Please help me understand this PRC circuit of a SMPS.

According to the NCP1606 PFC controller datasheet pin 4 is responsible for current sensing. Now, there are two resistors:

(red) in series to the source of the MOSFET source pin and

(blue) connected to ground.

At first I was not sure which of the two are serve for current sensing. Usually I follow two indicators to determine whether a resistor is a current sense resistor:

  1. The resistor is in series with the load or
  2. The resistor has a very low ohm value.

With the help of the example circuit on page 1 of the datasheet, I can say that the resistor connected to the ground plane is the current sense resistor. How would I could I determine the current sense resistor if I had no example circuit on hand?

Does the low ohm value trump the 'in series with load' argument?

My second question: what is the purpose of the red resistor? Would be safe to assume that it limits current?

PFC circuit

  • 5
    \$\begingroup\$ The "blue" one is the true current-sense resistor. The "red" is likely there for form a small filter with the CS pin capacitance unless there is a small cap. connected between CS and GND which will strengthen the filter effect. The 1606 is a constant on-time borderline PFC whose power depends on the capacitor connected at pin \$C_t\$. The sense resistor is there for cycle-by-cycle protection. \$\endgroup\$ Dec 8, 2023 at 19:40
  • 1
    \$\begingroup\$ @VerbalKint you should put it as an answer. \$\endgroup\$ Dec 8, 2023 at 21:00
  • \$\begingroup\$ Will do tomorrow! : ) Thanks Rohat. \$\endgroup\$ Dec 8, 2023 at 21:50

1 Answer 1


The NCP1606 is a constant on-time PFC controller operating in borderline conduction mode or BCM. There is no voltage sensing as in its current-mode version like the old MC33262 for instance. Capacitor \$C_t\$ sets the on-time that you will determine based on the transmitted power and inductance value. Assume the following values:

enter image description here

By measuring the inductance and the \$C_t\$ value, you will determine the nominal power of the board and then progress to the sense resistance value. That is the way I would do but there are probably different options also. Please check whether the original controller was a A or B version as the sensed voltage differs between the two parts.

The small resistance in series with the CS pin can be combined with a capacitor of a few tens of pF to form a low-pass filter, helping the internal leading-edge blanking (LEB) circuit included in the IC. It is good practice to keep this filter in place, with the \$RC\$ elements colocated with the controller.


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