Duty Ratio Stability and Average Steady-State Error of Two-Level Current-Mode Wide-Bandwidth Switching Power Amplifiers

Nowadays, high-frequency switching power amplifiers (SPA) have been studied widely; bulk of the researches were focused on audio power amplifiers. In active magnetic bearing (AMB) systems, high-frequency electromagnetic excitation caused by rotor imbalance can affect the performance of the SPA, so the traditional SPA cannot meet the requirements of the AMB systems. Therefore, on the premise of ensuring larger power, the frequency of the SPA should be improved to ensure the normal operation of the AMB system. Two-level pulsewidth modulation (PWM) current-mode SPAs are widely used in AMB systems. In order to broaden the bandwidth and to reduce the average steady-state error current, SiC power devices based SPA with a high bus voltage and a high switching frequency was designed. The basic principle and the linearized control model of SPAs are introduced. Then, the duty ratio stability and average steady-state error current of the SPAs are analyzed based on the output ripple current. When the PWM modulator is designed by either the discrete devices or the integrated chips, the critical gains to ensure the duty ratio stability are, respectively, analyzed based on the slope matching and the nonideal characteristic of operational amplifiers. Hence, two methods to reduce the average steady-state error current are proposed, respectively. One is to adjust the offset voltage of triangular carrier for the P controller, the other is to use a proportional integral (PI) controller. The offset adjusting method allows the SPA to achieve better dynamic performance and wider bandwidth. The proportional integral (PI) controller can reduce the average steady-state error current but narrows the bandwidth. Finally, the experiment results are in good agreement with the theoretical analyses.