The excerpt below is from the book Pulsewidth Modulated DC-to-DC Power Conversion: Circuits, Dynamics, and Control Designs by Byungcho Choi.
It looks a bit long but my main question is why the implementation of current sensing network (CSN) by sensing the switch current is simpler than that of sensing inductor current?
Current mode control can be implemented in many different forms. The most popular among them is the peak current mode control, illustrated in Fig. 10.6. In this control scheme, the switch current is utilized in place of the inductor current. The switch current, which corresponds to the on-time inductor current, is sensed through CSN and blended with the compensation ramp. The peak value of the switch current, or equivalently the peak value of the inductor current, is used to determine the instant to turn off the switch; thus, the control scheme is called the peak current mode control. It should be noted that the switch current sensing is functionally identical to the inductor current sensing because the peak value of the inductor current is employed as the criterion to turn off the switch. There are several advantages in sensing the switch current rather than the inductor current. The first is the simplicity in CSN. An implementation of CSN for the peak current mode control is illustrated in a later example. As the second advantage, the sensed switch current can be used for the over-current protection for semiconductor switches. Due to these advantages, the peak current mode control is widely adapted to modern PWM dc-to-dc converters.