E Series Ferrite Core for Switching Power Supplies TDK E Series Ferrite Cores for Switching Power Supplies are available in sizes from 8mm to 60mm and have a -30°C to +105°C temperature range. This series features a shape that conforms to JIS and IEC standards and has a standard form for use in the most commonly used transformers. The E Series from TDK is ideal for electronics, power adapters, and coils for chargers.