Active capacitor voltage balancing control of flying capacitor seven-level soft-switching power amplifiers

The flying capacitor multilevel soft-switching power amplifier (FCMLSSPA) has emerged as a critical component for driving the gradient coils in magnetic resonance imaging systems, enabling the delivery of high-precision current. However, the unbalanced characteristics inherent to capacitor voltages significantly compromise the accuracy of the output current. To mitigate this limitation, this paper proposes a novel capacitor voltage balancing strategy, employing a ladder filter inductor current and carrier phase-shift reconstruction. This paper establishes a mathematical model of the power amplifier topology that provides a detailed analysis of the multilevel voltage and inductor current characteristics under various operating conditions. Furthermore, the parameters of the trapezoidal filter inductor current are comprehensively examined. Based on the transition mechanism of the ten states in soft-switching, the six key time domains of the trapezoidal filter inductor current are dynamically allocated based on the current circulation conduction path and the constraint of minimizing the error in the equivalent average current of the trapezoidal filter. Considering the initial value of the flying capacitor, the carrier phase-shift modulation satisfies the initial voltage balance condition. Combined with the dynamically reconstructed trapezoidal filtering current, the stable voltage of the flying capacitor with multiple switching cycles throughout the entire operating range is achieved. A 700-W experimental prototype is implemented to verify the theoretical result.