Square-Wave Current Control Optimization Method of Variable Frequency Modulation Inverter for Two-Stage HMI Digital Ballast

When it comes to low-frequency-square-wave ballast for HMI lamps, the nonlinear volt-ampere characteristic of the lamp has introduced disturbance to the ballast and thus imposed great difficulty on current control. In this paper, an optimization algorithm based on PI controller is proposed. Building on volt-second principle of the inductor, base duty is added to balance the disturbance in lamp voltage. Analyzing the inductor voltage equation, feedforward duty is added to speed up transient process. Combining these compensation objects with conventional PI control, the algorithm shows high stability facing the nonlinear lamp characteristic. To confirm the effectiveness of the proposed algorithm, a 9kW prototype is built and tested. Perfect square-wave output current is achieved in wide frequency range (50Hz to 1kHz), while rising edge takes up less than 100 mu s.