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How to spec. out Transformer and Bridge Rectifier

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Jacob Harris

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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!
 

SunnySkyguy

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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.
 

Jacob Harris

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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.

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.
 
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Jacob Harris

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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?
 
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BradtheRad

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

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?

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

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