Minimize Ripple in Rectifier by Waveform, Polarity, Bridge-Type, Inductor, Trafo

theboom

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In a rectifier and RC filter, which combination of waveform, bi-polarity, inductor/cap, and transformer-type will give the least ripple in the output?

My source is 100 kHz AC supplying a transformer. I have control over the supply voltage, wave shape, and polarity, over the transformer type, and over the rectifier / filter design. This question is about how to minimize ripple through selection of those parameters.

Assume there is no regulator involved. Just AC source, transformer, rectifier, filter.

Ie, how is ripple affected by:
  • an AC source that goes negative on the troughs, vs an AC source that only goes positive, never below 0v?
  • sine vs square source?
  • pulse transformer vs non-pulse transformer?
  • full-bridge vs half-bridge?
  • Use an inductor in the filter? Or just a cap?

ChatGPT says:
To achieve the least ripple in the output of a bridge rectifier and RC filter:
  • Use a bipolar AC source (e.g., a sine wave) rather than a unipolar one, as it results in a higher ripple frequency. [specifically, 2 x f-in]
  • Use a sine wave input rather than a square wave for a smoother, more manageable ripple. [specifically, no harmonics]
  • Use a standard non-pulse transformer designed for power applications rather than a pulse transformer. [i need to understand more]
  • Use a full-bridge rectifier instead of a half-bridge rectifier to take advantage of the higher ripple frequency produced by using both halves of the AC waveform. [specifically, 2 x f-in]

This isn't for safety. My source is already isolated from mains. This is for operational needs.

My load wants 5V @ 3A (after the filter).
 

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theboom said:
The interior of the enclosure will be at room temp
Click to expand...
Physically impossible.
As soon as you have power dissipation inside the enclosure .. you get a temperature rise.
AND: Let´s say you have 5W of power dissipation ... it does not matter what the devices you use inside and how they are saped and how many heatsinks insdie you use ....the average temperature rise will always be the same.
To lower the inside temperature rise you only have two options:
* change the enclosure to get a lower R_th of the whole enclosure: using, vents, fins, different material, fans, bigger...
* or reduce the total power dissipation inside

As already mentioned: There is a good chance that the rectifier is the main source for overall temperature rise.
--> I can only recommend: Do your job, do the calculations.

Klaus
 

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KlausST said:
Physically impossible.
Click to expand...
It's not impossible to dissipate heat inside the enclosure without causing a problem. It depends on how much. I'd like to learn what the formulas are.

KlausST said:
I will be back when I see you do your job.
Click to expand...
It seems you can't resist. But you haven't added anything new with your new comment.
 

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theboom said:
It's not impossible to dissipate heat inside the enclosure without causing a problem.
Click to expand...
No, but impossible to dissipate heat without causing temperature rise as post #20 seems to suggest. When I read this, I thought of a typo, exterior at room temperature would make more sense.
I have to design for a sealed enclosure without a fan, or give up. The interior of the enclosure will be at room temp
Click to expand...

There are many devices with "sealed" enclosure out there, e.g. most notebook power supplies. If you analyze the thermal situation, you have natural convection inside and outside the enclosure and the enclosure wall inbetween. Heat transport can be calculated by using technical thermodynamic text book formulas or modelled exactly using modern finite element tools.
 
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D.A.(Tony)Stewart said:
When you need low EMI, low power, high effciency , no shielding , low mass magnetics, low cost AND high voltage , then you avoid SMPS and choose high frequency sine waves and linear control to regulate the voltage.
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In my application, i need a non-regulated output. No regulator needed.

D.A.(Tony)Stewart said:
Because a sine FW bridge is only active during voltage peak detector the current pulse width tends to-be <10% duty cycle or as small as the % ripple needs to be. Now you have 9 harmonics !
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I don't understand how we're getting harmonics.

FvM said:
No, but impossible to dissipate heat without causing temperature rise as post #20 seems to suggest.
Click to expand...
Post #20 doesn't say "without causing rise", it says the opposite:

theboom said:
a 55c/80f deg allowed rise above ambient
Click to expand...

FvM said:
When I read this, I thought of a typo, exterior at room temperature would make more sense.
Click to expand...
When power is off, i assume the interior of the chamber will be similar to the exterior ambient temp.

FvM said:
There are many devices with "sealed" enclosure out there, e.g. most notebook power supplies. If you analyze the thermal situation, you have natural convection inside and outside the enclosure and the enclosure wall inbetween.
Click to expand...

I'm thinking i may have to do something like that.

I hope no one minds that a post about ripple turned into a post about heat. I don't mind!


FvM said:
Heat transport can be calculated by using technical thermodynamic text book formulas or modelled exactly using modern finite element tools.
Click to expand...
Both would be challenging for me at the moment.

thx!
 
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theboom said:
When power is off, i assume the interior of the chamber will be similar to the exterior ambient temp.
Click to expand...
You didn't mention power-off in post 20, but apparently rejected KlausST's explanation about no heat dissipation without temperature rise.

Finally you seem to agree there's nothing wrong in post #19.

Your initial description says 5V/3A power, seems manageable. We see higher power in small wall-plug SMPS.
 

