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PMP3162: Purpose/sizing of R3

Part Number: PMP3162
Other Parts Discussed in Thread: UCC2897A

In another thread, R3 is mentioned as a sense resistor, but I'm unclear as to how it's used here (also new to sense transformers in general).  Shouldn't all of the power transformer's current flow through the current sense transformer's (CST) primary?  R3 sits across the primary of the CST, which can only serve to provide an alternate path for current.  R6 provides for the transformer reset, and R5 provides for the sense voltage on the UCC2897A.

Without understanding how it's being used here, I'm unsure how to calculate the proper value.