An initiator assembly with a housing, an exploding foil initiator, an input charge, which is formed of an energetic material, and a support plate. The exploding foil initiator has a base coupled to the housing, a bridge mounted to the base, a flyer overlying the bridge, and a barrel. The barrel is coupled to the base and defines a channel through which the flyer traverses when the exploding foil initiator is activated. The support plate supports an axial end of the input charge and is interposed in a path of travel of the flyer between the bridge and the input charge. A related method is also provided.