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:
- The resistor is in series with the load or
- 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?