TM81 is a wide-temperature mid-frequency low-loss MnZn power ferrite with an initial permeability (μi) of 3000, a saturation flux density (Bs) of 520 mT at 25 °C (420 mT at 100 °C) and a Curie temperature above 220 °C. Its loss characteristic is optimised for 300 kHz / 100 mT: 260 kW/m³ at 25 °C and 310 kW/m³ at 100 °C, with 230 → 270 kW/m³ at 200 kHz / 125 mT; the difference between the two temperatures remains within 20 %, so cold and hot-state behaviour are consistent. At 100 °C the remanence (Br) is 50 mT and the coercivity (Hc) 7 A/m, keeping hysteresis loss low. At 100 kHz / 200 mT it reads 340 → 400 kW/m³, comparable to TM80 with a slightly higher 100 °C value. Typical applications are 200–300 kHz LLC resonant and half-bridge transformers, server and telecom supplies and wide-temperature PFC front ends. Among the six mid-frequency grades TM81 is the wide-temperature choice for the 300 kHz region: TM80 (330 kW/m³ at 100 °C) is recommended for 100 kHz high-flux operating points and TM83 where the core remains at 120–140 °C. Selection should be based first on operating frequency and flux swing, then confirmed against the actual core temperature.