Calculating output of a CLC filter for to design AC-DC Converter

[varunkoganti]

Good day everyone,

I need help trying to calculate the output voltage of a CLC/pi- Filter.

Firstly objective of the project-
I have a DC motor rated for 12V/1.2A which i would want to use to cut PCBs & as a small dremel.

The design
the design has the following.
Stage 1 - Stepdown transformer, 230Vrms/50Hz - 18Vrms/50Hz.
Stage 2 - Bridge rectifier - GBU6J (datasheet - https://www.vishay.com/docs/88615/gbu6a.pdf)
Stage 3 - Filter to smooth out bridge rectififer output. (C Filter/ LC Filter /CLC Filter) - I have selected a CLC filter, as it has best of both C filter & LC filter.
Stage 4 - DC-DC stepdown converter using PWM.
Stage 5 - Output Stage.

I need help specifically in Stage 3, that is CLC Filter design.
Firstly what is an acceptable ripple voltage in this circuit?
Secondly, How does one design CLC-Filters, cascaded CLC-Filters as well.
How to calculate (Vout/Vin) or gain of these filters? I would like to write a C program to calculate gain & plot graphs using root.

[Jwillis]

I found the math to be rather over whelming .
What I did may not be the correct way but it worked in my case .
I broke the pi filter into 2 sections . The first capacitor then the low pass filter .
The first capacitor does most of the filtering so I worked it out for a 1Vpp

C = I / (2 x f x Vpp)  where I is the maximum current , f is the rectifier frequency (120Hz or 100Hz) , and Vpp is the desired ripple voltage of 1

Then I worked out the load resistance  E/I and divided that by 1200 . I can't remember where I obtained that value .
This gives you the choke value in Henries .
Then I worked out a cut off frequency of the low pass filter at about 2/3  the rectified frequency with  Fc= 1/2 * pi * sqrt L * C

There is a calculator here Chebyshev Pi LC Low Pass Filter Calculator
But it doesn't  explain how the calculation works  and I got lost on the impedance and frequency response ripple in db
I could sure use some help understanding it.

[varunkoganti]

Hi,

the mentioned link | http://www.calculatoredge.com/electronics/ch%20pi%20low%20pass.html
my cutoff/3db frequency should be less than 100 considering cutoff/3db means it is 3db of the actual value at this frequency.
So maybe 75-70 Hz?

I will test it tomorrow morning & post the ripple filtering observation.

[tunk]

No expert on this, but I think that there's a rule of thumb
that says you should use ~2000uF/A in a unregulated PSU.
This will give you ~4V ripple. Then your C1 at 100nF is far
to small. And are you sure you need the LC part at all, the
DC-DC stepdown may (possibly) handle this ripple?

[Jwillis]

my cutoff/3db frequency should be less than 100 considering cutoff/3db means it is 3db of the actual value at this frequency.
So maybe 75-70 Hz?

Oh cool . OK so 2/3 isn't to far off then . AT 120Hz I worked it out to be cutoff of 80Hz