How to spec. out Transformer and Bridge Rectifier

I have a project that for past versions I have been using a DC power supply. To take it to the next level, I would like to the AC to DC conversion on my PCB. This is my first time ever specking out parts to do this and I am wondering if I have the right process. I will take you through my thoughts.

I currently use a LM2596-ADJ switching voltage regulator with an input voltage of 12.10 VDC I saw on average a 78 mA current from my circuit. The highest peak I saw was a fairly rare 140 mA. So that brings me to a peak power consumption of 1.69 Watts.

With that I went to mouser and filtered for any transformer that accepts a 110-120VAC input and has anything between a 2-3.5 Watt rating. The cheapest one was the F36-065-C2 datasheet https://www.mouser.com/ds/2/410/36-065-C2-263502.pdf.

Now for the bride rectifier. I discovered that bridge rectifiers are cheap and can be small with a high amperage. With that I chose a large 1.5 A in a SMT package and chose the cheapest one the ABS15M RGG. datasheet https://www.mouser.com/ds/2/395/ABS15J SERIES_A13-335344.pdf

Okay then I drop a 470uF cap on the input line to my voltage regulator and I should be good to go right????

So my questions:
Was my thinking correct with the transformer and rectifier?
How big of a filter cap should I put on the incoming line? Is there a rule of thumb or calculation?
Why is it that my 1 cubic inch tablet charger can put out 5 watts and I spec out a transformer twice that size and it is much lower power? How do they do they make it so small?
In the end I want a cheap AC to DC conversion and not have it be too massive. (I can make the size of what I specked out work if needed)

Thanks for your help!

There are many criteria not used in your assumptions.

VA ratings must be much higher for line rate rectified output because the transformer efficiency is only 60~70% when used for pulse charging with low ripple.

The ripple current peak/average is related to the 1/(%pulse width) and 1/% ripple.

The equivalent R of worst case Vin and max load must be considered for droop and meet the regulator Vmin dropout requirement.

If you prefer to use a reliable 1" cubic SMPS regulator, buy one. It's smaller , more efficient and regulated.

SunnySkyguy said:

There are many criteria not used in your assumptions.

VA ratings must be much higher for line rate rectified output because the transformer efficiency is only 60~70% when used for pulse charging with low ripple.

The ripple current peak/average is related to the 1/(%pulse width) and 1/% ripple.

The equivalent R of worst case Vin and max load must be considered for droop and meet the regulator Vmin dropout requirement.

If you prefer to use a reliable 1" cubic SMPS regulator, buy one. It's smaller , more efficient and regulated.

Click to expand...

Great SunnySkyguy this is what I need to learn.

ripple current peak/average: 0.140A/0.078A = 1.795 ...... Well for the pulse width...Hmmm....I am regulated to 4V and lets say that out of the transformer I get 36V/sqrt(2). so to get an average of 4V from 25.4 VRMS it would be 4/25.4VRMS = 0.157 (%pulse width??) 1/%pulsewidth = 6.35? ..... 1/%ripple = 1/1.795 = 0.5571 ...... If by some miracle I did that right you say that these two numbers are related...how? Please feel free to mock my ignorance! I need to learn.

Well the Vmin Drop out for the LM2956-ADJ https://www.ti.com/lit/ds/symlink/lm2596.pdf is 1.3 V differential voltage. So if I am going for 4 volt output then my Vmin = 5.3V. Now we want to find the worst possible resistance. (Of the transformer?) Well....hmmm... the data sheet I have doesn't explicitly say. Then again the regulator I chose according to the your first point is undersized so I may need to go hunting again.

This is a edit to my last post

I just found something online that talked about sizing the cap. It said that C=(Current*period)/how big your voltage dip can be so C = (2 Amp * ((1/60 Hz)/2)/(25.4-5.3)V= 829 uF. Bump it up to 1200 uF to be safe. Sound sane?

Jacob Harris said:

How big of a filter cap should I put on the incoming line? Is there a rule of thumb or calculation?

Click to expand...

One Ampere for every 1000 uF is a common round figure. This is for rectified 50/60 Hz and typical low voltages.

The RC time constant can tell you how much the volt level will drop as the capacitor powers a given load, between peaks?

Your 470 uF is a reasonable estimate to start out.

Why is it that my 1 cubic inch tablet charger can put out 5 watts and I spec out a transformer twice that size and it is much lower power? How do they do they make it so small?

Click to expand...

Such power supplies run at a high switching frequency, allowing small transformers.