How to design open circuit and short circuit fault in switch of buck converter in simulink/matlab?

I am working on the project in which i am controlling the switch (MOSFET) of buck comverter to get desired output. But now i want to create the fault at the switch like open circuit and short circuit fault for certain period of time in the switch. So, how i can design such faults in switches in simulink?

[KALYAN ACHARJYA]

Basicly you want randomly change the duty cycle of MOSFET output?

[Nikita Joshi]

SIr i need some additional input with my running duty cycle for mosfet which can creat such open and short circuit fault in the mosfet.