I'm designing a buck system using TPS40304. When input and output are given, an automatically circuit will be generated and I can see these functions as the Fig1 below. 
However, after I modified some of those components, I can only see these functions as Fig2. Where are the left gone and how can I judge the loop stability of the whole system?
Hongjia,
You have used the Schematic Editor to create a copy of the original design. This editor should only be used if you need to change the topology of the circuit.
If you only want to change component values, you should used the BOM or the schematic. You can select the new values from the Select Alternate Component list:
You can change component values and still use the full feature set of WEBENCH® Power Designer. The Schematic Editor allows you access to a more fully features simulator, but you lose the access to the built in WEBENCH® features.
You can also only simulate a startup simulation in the Schematic Editor, however, you can set up your simulations to do load and line transient simulations as well. Since it is a simulator interface, you can add and configure sources and do a lot of different simulations as well.
