Besides the choice of AC flux density (ΔB) with L_M and N_P , the core loss of the transformer can also be significantly reduced by a proper selection of the magnetic core material. For converters operating at full-load switching frequencies up to 250 kHz, ferrite materials such as 3C97 and 3C98 (by Ferroxcube) exhibit low core loss density. For converters operating at full-load switching frequencies over 400 kHz, materials such as 3F36 from Ferroxcube and N49 from TDK/Epcos exhibit low core loss density. Other ferrite materials with equivalent or similar loss characteristics may also be used. Litz wires are recommended for both primary and secondary windings, in order to reduce the R_AC winding loss caused by the proximity effect and the skin effect of the transformer windings.