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Just want to make sure that I understand how the current sense resistor works in this circuit:

enter image description here

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?

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  • \$\begingroup\$ Rcs has a low value because any current through it will generate power that's wasted as heat. The more power, the bigger the resistor power rating. Meanwhile: can you describe (or give examples of) the circuitry you'd use to monitor the current directly, without a resistor like Rcs? \$\endgroup\$
    – TonyM
    Apr 21, 2021 at 16:56

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Rcs is exactly how you monitor the current directly. It gives a measurable voltage proportional to the current.

Of course you could replace Rcs with something like a current transformer but it would be more expensive and possibly not as reliable.

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