How to Speck out Transformer and Bridge Recifier

[jacob84401]

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 http://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 http://www.mouser.com/ds/2/395/ABS15J%20SERIES_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!

[mariush]

The transformer you chose has two 18 Vac secondaries, which you could connect in parallel to obtain a single 18Vac output.

After the bridge rectifier, you'll have a DC voltage with a peak value equal to  1.414 x Vac  - 2 x Vdiode , where Vdiode is the forward voltage of one diode in the bridge rectifier you choose. In your case, you will have about 1.414 x 18Vac  - 2 x 0.7v (see graph on page 3 ) = ~ 24v

However, please note that you may have excess input voltage (let's say up to 125v) instead of the 115v the transformer is designed for, so instead of 115 : 18 you may have 125 : 19.5v
Also, please note the line : 4. Voltage Regulation: 25% TYP @ full load to no load  in the transformer datasheet - this means the output voltage on the secondary may vary as much as 25% depending how much it's loaded, so your standard 18v may actually be about 22-23 Vac. This happens because the transformer is very small, with larger transformers the percentage is much lower.
In the worst case scenario, you could have both of the above at the same time, so the point is don't do your design expecting 18Vac at the output of the secondary, and expect a much wider range of voltages at the output of the bridge rectifier, somewhere in the range of 22-32v, which is important when you decide what capacitor to use ... you'll want to use one rated for 35v or more.

As for the value of the capacitor, that depends on lots of things. You need to know the current and how much you're willing to let the voltage sag, and then you know the capacitor will always keep the voltage above that threshold.

Your transformer is rated for 2.5 VA, which means 2.5VA / 18Vac = ~ 0.14A. There's a formula which approximates how much current you have after rectification, which is Idc = 0.62 x Iac , where 0.62 is a constant that works best with transformers around 20-100VA. With smaller currents, the value may be around 0.75-0.9. 
Anyway, let's say you'll have Idc = 0.62 x 0.14 = 0.0868  but let's round it up to 0.1A and be happy with it.

So after rectification, we have 22-32v depending on mains voltage and transformer load, and we know the transformer is definitely capable of 0.1A .

The capacitance after rectification can be approximated with the formula  C = Current / [ 2 x Frequency Mains x (Vdc peak - Vmin) ] so now you just have to figure out the minimum DC voltage you'd be comfortable with.

For the sake of the example, let's say you want at least 15v DC in even the worst case scenario, so then the formula becomes  C = 0.1 A /  2 x 60 Hz x (22v - 15v) = 0.1 / 840 = 0.00011904761 Farads or about 120uF.  That's the minimum you would need to keep the voltage above 15v all the time. I would just go with a 220uF or 330uF 50-63v rated capacitor and be done with it, as capacitors with these parameters will have pretty much the same diameters or heights.

Now, you can tweak your LM2596 dc-dc converter knowing you'll have at least 15v at the input, with potentially up to about 32v depending on the load (if your device uses little power, the capacitor will charge up to the peak dc voltage)

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?

Those chargers use high frequency transformers and switching ICs that work at high frequencies (70kHz and higher) like LNK4022 for example: http://www.digikey.com/product-detail/en/LNK4022S-TL/596-1491-1-ND/5171431 (see  datasheet linked on the page).  The transformers used with such ICs are often custom (as they require a feedback winding on the primary or secondary side or a particular number of windings) so it's heavier for a beginner to design something like this.  Also, unlike classic transformer+bridge+capacitor solution, such circuit would be harder to test for EMI (noise, radiation, interference etc) and you may have a harder time selling this commercially (more approvals, certifications etc)

The transformers like the one you linked to are designed to work at 50-60 Hz, so obviously they're much heavier and more expensive due to the copper in them and due to limited quantity sold and due to cost of shipping them.

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)

The cheapest would probably be to buy a ready made phone/usb charger or a wallwart (universal ac-dc adapter).

As an observation... for smaller amounts of power, and not cheaply, it would be possible to make an ac-dc power supply without a transformer. First, you would use a capacitive or resistive power supply to get a lower voltage ( see this application note: http://ww1.microchip.com/downloads/en/AppNotes/00954A.pdf ) and then you would need to add insulation by using a digital power isolator IC like ADUM5000 for example : http://www.analog.com/media/en/technical-documentation/data-sheets/ADuM5000.pdf
The downsize is that such ICs are expensive ($5-7 in small quantity), limited in output power (5v at maximum 100mA for Adum5000) and they have a lousy efficiency (up to about 33%), so you'd need 5v @ 300-350mA at input to get 5v @ 100mA at output, which in turns means big resistors and zener diodes for the transformerless power supply.