how can I calcukate a maxumum current that is possible to get with LTC7801? What are thelimitations for MOSFETs?
LTpowerCAD is an excellent tool to estimate power loss in MOSFETs. Output current depends many things including switching frequency and maximum input voltage. Looking at various 60V to 150V Buck controller Data Sheets and Demo Boards (LTC3871/3890/3891/3892/7801/7810, etc.) for 72Vin, output current is typically <20A/phase (for example, two-phase LTC3871 30-72Vin to 12V/30Aout = 15A/phase).