Calculating output of a CLC filter for to design AC-DC Converter - Page 1
Forum section: electronics / beginners

author: varunkoganti
author_rank: Contributor
timestamp: May 17, 2021, 02:30:24 am
content: 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 designthe 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.

author: Jwillis
author_rank: Super Contributor
timestamp: May 17, 2021, 05:03:14 am
content: 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 1VppC = 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 1Then 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 * CThere is a calculator hereChebyshev Pi LC Low Pass Filter CalculatorBut it doesn't explain how the calculation works and I got lost on the impedance and frequency response ripple in dbI could sure use some help understanding it.

author: varunkoganti
author_rank: Contributor
timestamp: May 17, 2021, 05:39:24 pm
content: Hi,the mentioned link |http://www.calculatoredge.com/electronics/ch%20pi%20low%20pass.htmlmy 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.

author: tunk
author_rank: Super Contributor
timestamp: May 17, 2021, 05:58:18 pm
content: No expert on this, but I think that there's a rule of thumbthat says you should use ~2000uF/A in a unregulated PSU.This will give you ~4V ripple. Then your C1 at 100nF is farto small. And are you sure you need the LC part at all, theDC-DC stepdown may (possibly) handle this ripple?

author: Jwillis
author_rank: Super Contributor
timestamp: May 18, 2021, 07:40:02 am
content: Quote from: varunkoganti on May 17, 2021, 05:39:24 pmOh cool . OK so 2/3 isn't to far off then . AT 120Hz I worked it out to be cutoff of 80Hz