If you mean that you want a solar cell emulator based on a dc-dc converter, then first start by finding an appropriate circuit that gives the correct output I-V. E.g. I think I remember that the following circuit should work for a 20W 12V (19V MPP), where V4 signifies the output. Together, R5 and R1 control the current and voltage.
Once you've found the circuit that gives the response you want, you can add the DC-DC converter, and feed an inverted signal from the emulator output into the DC-DC converter feedback pin.