How to reduce the number of nodes in Pspice model

Hi Friends

When I am creating a PI controller for my buck converter,I had a problem when I go for simulation.It shows that "ERROR -- EVALUATION VERSION analog Node Limit (64 Nodes) Exceeded!".
Currently I am using Pspice 9.1

your help will be greatly appreciated

You don't.

You either buy the full program or download some free simulation software which doesn't impose silly restrictions.

EDIT:
Try LTSPICE.

Break the circuit into smaller parts. I dunno about PSpice, but in LTSpice you can write the voltage at any node to a file. That file can be as stimulus as input to the next circuit section being simulated. However, LTSpice has no hardwired node limit, so you dont have to do this very often. Another trick is to use "ideal" models for opamps, which have fewer internal nodes, simulate faster, and dont use use up the limit as fast.

Of course using LTSpice directly solves the problem completely =)

Thanx a lot one and all for your suggection. Now I am working on LT spice.

If any one give me suggestion How to select OP-amp to design PID controller for 5.1 volts to 2.5 volts step down Buck converter??

Your help will be really appreciated.