the output in buck converter multi sim 14 is too low
it is required to use UC3842 to make a Buck converter that has 68V
input and 29 output.

Read the &quot;featured&quot; data sheet: -

Your supply is 68 volts and it will blow the back off the device (and probably the front). Also, it is intended only for low power buck operations without a transistor but, even if it could support an output transistor (for buck operations), using an N channel MOSFET as a source follower is missing the point entirely.