The multi-winding transformer in a Multiport-Ac-Coupled (MAC) converter interfaces with a large number of ac-dc ports and processes multi-way power flow. This paper investigates the "sparse operation" of multi-winding transformers, i.e. when only a few windings deliver power and a majority of the windings remain open-circuited. These open-circuited windings induce additional leakage inductance and skin proximity effects in planar magnetics, change the circuit behavior, and increase the losses. By modeling a ten-winding PCB-embeded planar transformer in SPICE, the impact of "sparse operation" on transformer performance is predicted. To verify the theoretical modeling and simulation results, the performance of a 10-winding planar PCB transformer is experimentally compared to the performance of a 10-winding litz wire transformer in a 10-port MAC converter while transferring power from one winding to another. In the planar transformer, it is observed that windings that are physically closer to each other can transfer more power at higher efficiencies than those that are further, while this trend is not present in the litz wire transformer.