Just want to make sure that I understand how the current sense resistor works in this circuit:
Basically, we want to monitor the current through the primary side of the transformer, so we know when to switch the MOSFET on and off.
Rcs creates a voltage based on the primary side current and the value of the resistor (V = IR), and the CS pin of the flyback controller measures that voltage.
We have to choose an exceptionally low value for Rcs because we need to be able to generate a high enough current through the primary winding to achieve the output voltage that we desire.
What I do not understand is why can't we just monitor the current directly? Why do we need Rcs at all?