Parallel RL circuit in Power supply

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myatham

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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?
:?
 

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.
 

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.
 

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