Parallel RL circuit in Power supply

[myatham]

Hi,
I found in a few designs, designers kept parallel RL filter (R=50 ohm and L= 0.68 uH with high current rating) in series to input voltage of a regulator. What is the main function of this parallel circuit? Is it to minimize inrush current?
:?

 

[FvM]

I would expect it intended as an EMI filter, with dampening resistor to reduce the Q of th LC circuit (provided, there's an input capacitor, too) and avoid oscillations. Sounds a bit unusual, ferrite beads would be mostly used these days. An effect on ínrush current can be surely neglected.

 

[myatham]

Yes,
you are right..there is an input capacitor shunted to ground before the inductor and after the inductor like Pi filter. Can you explain it in detail.
I know that if we add a resistor in series it will decrease the Q. Correct me if I am wrong.

 

[FvM]

I know that if we add a resistor in series it will decrease the Q.

Of course, a series resistor (except for the unavoidable windings resistance) is unwanted for a filter choke. So a parallel resistor has serve the purpose. Unfortunately it will also reduce the filter attenuation. You can calculate the filter behaviour from simple AC network relations, or use a circuit simulator. But you can assume Q = Rp/ZL as a first estimation.

 

[myatham]

Thanks for your help!