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FvM said:
You didn't talk mention power-off in post 20, but apparently rejected KlausST's explanation about no heat dissipation without temperature rise.
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i was misunderstood. I wouldn't have said there could be heat dissipation without temperature rise. That doesn't make sense.

Where i disagree with @KlausST is his apparent view that we cannot allow the electronics to get even 1 degree hotter than ambient in a sealed enclosure. That seems incorrect to me. In my application, i can allow a 55c rise above ambient of the air inside the chamber. I believe that if the electronics get 1 deg hotter than ambient, that will not cause a 55c rise. The question is, how hot will the electronics get? I hope i can find the math, because i can't simulate or build it at this moment.

FvM said:
Finally you seem to agree there's nothing wrong in post #19.
Click to expand...
#19 is one of my favorite comments in this thread. Very informative.
 

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To reduce losses for 15W transfer you may start a power loss budget and identify your goal for max temp rise like Klaus suggested +15'C for MTBF then max Rth conduction resistance and max Pd losses such that Rth*Pd= 15'C Then identify all sources of both variables. Since I^2R=Pd determines loss consider what shape is the optimal current flow for 100KHz fundamental (which may include many harmonics for I)

Sinusoidal AC to DC only charges current during the rise of charge current otherwise no current, so high peak/avg= crest factor currents will be poor efficiency. Pulse transfer efficiency is higher.

Since Rs/(Rs+RL) source/load impedance ratio determines all factors for ripple, crest factor, and efficiency as well as the size of magnetics for R/L~3f and choice of R,L variables and materials.

So start a design budget then choose materials to efficiently store say 5 to 9 harmonics of fundamental 100 kHz. (more than 2 freq. decades is very difficult in magnetics) while sine AC2DC is low efficiency. Thus Flyback transformer design is the most common choice for <100W and may be used to < 250W but not much higher. For high power, tight magnetic transformers work with higher efficiency by not storing the energy (as flyback does) instead using a forward converter method.

Now start your power distribution budget and design, learning from examples before dictating rigid parameters like 100 kHz. You might use a planar PCB magnetics or air core or Off-the-shelf solution with a >1MHz SMPS. Who knows?

So when asking a question about reducing voltage and current ripple , why assume we know your specs or that they do not matter?
Everything interacts in the SMPS analog world.

We do not know your topology or why it must be sealed or what pressure it experiences under air or water or why ripple matters and we do not know what you do not know.

Idea (!) for all readers of this advice who may pose questions.

Always provide the big picture (Purpose, environment,qty, budget) with all relevant top-down expectations (or specs) so we can analyze your question with a bottoms-up design approach for a better answer.
 
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    theboom

    Points: 2
    Great info and detail!
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Hi,

The interior of the enclosure will be at room temp
Click to expand...

--> AT room temperature? ( = 0°C rise!!)
or
--> room temperature +1°C? ( = 1°C temperature rise)

I did not write the quoted sentence ... I just reacted to it.
And also you talked about power dissipation .. which makes sense.

Later you talk about "POWER OFF" ... the nect time you talk about power dissipation.
HOW can an electronics device have power dissipation while powered OFF?

I´ll leave. You may go on annoying other people. If you think it brings you one step further regarding your technical problems....

Klaus
 

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Great! This will take me some time to understand.

My requirement is for an unregulated design.
"tight"?

D.A.(Tony)Stewart said:
learning from examples before dictating rigid parameters like 100 kHz
Click to expand...
100 kHz is not a rigid parameter. I mentioned a freq range and reasons:
I'm targeting 100 kHz - 400 kHz to avoid audio and commercial radio freq's, and to keep components small.
Click to expand...
but i welcome alternatives.

D.A.(Tony)Stewart said:
why assume we know your specs or that they do not matter?
Click to expand...
I make no assumptions, but in my lack of expertise i might fail to provide essential info. Don't get mad, just ask.

D.A.(Tony)Stewart said:
We do not know your topology
Click to expand...
Converter topology? Any unregulated "transparent" design. My current choice is a half-bridge converter, but i welcome alternatives.

D.A.(Tony)Stewart said:
or why it must be sealed
Click to expand...
Because it must operate in the rain.

KlausST said:
HOW can an electronics device have power dissipation while powered OFF?
Click to expand...
That's not what i meant. Sorry if i was unclear. I'm comparing heat while powered-on to heat while powered-off. Is that the wrong thing to compare?
 
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Here is a random implementation of a 15W 5V linear unregulated ACDC converter/

  1. Can you compute efficiency and breakpoint f from this schematic?
  2. Thus from -40 dB/dec, what is ripple attenuation?
  3. What Vpp is good enough at 1.5 ohms and 50 mV/div?
  4. What is fo, Zo? of transformer Ls, C filter?
  5. how can I make this >10 times lower ripple with negative feedback? with > lower loss than diodes?
  6. Can you estimate the harmonic content from Vcap? (sawtooth) Do you need a 4th order filter?
  7. Do you know how to estimate low ESR from C and low Rs from Pd rating in PN jcns?
  8. How do you write design specs or performance expectations?
  9. Can you read these plots?
  10. What do you see here that is unexpected?
  11. What is unrealistic?
  12. Why unregulated?
  13. What is the order of your design priorities when there is a tradeoff? {Cost , Reliability, Performance} Choose here. Then for each what is your goal "target' and "deal-breaker" threshold?

 
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