Buck converter vs an LC filter - Page 1
Forum section: electronics / beginners

author: drogus
author_rank: Contributor
timestamp: December 01, 2017, 02:28:09 pm
content: Hi,I'm working on a project where I need to control a peltier element to keep a stable temperature. At the beginning I wanted to use a PWM signal, but then I found out that it's more efficient to use stable voltage, so I came up with a dc-dc buck converter design that I can control from Arduino to set a stable voltage.Recently I also read about smoothing voltage with a LC filter for a similar use case ina thread on reddit. Now I'm thinking - what's the difference between the two approaches? I know roughly how both things work, but my understanding is still limited.So, what are the theoretical and practical differences between using an LC filter and a DC-DC buck converter for driving a high current load like a peltier element?

author: dmills
author_rank: Super Contributor
timestamp: December 01, 2017, 02:35:48 pm
content: The output stage of a classical buck converter **IS** an LC filter!Regards, Dan.

author: drogus
author_rank: Contributor
timestamp: December 01, 2017, 02:39:25 pm
content: Quote from: dmills on December 01, 2017, 02:35:48 pmSureBut then you also have a diode in a buck converter and I'm not really sure what's the real difference between a PWM smoothed out with an LC filter and a buck converter with a diode.

author: Belrmar
author_rank: Regular Contributor
timestamp: December 01, 2017, 02:49:01 pm
content: the diode is just to get rid of the back emf of the inductor, if you want to build one of those i recommend you to watch Greatscott's video on this matter

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: December 01, 2017, 03:18:19 pm
content: Quote from: Belrmar on December 01, 2017, 02:49:01 pmNo, the diode is to allow a path for the circulating current through the inductor and load when the switching element is off. Which, if you have a high voltage in, low voltage out, will be most of the time. Without the diode you'd be throwing away most of the energy delivered to the inductor as heat in whatever component limited the backswing voltage.Ideally in a simple Buck, the series inductance is high enough that the inductor circulating current doesn't vary too much throughout a switch cycle. That way there's not too much smoothing to do at the output. Of course if you put too many turns on it it will saturate though, so it's a balancing act between enough inductance and not saturating at max current and voltage drop.This differs from a flyback converter, where it's desirable that the primary current does fall to zero before another on cycle commences.

author: MrAl
author_rank: Super Contributor
timestamp: December 01, 2017, 05:54:28 pm
content: Quote from: drogus on December 01, 2017, 02:28:09 pmHello,You seem to want to use an LC filter on a PWM output vs a buck converter?A buck converter is basically a PWM output that is filtered with an LC filter. The main difference is just that you can control the output voltage of the buck converter. That means you can raise or lower it as needed.If you can control the buck circuit then you can measure temperature and make adjustments to the buck control so that you can maintain the temperature to within some upper and lower limits.Forget about the diode as that is just there to make the circuit work right. A synchronous buck has no diode.