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A Distributed Hierarchical Digital Control System for Medium-Voltage Modular Converters Enabling Peak Current Mode Control for Parallel Operation

Hierarchical digital controller
Fig. 1. Hierarchical digital controller of modular converter operating PCM.
  One of the key characteristics of modular converters is scalability, which requires precise parallel operation of multiple modular converters with current mode control. Furthermore, a well-structured digital control scheme allows for the programmabil- ity of the modular converter to achieve different functions and thus must be carefully defined. This paper introduces a distributed hierar- chical digital control system for medium-voltage modular converters using gate drivers for PWM modulations. The gate driver, which features a PCB-embedded Rogowski current sensor and local FPGA with peripherals, is capable of directly sensing the switching currents of SiC devices and locally implementing PWM modulations. To provide an application example, a modular converter system is constructed and operated as paralleled-synchronous buck converters with peak current mode control.
  As shown in Fig. 1, for each converter in parallel operation, current loop reference is generated from the controller based on output voltage sensing information and is transmitted digitally through an optical cable to the gate driver, which further increases the noise immunity of the modular converter. After deducting external ramp values in the local FPGA for stability consideration, the current reference is sent to the digital-to-analog converter (DAC), and the DAC finally generates the analog signal to be compared with the sensed top switch current from the Rogowski coil sensor directly. In this way, not only is the modulation delay for the current loop minimized, it also helps to eliminate the need for extra current sensors in the modular converter and thus demonstrates the potential for a more compact design. The schematic diagram for the medium-voltage modular converter in parallel operation is shown in Fig. 2. The output inductor currents, labeled as iL1 and iL2, are captured. It can be seen from the experimental waveforms that good load current sharing between the paralleled modular converters is achieved.
Current sharing performance
Fig. 2. Current sharing performance of modular converter in parallel operation with PCM.