Trouble calculating L

[dernyarn]

In this circuit im trying to calculate the inductor value required to get a frequency cut off of 4khz, how do i go about this? I cant seem to get an equation as i cant use the basic passive equations since R2 is not in its 'normal place'.

 

[MikeMl]

The inductor requirement for a switching power supply not calculated based on a "cutoff frequency"

 

[dernyarn]

Im not quite sure what you mean sorry. With this circuit i was given this task:
"9. Change the cut-off frequency of the low pass LC filter to 4 kHz with C1 = 100 µF. State the value of L you used, and describe the effect that reducing the cut-off frequency has upon the output voltage waveform. "

 

[JimB]

The circuit as drawn is confusing on several levels.

The question seems to be asking about frequency response.
The circuit looks like a fragment of a switching power supply.
The capacitor C2 is effectively out of circuit due to the apparently very high value of R2 (100 M Ohm).
Some context for the question may help to remove the confusion.

JimB

 

[dernyarn]

Heres the original question. It has the info for how we had to set up the switching power supply

 

[Ylli]

Is R2 100 Megohms, or 100 milliohms?

 

[dernyarn]

im pretty sure 100 milliohms

 

[dernyarn]

looking at a textbook i found; would i have to make a transfer function for the circuit, then equal it to 1/sqrt(2) to get an equation for cutoff frequency, then rearrange that equation to get the inductor value for the desired cut off frequency?

 

[unclejed613]

calculate the reactance of C1 at 4khz, then use reactance and frequency to solve for L.