Average Model of Peak Current Mode Buck Converter

Question

I am trying to understand average model of Buck Converter for Peak Current Mode Control Topology. Below is a snapshot taken from the Christophe Basso Webpage, from file “PWM Switch LTspice: Mark Dimattina”:

Below image is the expanded view of Transconductance Amplifier:

Now I have the following questions:

1. Why do we need to use a limit function at the input?
2. What is the purpose of B1 & B2?

I am unable to understand the purpose of above two functions, it would be really nice if someone can help me.

Answer 1

B1 and B2 are modelling part of the internal gm amplifier, with G1. The amplifier can't pull Vc below ground or above its supply voltage, so it keeps the comp pin Vc from winding up or down to large positive or negative voltages that wouldn't be physically possible in the actual device.