 250V, Vs may be chosen as high as 170VRMS. However there are some disadvantages to working at such high ratio as we shall see.

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