Output LC filter help

[krish2487]

Hello all..

while writing down the specs of the 100w 36 to 400 boost converter i wanted to build

i ran into a minor roadblock

The output filter design.

This is how i arrived at the present values

I am using a 3525 clocked at 100Khz to drive a push pull stage
obviously the trafo secondary will have ripple at 200khz

i used the formula L= (V_out*(1-D)*T)/ΔI

where vout is the output voltage ie 400 v

D is the minimum duty cycle wrt the frequency on the secondary side
i am assuming the minimum value of D to be 0.1 -0.2

T is the time period = 1/200 Khz= 5 Us

ΔI= current ripple
I assume that the ripple here decides the mode of operation DCM or CCM
i assume that more the ripple then the operating mode goes into DCM and lesser the ripple it shifts into CCM.

however for 250ma(100W @ 400V)
i chose a ΔI of 20% = 50ma

i ended up with a value of 4mH.


Then i used the formula f=1/(2*pi*(sqrt(LC)))
to arrive at the capacitance value.
the frequency specified here i assume is to filter out the high frequency ripple
i chose this f as 1/2 the trafo output ripple freq
ie f= 200khz/2=100khz

therefore C=1/(4*pi*pi*f*L)
= 63Uf

is my method of calculating the values of the filter components correct??

and help is highly appreciated

[darkfenriz]

The inductor choice seems fine.
In the capacitor choice you've made a double mistake, first in your equation f should be squared, and C=1/(4*pi*pi*f*f*L).
Second, the corner frequency of 1/2 of switching frequency is not a very good choice, because LC filter will have only around -10dB, ratios of 1/100 to 1/10 of switching frequency is more common.
Also the "better" approach is to choose the capacitor basing on voltage ripple on the output. For example if you have current ripple of 50mA and assume 50mV of output ripple, then a capacitor of 1 ohm at 200kHz is good enough, and 0.1 ohm is preferable for 5mV ripple. Add the ESR and 1/(2*pi*f*C) to calculate the capacitor impedance.

[krish2487]

@darkfenriz

thank you
i did not realise abt the f squared in the equation till now.

and i was doubtful about the choice of corner freq
thank you very much for clarifying.