EPCOS B66423 EFD 30/15/9 Transformer Ferrite Cores B66423 EFD 30/15/9 transformer ferrite cores from EPCOS are ideal for DC-DC converters. These are E cores with a lower centre leg that has been flattened, and are suitable for flat transformer designs. Ungapped and gapped versions are available, and these cores are offered with construction from either N87 or N97 ferrite material.