PQ Ferrite Cores for Switching Power Supplies TDK PQ Series Ferrite Cores for Switching Power Supplies are available in TDK's original shapes and feature a -30°C to +105°C operating temperature range. Compared to E-core and EER-core, the PQ-core occupies a smaller mounted area as a transformer. TDK PQ Series Ferrite Cores are ideal for applications with high mounting density or low-profile requirements.