Author Topic: Buck converter vs an LC filter  (Read 444 times)

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Offline drogus

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Buck converter vs an LC filter
« on: December 02, 2017, 01:28:09 am »
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 in a 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?
 

Offline dmills

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Re: Buck converter vs an LC filter
« Reply #1 on: December 02, 2017, 01:35:48 am »
The output stage of a classical buck converter **IS** an LC filter!

Regards, Dan.
 

Offline drogus

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Re: Buck converter vs an LC filter
« Reply #2 on: December 02, 2017, 01:39:25 am »
The output stage of a classical buck converter **IS** an LC filter!

Sure :)

But 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.
 

Offline Belrmar

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Re: Buck converter vs an LC filter
« Reply #3 on: December 02, 2017, 01:49:01 am »
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
 

Offline IanMacdonald

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Re: Buck converter vs an LC filter
« Reply #4 on: December 02, 2017, 02:18:19 am »
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

No, 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.
« Last Edit: December 02, 2017, 02:21:09 am by IanMacdonald »
 

Offline MrAl

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Re: Buck converter vs an LC filter
« Reply #5 on: December 02, 2017, 04:54:28 am »
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 in a 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?

Hello,

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.

 


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