MC33Switching:

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   3-Phase to 3-Phase Matrix Converter Switching Functions.
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   ------
   Inputs
   ------
   Vin         (3,1)  Input voltage in abc reference frame variables
   Vref        (3,1)  Reference voltage specification: [magnitude (Volts);
                                                       frequency (rad/s);
                                                       phase (rad)]
   t           (1,1)  Time (s)

   -------
   Outputs
   -------
   S           (3,3)  Switching function matrix
                      [S1  S2  S3;
                       S2  S3  S1;
                       S3  S1  S2]

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   Reference:  Nikkhajoei et al "Dynamic Model of a Matrix Converter for
               ControllerDesign and System Studies", IEEE Transactions on
               Power Delivery, vol. 21, no. 2, April 2006, pp 744-754
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