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Rogowski Switch-Current Sensor Self-Calibration on Enhanced Gate Driver for 10 kV SiC MOSFETs

enhanced gate driver
Fig. 1. 10 kV SiC MOSFET enhanced gate driver with indicated constitutive parts.
  Modular multilevel converters (MMC) are increasingly considered in medium-voltage (MV) applications due to features such as modularity, voltage scalability, and transformer-less operation. If the power cell's kernel piece is SiC MOSFET, design will be able to meet high-density and efficiency. The pinnacle of SiC technology is the 240 A, 10 kV SiC MOSFET XHV-6 module. For switching cycle control of MMC requirement is accurate analog information. Thus, a high-bandwidth, Rogowski current sensor (RSCS) is developed and integrated on the enhanced gate driver (eGD) to serve as peak-current-mode controlsensor, short-circuit detector, and as a phase-current sensor by sampling the switch current (prototype shown on Fig. 1.). Each switch position requires 3 RSCSs.
  Due to known non-idealities of the OpAmp integrator circuit such as input offset voltage and input bias current, an offset calibration circuit is necessary for successful integrator use. Mechanical potentiometer is not ideal due to temperature swings and mechanical parts that reduce usable lifetime. Generally, electrical parts are more reliable and have longer lifetime. Therefore, digital potentiometer instead analog is proposed. Calibration circuit with ADC, FPGA, and DPOT is implemented during start-up, shown on Fig. 2(a) for one out of three RSCS. SRST sequence is first initiated. Due to existence of non-idealities we will see ramp increasing until the reset occurs. Towards the end of the SRST enable, FPGA will initiate ADC to sample. If sampled value is higher than maximum allowable error, FPGA will reconfigure the digital potentiometer (DPOT), thus reducing the influence of non-idealities in the next cycle. This process will be repeated until the error becomes satisfactory. Verification of the implemented algorithm is shown at Fig. 2(b). Good resolution of the system is achieved, with maximum error of the system of ± 2.5 A. Self-calibration process is finished relatively quickly during start-up, within 12.4 ms maximum.
Rogowski switch-current sensor fundamentals
Fig. 2. (a) Rogowski switch-current sensor fundamentals and digital offset calibration, (b) Self-calibration experimental results at startup of GD.

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