Inductor for current smoothing

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Hey guys.

I know we typically use large capacitors to smooth out the output ripple from a rectifying/switching power supply.

My question is: Is it ever practical to use an inductor to smooth the output ripple? Is there any advantages or problems?

My follow up question:

Why do output capacitors always come in pairs? A big electrolytic and a small ceramic? why?

Cheers,

Kevin Jaako

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Good questions!!

The answer to the large electrolytics is a tad tough. the simplest answer is that the caps are based on the design. Sometimes two caps and an inductor are used in a 'pi' filter. Sometimes it is easier to use two caps to create one big one rather than the expense of buying one big cap. There is nothing wrong with using an inductor, but it is also based on the design.

Second question regarding a big/small pairing
The typical Electrolytic/tantalum or ceramic cap combination is simple, the electrolytics can handle low frequencies, the tant's/ceramics handle high frequencies, therefore you are improving decoupling. Electrolytics can also be a reserve energy source for current demand 'spikes'

Great questions

Jim

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Until recently, a capacitor was usually chosen over an inductor because of weight.

Also, from a storage point of view, an inductor stores it's energy in the form of a magnetic field. When the current stops flowing, the magnetic field collapses. When the magnetic field is fully dissipated, the stored energy is depleted. This happens whether or not anything is utilizing the stored energy or not.

A capacitor stores energy in the form of accumulated electrons (or absence of) on the plates of the capacitor. A capacitor charge will deplete in direct proportion to the amount of current drain between it's terminals. This current drain is in the form of both, the internal resistance of the capacitor and the load resistance externally connected to the capacitor terminals. But the capacitor, unlike the inductor, will hold the accumulated charge if not connected to an external load - only to be discharged by the internal resistance of that capacitor.

There have been substantial improvements made in the areas of capacitor and inductor technology. As such, the advantages and disadvantages of both seems to be blurring. I think that the choice of the use of an inductor or capacitor in a power supply today depends on the type of power supply. Pass transistor (linear) type power supplies tend to use large capacitors and small inductors (the weight thing). Switching power supplies, on the other hand, tend to use larger inductors and smaller capacitors - where the inductor is the voltage translation device and the capacitor is the storage device.

Pass transistor type power supplies operate at 50Hz, 60Hz and possibly 400Hz. This requires large capacitors and inductors for filtering at these frequencies.

Switching power supplies operate at much higher frequencies - say, 15KHz. At these frequencies, both the capacitors and inductors can be much smaller for a given amount of energy translation.

I hope this helps provide a little better understanding.

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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Also inductors resist current change and thus smooth current ripple, capacitors smooth voltage ripple.

Edward

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For a switched mode powersupply you need both an inductor and a capacitor.

Old tube based radios usually used an extra inductor after the rectifier, that is not found in many newer ciruits. This was because large capacitors were expensive and the peak current to recitfiers is smaller this way.

Due to new EMV regulations there are now lower limits to the input power factor. The old solution with the extra inductor has a better power factor than the simple large capacitor behind the rectifier. So we may see more inductors in the future.

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Thanks guys.

I'm getting interested in making my own switching mode boost power supply using an ATtiny and a pwm cycle through an inductor.

I'm reading a few examples online:
http://spritesmods.com/?art=ucbo...
(i linked to page 3, the article starts earlier.)
This example is a little lacking, there's no feedback into the AVR and it's lucky didn't explode.

I've seen some others on the forum too,
(https://www.avrfreaks.net/index.p...)

I found another great example a while ago, but I didn't bookmark it and sadly lost it.

Does anyone know any good examples of an AVR-controlled boost or buck-boost power supply? I've read the AVR app-note about the LEDflashlight and I'm intruiged to make my own. Maybe using mosFETs instead of diodes.

Thanks for your help guys!

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https://www.avrfreaks.net/index.p...

I just made a boost converter using some spare parts to make a GAL programmer.

Math is cool.
jevinskie.com

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Here is one of our long-runners

https://www.avrfreaks.net/index.p...

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_jaako wrote:
Why do output capacitors always come in pairs? A big electrolytic and a small ceramic? why?

In some cases the electrolytic is for bulk storage and the ceramic is to get the ESR down. It's all for improved efficiency.

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Thanks for the links guys.

hmmm. would ESR have anything to do with a cap's reaction speed? Is that why ceramics have good high-frequency response, or is that just another property of ceramics? I guess, then, the electrolytics are slower to react, but carry enough juice to smooth out the low frequency spikes?

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_jaako wrote:
hmmm. would ESR have anything to do with a cap's reaction speed?
Absolutely, the ESR creates an RC filter with a time constant proportional to R x C.

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I guess that makes perfect sense, now doesn't it.
How about choosing values?

Is there a rule of thumb to how much smoothing capacitance should be ceramic vs. electrolytic?

KJ

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The ESR also causes a voltage drop depending on the current that flows into or out from the capacitor. ESR is basically a resistor in series with the capacitor. It also dissipates power.

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_jaako wrote:
I guess that makes perfect sense, now doesn't it.
How about choosing values?

Is there a rule of thumb to how much smoothing capacitance should be ceramic vs. electrolytic?

KJ


In my experience, the best thing to do is read the datasheet. Linear Technology probably makes the best datasheets of all the chip manufacturers IMHO, and they will go into great detail about what you can and can't do for the input and output capacitors. At work we have an LTC1871 design that uses between 200-300uF on the input and output and they're all ceramic. The efficiency is like 95% and I'm told it's because the ESR is so low. That approach isn't exactly what the datasheet says to do, but the datasheet would be a good starting place.

If you really need to smooth voltage ripple, I would definitely go with a pi filter - two big electrolytics and an inductor.

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Speaking of ESR I can easily find ESRs and ripple currents for electrolytic caps, but none of the ceramic datasheets I've seen list those statistics just "low ESR, High Iripple". Anyone know the sorts of values you can expect for ceramic capacitors?

Edward

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As low as 0.001 Ohms, but usually <0.01 ohm.