How do the current-sense resistor (Rs) and the CS pin work?

On the MOSFET's source you'll see small-value resistors, often several in parallel. The IC watches the current through the MOSFET using them.

What does it do? The current-sense resistor turns MOSFET current into a small voltage. The IC watches it on every pulse and ends the pulse when it reaches the threshold. It's both overcurrent protection and, on current-mode ICs, the output regulation itself.

⏱ Slow motion: what the driver IC outputs and what the circuit does (the highlighted slice is the moment shown on the schematic)

The gate falls the moment the CS ramp reaches the threshold. More load raises the threshold, the ramp runs longer and the pulse lengthens.

Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.

Phase 1/2: Q on

Primary current flows through the MOSFET and Rs. As the current rises linearly, the voltage across Rs ramps up too; the IC watches it on its CS pin.

PWM output: HIGH (≈12 V, switch on)

1. Voltage across Rs (unfiltered)

There's a short but high spike at the start of the ramp: as the MOSFET turns on, the transformer and snubber capacitance draw a brief current. Without the filter, the IC would mistake it for overcurrent and cut the pulse at once.

On the scope

Multimeteruse the scope

🔍 How to spot it on a board

•One or more 0.1–2 Ω resistors in parallel between the MOSFET's source and ground
•A resistor (≈1 kΩ) from there to the IC's CS (ISENSE) pin, and a small capacitor (≈1 nF) from the pin to ground

🔧 How to measure it

•Unplugged, on the lowest resistance range (short the probes first to zero).
•Running, scope the CS pin: you should see a clean ramp staying below the threshold on every pulse.

⚠ Common faults

•Often opens when the MOSFET blows; always measure it before replacing.
•If its value rises, the device shuts down under load or loses power.
•If the CS filter capacitor shorts, current limiting stops working and the new MOSFET blows too.

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Go further

UC3842: the ISENSE pin The real flyback circuit Concept: snubber