determining load value for mppt buck converter

hey guys i have a question which may be simple however it confused me. i am trying to build a simulink project about MPPT on PV array. i want to use Buck converter and a algorithm in MPPT to adjust duty cycle of converter. i know the buck converter adjust output voltage by changing duty cycle to adjust output impedance seen by PV panel to draw maximum power. i use matlab 2016a and because library has a predefined PV array (which constantly gives algebraic loop error thanks to designers :| ) i choose it for my project. the problem i have is to set a value for output load (resistance) to run. i mean the original buck converters have not a resistor inside but i have to use a resistance to draw current. if i choose low value for R, duty cycle reaches to 1 before desire value for output voltage. if i choose high value for R, current through R is way below than desire output current.