Active Thermal Control for Active Neutral-Point-Clamped Converters with Hybrid Modulation

Thermal stress imbalance among power devices is a major factor limiting the reliability and power density of three-level active neutral-point-clamped (3L-ANPC) converters in more electric aircraft applications. To address this issue, this paper proposes an active thermal control method for 3L-ANPC converters based on discontinuous pulsewidth modulation (DPWM) and hybrid switching frequencies. The proposed method exploits the opposite temperature characteristics of the outer devices and the inner devices under different switching-frequency conditions. A Foster thermal model is employed to estimate the junction temperatures of the IGBT and the freewheeling diode, and the switching-frequency state is selected according to the thermal-imbalance variable between the outer and inner devices. Based on the thermal crossover point of the selected IGBT module IKFW40N65ES5, 30 kHz is chosen as the boundary frequency, while 20 kHz and 50 kHz are selected as the low and high-switching-frequency states, respectively, under the combined constraints of thermal redistribution and total harmonic distortion (THD). Simulation results show that the proposed method effectively redistributes thermal stress during switching-frequency transition and under load-step operation. The temperature difference among the power devices is limited to within 0.5 °C, while the output THD remains below 5%. These results verify the effectiveness of the proposed method in alleviating thermal imbalance while maintaining the required output power quality.