buck converter in TCM or CRM
To model the buck operated in borderline or boundary conduction mode (BCM), you need to resort to a specific PWM switch model that I derived some years in my book. From that model, you can linearize it and determine the control-to-output transfer function of the BCM buck converter. That is what I did in my newly-released book dedicated to small-signal modeling of switching converters:

If you derive the transfer function, you'll find a simple 1st-order response which is easy to stabilize.
I have released a set of free SIMPLIS simulations templates described here and one of them is a BCM CM buck converter:

It works on the free demonstration version and you have the results in a few fractions of seconds. As you can see below, the part operates in second valley and turns the switch on at almost 0 V.