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 input 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:

1. an AC source that goes negative on the troughs, vs an AC source that only goes positive, never below 0v?
2. sine vs square source?
3. pulse transformer vs non-pulse transformer?
4. full-bridge vs half-bridge?
5. 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).


(also asked on EDA Board, EE Web, All About Electronics, electro-tech-online.)

 

ChatGPT says:

To achieve the least ripple in the output of a full-bridge rectifier and RC filter:

• Use a bipolar AC source (e.g., a sine wave) if possible, because it results in a higher ripple frequency (twice the AC frequency).
• Use a sine wave input rather than a square wave, as sine waves produce a more manageable ripple.
• Use a standard non-pulse transformer designed for power applications rather than a pulse transformer.

Almost!
Full bridge rectifier is better than half-wave (which is what I presume it means by bipolar AC source, but it was rather less explicit that it should be).
Square wave is better than sinewave, because the voltage is stable for all the period of the squarewave, whereas a sinewave varies (so that one was wrong)
Pulse transformer? What does it mean by a pulse transformer?

 

What does it mean by a pulse transformer?

(not claiming i fully understand this yet)

The main difference between a pulse transformer and a normal transformer is that a pulse transformer has an inductance value that is very high compared to its resistance, which allows it to store energy for only a short period of time. The inductance value is usually 0.5 Henry or higher for most applications. As a result, pulse transformers are used when there is a high current demand for a short duration. These types of applications include medical equipment like defibrillators and industrial equipment like welders and arc furnaces.
https://amgistoroids.com/what-is-a-pulse-transformer/

In my application, I need don't need high current for a short duration. I need a steady current output for a long duration. So pulse might not be right for my need.

Pulse transformer designers usually seek to minimize voltage droop, rise time and pulse distortion. Droop is the decline of the output pulse voltage over the duration of one pulse. It is caused by the magnetizing current increasing during the time duration of the pulse. The magnetic flux in a typical A.C. transformer core alternates between positive and negative values. The magnetic flux in the typical pulse transformer does not. The typical pulse transformer operates in a unipolar mode (flux density may meet but does not cross zero).
https://www.gowanda.com/application...rmers/pulse-transformer-operating-principles/

Here, Fez is using a pulse trafo to gate a power mosfet, but since my power needs are relatively low (15W) i don't need the second power stage.

Square wave is better than sinewave, because the voltage is stable for all the period of the squarewave, whereas a sinewave varies

I can see how that makes sense, but what about harmonics? Also, do transformers like those sharp corners?

 

(not claiming i fully understand this yet)

The main difference between a pulse transformer and a normal transformer is that a pulse transformer has an inductance value that is very high compared to its resistance, which allows it to store energy for only a short period of time. The inductance value is usually 0.5 Henry or higher for most applications. As a result, pulse transformers are used when there is a high current demand for a short duration. These types of applications include medical equipment like defibrillators and industrial equipment like welders and arc furnaces.
https://amgistoroids.com/what-is-a-pulse-transformer/

Pulse transformer designers usually seek to minimize voltage droop, rise time and pulse distortion. Droop is the decline of the output pulse voltage over the duration of one pulse. It is caused by the magnetizing current increasing during the time duration of the pulse. The magnetic flux in a typical A.C. transformer core alternates between positive and negative values. The magnetic flux in the typical pulse transformer does not. The typical pulse transformer operates in a unipolar mode (flux density may meet but does not cross zero).
https://www.gowanda.com/application...rmers/pulse-transformer-operating-principles/

In my application, I need don't need high current for a short duration. I need a steady current output for a long duration. So pulse might not be right for my need.

Not sure I understand that either!
A "ratio of inductance to resistance" is nonsense. They don't have the same units.
A ratio of inductive reactance to resistance makes more sense, but that is dependent on frequency. A mains 50Hz mains transformer would have the largest ratio of inductive reactance to resistance.

 

Check out the Fez video above, it might help.

He is using the term to describe a gate-drive transformer. That sort of transformer can also be used to galvanically isolate data, but the two aren't quite the same. The gate drive transformer needs a lower impedance than the data isolation transformer but they are both pulse transformers, but they are not power transformers, although if you only need a small amount of power they could be used as such.

 

So, go back to the beginning and state what you are trying to achieve.
So far we have 100kHz and 5V @ 3A, but what is the input voltage?

Have you read Hitchhikers' guide to the galaxy? The answer to "life the universe and everything" was 42. The answer was nonsense because the question was ambiguous.

 

Well, you could choose 5V as the input voltage then you would need no circuitry whatsoever. I'll ignore that idea.
If your input is between 6V and, say, 30V, then you would use a buck regulator, unless you wanted it isolated.
If you needed isolation, then for 15W you would almost certainly use a flyback circuit, just because there are so many device out there to do that job. The ripple is poor but the circuit is simple, and the extra filtering you might need still makes it cheaper than any alternative.

 

My input is AC supplying a transformer, as described in the question.

Where in the question does it say that?

 

In general, the higher the AC secondary voltage, the lower (as a percentage) the rectifier losses,
however, the greater the stepdown ratio of the buck regulator the lower the buck-regulator efficiency.

So, as the secondary voltage goes up the rectifier losses reduce but the buck regulator losses increase.
At some point the losses will be at a minimum, but where that point is will depend on the particular buck regulator you use.

 

Question updated.

Is your AC 100kHz?

 

Is your AC 100kHz?

Yes. I updated the question. Feel free , if you want, to delete our back and forth messages on these clarifications, since the question is now clarified.

Sorry for not being clear!