Power amplifiers have a wide range of applications in civilian and military areas. In this article, a modular multilevel switching power amplifier (MMSPA) with high power and high output performance is investigated. Due to mismatched parameters or proper control signals among the modules, there may be significant temperature differences among the modules of MMSPA under long-term operations. Uneven temperatures among modules directly lead to premature aging and failure, which in turn affects the overall system reliability. Aiming at the thermal stress imbalance among modules of MMSPA, a model predictive control based on the thermal stress balance is proposed. First, the mathematical model is constructed for the MMSPA. Then, the junction temperature estimation method based on the power loss calculation is presented. On this basis, the finite set control switch state optimization and the switch distribution algorithm for thermal balance are presented. Finally, the proposed active thermal control is verified by the simulation and the experimental results.

In the extremely low-frequency electromagnetic wave communication system (ELFEWC), the power amplifier is required to output extremely low-frequency voltages, which makes the junction temperature swing of the power device significantly large. It brings great challenges to the overall reliability of the system. In this article, an active thermal control strategy based on modulated model predictive control (M2PC) is proposed for the extremely low-frequency power amplifier (ELFPA) in ELFEWC to reduce the junction temperature swing of the power device and extend the life of ELFPA. Specifically, the proposed M2PC can guarantee the output accuracy of the power amplifier with the fixed switching frequency. Furthermore, the appropriate switching frequency is selected by considering the tradeoff between junction temperature swing and total harmonic distortion to improve the reliability. The proposed control strategy has the advantages of reducing the junction temperature swing of the power device while meeting the output accuracy. Finally, simulation and experimental results of a 10kW ELFPA demonstrate the effectiveness of the proposed control strategy on the junction temperature swing reduction of the power switch.

The inner winding of the power transformer is subjected to uniform compressive stress in the radial direction during the short circuit, which makes circular radial stability a crucial component of reliability. The radial stability evaluation system currently needs more state characteristic judgment quantity considering the manufacturing deviations(MD) and assembly gaps. Thus, there is a significant deviation in the transformer design, which seriously endangers the operation safety of the power system. This paper proposes an improved radial buckling analysis method (IRBAM) to investigate the circular radial stability of inner winding. The IRBAM is implemented by defining two characteristic parameters: equivalent flexibility and MD. Besides, the relative change ratio of impedance (RCRI) is proposed as the buckling judgment for ending each short-circuit test. The destructive short-circuit tested transformer is designed with the rated power of 50 MVA and the high-voltage (HV) rated voltage of 110 kV. Then, 73 cumulative short-circuit tests are performed until impedance exceeds the permissible value based on the judgment. Finally, it is verified that IRBAM can effectively match the winding failure modes by comparing the buckling phenomena and calculation results. Further, the IRBAM can provide a reference for similar designs and verification with more minor deviations.

The modular multilevel railway power conditioner (MM-RPC) can effectively improve the power quality of the railway power grid. However, since the operating conditions of MM-RPC frequently vary with the traction load, the junction temperatures of semiconductor devices fluctuate very drastically. Severe junction temperature swings can easily affect the service lifetime and reliability of the MM-RPC. In this article, a power hysteresis based on incomplete compensation control strategy is proposed. The proposed control can effectively reduce junction temperature fluctuations under complex mission profiles and increase the overall life of the system while meeting the power quality requirements. Furthermore, the effectiveness of the proposed control is verified in MATLAB/PLECS electrothermal cosimulation under a section of the load curve of the Beijing–Shanghai high-speed railway traction substation. Finally, the experimental thermal platform is built to validate the effectiveness of the proposed control strategy.

High-efficiency high voltage dc power supply is the core equipment for magnetron in the microwave industry. This article presents an input-parallel output-serial modular high isolated resonant converter for the magnetron. However, the high stray capacitance of the high isolation transformer brings some challenges, such as high turn- off current, longer dead time, increasing gain, and transformer optimization. A simplified model including the stray capacitances is built to deal with these problems, and the minimum turn- off current is derived after the detailed time-domain analysis. The appropriate deadtime and the maximum magnetic inductance needed for the proposed converter are obtained with the minimum turn- off current. Then, a 15 kW, 60 kV isolation multiple-windings output transformer is designed and optimized for the converter. The insulation design, core shape, losses, and parasitic parameters are calculated thoroughly. The Pareto optimization process optimizes the switching frequency and the transformer's turns to obtain higher efficiency and a more stable gain. The finite element method and time-domain simulation verified the optimized design results. Finally, a 0.8/2.5 kV, 15 kW all-SiC dc–dc converter module is developed to validate the proposed design. The results indicate that the module efficiency is as high as 98.6%.