I'm trying to get a 350V DC output from a 24V supply,but my output doesnt go higher than 24V. Could someone explain where im going wrong. This is the first time im doing something like this.View attachment 366169

I just made another simulation based on your model:

 

X3 drain voltage (Green)
X1 drain voltage (Blue)
Output voltage (Red)

Are you showing that the Green MOSFET is working and the Blue one is not for most of the picture?

 

Hi Peter,
Welcome to AAC.
This is a breakdown of the LTSpice simulation circuit you posted.
What is the specified Voltage Output requirement for your project?
E

 

See. 300 V rectified cannot be obtained with 150‑volt diodes! Also, when the coupling coefficient is less than 1, a high voltage develops across the transistor drain terminals.