how to transform 120V AC to 5V DC 5A load using flyback transformer circuit? Setting frequency - low cost, low footprint - change load capacity

Question

i am using a flyback transformer circuit to keep the footprint small and cost efficient. Its been suggested to me to keep the frequency high so the transformer can remain small.

how is the frequency set in a flyback converter circuit?

Which components are different when transforming a load of 400mA or transforming a load of 5A? (i am using 5A, though most of what i see on Google is people looking to transform to 400mA for 30-60V DC)

is this something i can find load measurements on my components for, if so, which components measurements should be reviewed when changing the load capacity to a greater amperage?

this is my application .. 120V AC -> flyback transformer circuit -> 5V DC 5A power draw

the others i have seen are 30V DC -> flyback transformer circuit -> 5V DC 400mA power draw

the power draw is much greater in my circuit, i am looking to find what changes there are.

the flyback converter circuit i am designing is going to change AC to DC also. I think (don't know, looking for confirmation on) a typical flyback transformer changes DC to AC and then back again in it's typical application. So it does not matter whether it is AC or DC on its first input?

i am going to use a tool called WEBENCH - its available from Texas Instruments https://webench.ti.com/ . suggested to me by @user4578 on the post that lead to this Which buck converter can be used for 120 V AC to 5 V AC rated at 5 amps? - on that post it was decided to use an isolated transformer circuit to step down from 120V AC instead of the buck converter i had originally intended.

Answer 1

how is the frequency set in a flyback converter circuit?

The frequency is set by the switcher IC. To find out the details you must look at the datasheet for specific chips. Its going to be different for every one of them.

• Sometimes its a fixed frequency determined by the chip design and you cant change it.
• Sometimes you add an external component (resistor or capacitor) to set the frequency.
• Sometimes there is a pin that can select from a set of frequencies by applying a voltage.
• Sometimes you use a separate oscillator to set the frequency.

As far ad determining what a reasonable frequency is...

• Somewhere in the 100KHz to 1MHz range is pretty typical, but higher and lower is not impossible.
• It depends on your transformer. The transformer will probably only be rated at a specific frequency, or range of frequencies. Using it outside of whats specified you don't have any guarantee on performance. So use what the datasheet says if you can.
• Your switcher IC will also have some frequency limitations that are stated in the datasheet.
• Higher frequencies can use smaller transformers but have higher switching and core losses. so its a trade.

Which components are different when transforming a load of 400mA or transforming a load of 5A? (i am using 5A, though most of what i see on Google is people looking to transform to 400mA for 30-60V DC)

All other things being equal the transformer is going to be 12.5 X larger for 5A vs 400mA.

Wurth electronics has a great tool called "Red Expert" that can help with transformer selection. They have a whole section in the tool to select fly-back transformers for offline switchers.

https://redexpert.we-online.com/redexpert/

is this something i can find load measurements on my components for, if so, which components measurements should be reviewed when changing the load capacity to a greater amperage?

When increasing the amperage, all the power path components need to be reviewed.

• Rectifier Didoes, wattage rating increases proportional to current.
• Transformer, volume increases proportional to output power
• Switcher IC, need to check that it can handle current levels.
• Switching transistors (if any)
• Bulk holdup capacitors probably increase in size proportional to output power level.

the flyback converter circuit i am designing is going to change AC to DC also. I think (don't know, looking for confirmation on) a typical flyback transformer changes DC to AC and then back again in it's typical application. So it does not matter whether it is AC or DC on its first input?

Some converter designs work with either AC or DC input, some only on AC. There is no universal answer here. Usually if there is a full bridge rectifier at the front end of the design, then there is a good chance it will work with DC also.

Answer 2

how is the frequency set in a flyback converter circuit?

There is a chip (usually) that operates at a certain frequency, quite often 100 kHz but can be significantly lower or, up to a few hundred kHz higher.

Which components are different when transforming a load of 400mA or transforming a load of 5A?

If the converter is designed to deliver up to 5 amps then there is no difference. If designed to only output 400 mA then, to use it for 5 amps will pretty much change every component.

is this something i can find load measurements on my components for

You need to look at various flyback converter designs to get a feel for things.

which components measurements should be reviewed when changing the load capacity to a greater amperage?

I wouldn't recommend this - trying to make a low power flyback converter suitable for a significantly higher power will require wholesale changes and is much more trouble than it's worth. You would redesign completely.

So it does not matter whether it is AC or DC on its first input?

Usually it doesn't matter but, you do need to put a bridge rectifier (and capacitor) on the front end of a flyback DC to DC converter to make it suitable for AC applications but, there may be a lot of other details that need to change too in order to meet EMI specifications. A 30 volt rated DC to DC converter with a bridge rectifier and smoothing capacitor will not be able to work with AC voltages higher than about 20 V. It has to be designed properly as in all circuits.