A modular multi-level converter connects several half bridges or full bridges in series .
When connecting the modules in series, you can either phase-shift the PWM signals for each module or level-shift as used here. This reference application uses level-shifting to balance capacitor voltages more easily. This minimizes component degradation and improves the overall quality of the voltage and current response.
As the number of modules in an MMC increases, you can take advantage of the Simulink API in MATLAB to programmatically build your architecture. For example, when you use more cells, leveraging a converter library can help you to model more efficiently and avoid errors when building models manually.