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TIDA-01604: question about AC voltage zero-crssoing

Part Number: TIDA-01604

Hi EXpert,

I have some questions about below description. 

  1. "When the AC input changes from the positive half line to the negative half line at the zero-crossing, theduty ratio of the low-side high-frequency switch SiC2 changes from 100% to 0%, and the duty cycle of SiC1 changes from 0% to 100%. "  Why the PEM duty for SiC is 0% or 100%? I suppose they are not 0 or 100% when normal working. 
  2. "Because of the slow reverse recovery of the high-side diode (or body diode of the MOSFET), the voltage at the cathode of D2 cannot jump from ground to DC+ voltage instantly (this causes a large current spike)." I can understand the issue cause. But I suppose we need to improve D1 and D2 reverse recovery. Why the solution becomes change S1 and S2 to SiC or GaN? Is these related? Thanks.

BR,

Elec Cheng