LC circuit of a full wave rectifier - Page 1
Forum section: electronics / projects

author: psydaddy
author_rank: Contributor
timestamp: January 06, 2025, 10:08:55 pm
content: How do i calculate this LC circuit of a secondary side of an SMPS? Been searching for quite awhile, I know how it works, but still don't know how to calculate it.

author: moffy
author_rank: Super Contributor
timestamp: January 06, 2025, 10:41:51 pm
content: It's a filter with a known input, what level of ripple on the output do you want? Also you need to factor in cost and size into the final choice, it needs to be practical and cost effective.

author: psydaddy
author_rank: Contributor
timestamp: January 06, 2025, 10:58:57 pm
content: Thanks. Cost is not a problem, I'm just learning at this stage, this is not for selling. Speeding a bit more is not a problem.That's a great question. I would like to achieve the best ripple so less than 50mV for an 24V 41A? Am I dreaming too much?

author: TimFox
author_rank: Super Contributor
timestamp: January 06, 2025, 11:04:15 pm
content: Traditionally, a "choke-input" filter gives an output DC voltage approximately equal to the "mean absolute value" of the input sine wave, so long as the output current is above a "critical value".Past that current, the voltage drops more slowly with current than with a capacitor filter.Below that current value, the inductor doesn't do much and the output DC voltage rises to approximately the "peak value" of the input sine wave.Choke-input filters were common with high-voltage supplies with 50 or 60 Hz (100 or 120 Hz ripple) frequencies, and the rule of thumb for critical value of the choke wasL=R/(900 or 1000).That inductance value scales down with increasing frequency.

author: moffy
author_rank: Super Contributor
timestamp: January 06, 2025, 11:04:24 pm
content: 50mV ripple @ 41A would be impressive, a multistage filter would be a better approach, but switching glitches are a pain and hard to eliminate. If you want low noise perhaps a resonant topology might be best.

author: psydaddy
author_rank: Contributor
timestamp: January 07, 2025, 09:09:39 pm
content: Thank you guysQuote from: moffy on January 06, 2025, 11:04:24 pmResonant topology is very hard to implement. In your opinion, what might be the maximum ripple value i could have with the characteristics I've mentioned?

author: moffy
author_rank: Super Contributor
timestamp: January 07, 2025, 11:17:27 pm
content: Quote from: psydaddy on January 07, 2025, 09:09:39 pmWithout an in depth analysis, I have no real idea, I agree resonant is complicated but it depends on your design objectives. With 41A load currents layout, grounding and magnetics design is very important, and will be vital to a succesful outcome.P.S. 1m Ohm @ 41A = 41mV

author: moffy
author_rank: Super Contributor
timestamp: January 14, 2025, 09:58:04 pm
content: Found this from Analog Devices about high current low noise power supply devices, not sure how relevant it is but might be of help:https://www.analog.com/en/solutions/power-solutions/silent-switcher-technology-solutions.html?lid=j1510bbyl2al&ADICID=email_ww-_22janlownoise_wb_braze_pwr-auto-ind-intbld-inst_20250114_01

author: psydaddy
author_rank: Contributor
timestamp: January 24, 2025, 07:51:18 pm
content: Thank you guysIt was a great help.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 28, 2025, 08:38:51 pm
content: Reads this interesting artice from TI.Its worth noting that poor pcb layout technique and poorly chosen components can undo all the filter goodness.Not all Ls and Cs are born equal.