EPCOS B66421 EFD 25/13/9 Transformer Ferrite Cores. B66421 EFD 25/13/9 transformer ferrite cores from EPCOS are ideal for DC-DC converters. Featuring an E core with a lower centre leg that has been flattened, these ferrite cores are suitable for flat transformer designs. Ungapped and gapped versions are available, and these cores are offered with construction from N87 or N97 ferrite material.