EPCOS B66363 ETD 39/20/13 Transformer Ferrite Cores B66363 series ETD 39/20/13 transformer ferrite cores from EPCOS are produced to IEC 61185 standards. B66363 transformer ferrite cores are ideal for switch mode power supplies and at small volume offer optimum weight to performance ratio. Available in gapped or ungapped varieties, you can also choose from N27, N87 or N97 ferrite material.