DC-DC switch mode forward convertor help

[IA52]

Hello, I am a college student and I have been given an assignment as follows:

Draw a circuit diagram of, and calculate the values of inductance and capacitance require in the output filter of a DC to DC convertor which is required to operate from an unstabilized input supply of 100v and capable of supplying an output voltage of 15v at a current of 2A. The peak to peak output ripple voltage should not exceed 20mV and the circuit must provide input-output isolation. It may be assumed the duty cycle is 0.3 at a frequency of 50kHz.

I can do the circuit schematic but I cannot work out the value of L and C at all, from multiple equations its possible L = 52.5uH but it isn't working out with other equations. Any help would be greatly appreciated as I do not understand this at all, more so to do with the forward convertor part as I don't know how that affects the L and C parts. Not sure if i'm over complicating it or not, I was fine with Linear regulators and a more basic Switch mode regulator but this one is confusing me. Thank you for helping

Here is the circuit schematic
https://obrazki.elektroda.pl/8206108300_1426264980.png

If anyone has any information for me at all about the circuit I would greatly appreciate the help, even just letting me know how much the transformer affects the currents across the inductor and capacitor etc. Thank you again

 

[asdf44]

Well if a duty cycle of 0.3 is producing 15V out then I assume you have a 2:1 turns ratio stepping the 100V down to 50V?

If so then you know you have 0V on your LC input in OFF state and 50V on the LC input in the ON state. Vout is 15V so in the ON state there is 35V across the inductor and in the off state there is -15V across the inductor. For any given inductor this produces a trianglular ripple current where the slopes in the ON and OFF state are defined by V=LI' (it really helps to actually draw this). Pick a value of L that keeps ripple to ~10% or so of your 2A output, so keep it between 1.8A and 2.2A (combine period with the slope information to calculate how much the current increases or decreases during each phase and keep this to <0.4A. Note that the ON phase and OFF phase will be equal and opposite if everything is right).

Now draw a line representing 2A through the triangle wave. See the areas of current that are above and below the 2A? Calculate the amount of charge here using basic geometry (assuming 2.2A peak it's: (2.2A-2A) * period / 2). This amount of charge must not raise or lower your output cap voltage more than 20mV. Chose a cap that makes this happen.

For reference current will look like this centered on 2A:

 

[IA52]

Thank you, the turns ratio is 2:1 yes, everything makes much more sense now Thank you. Will update with answer

 

[IA52]

Okay, so i've possibly worked out L and C



Thank you for the help

Learned alot in the last few hours about the whole circuit, thank you really