Your question has ambiguity. If you are asking about mppt loop then it depends on what type of mppt you want to implement. For example in voltage based perturb and observe method, P&O is used to generate Maximum power point voltage reference which is further regulated by PI controller to regulate the output voltage of the PV according to the MPPT desired power level.

But if you are asking about how to further generate pwm signal then the output of your PI should be compared to a sawtooth signal. In simulink you can just input the output of PI to the DC-DC PWM block and feed the output to the switch of boost/buck converter.

If you still have questions follow these papers for details.

Article An Advanced Two-Stage Grid Connected PV System: A Fractional...

Conference Paper Particle Swarm Optimization Based DC-Link Voltage Control fo...

I think you might as a first step:

1- calculate it between the relationship the rated output power from the DC-DC converter and the input voltage

2- decide the MPPT algorithm and implement it, because it will produce the dc variation if any variation in the input is produced

3- test it under the scenario choosen by you

Best wishes,

this paper may be useful for you Chapter Bio-inspired Approximation to MPPT Under Real Irradiation Conditions