Power Supply from Mains Voltage

[dreiercarr]

Hi all,

I'm looking to design a power supply that will drop 120 V AC down to 5 V DC and 3 V DC requiring no more than 500 mA of current. I've successfully prototyped a basic transformer with a bridge rectifier and a 5V regulator, but I'm really looking to get the size and cost of the design down as much as possible. Efficiency is pretty important as well and right now the 5V reg is dropping almost 20V so it is dissipating quite a bit of heat.

I know it's possible to get down to the size and price I'm aiming for, I ordered a knock-off iPhone wall charger on eBay and tried to reverse engineer it but most of the components were unlabeled and it was also a fairly shoddy knock-off. I can try to go back to it, post photos, and try to follow the traces if anyone thinks it would help. My hunch is that they are multiplying the 60Hz by something much higher so they can have a much smaller and cheaper transformer to step the voltage down at a low current, but I'm just guessing.

As an undergrad we haven't learned much more than basic transformers and rectifiers for power supplies, especially dropping down from mains voltages to digital logic. I've also had a bit of trouble finding any solid resources online, maybe I haven't been looking in the right place. What should I be reading up on? Switched mode power supplies? Something else?

Any help would be greatly appreciated!

Thanks a ton!
- Dreier

[franklin97355]

Dangerous but works.....http://www.google.co.uk/search?client=opera&rls=en&q=transformerless+power+supply&sourceid=opera&ie=utf-8&oe=utf-8

[uoip]

For practical purposes, buy a power supply off the shelf. Even if you know how to design one better than the big manufacturers, you can't possibly compete with their cost structure, because they have the volume advantage.

If you have unusual requirments that aren't met by off-the-shelf designs (adjustable voltage, multi-voltage output, constant current limits, etc.), it's probably easiest and cheapest to start with an off-the-shelf power brick or wall wart, and then regulate its low voltage to do what you need. Let someone else deal with the issues around the mains power. That's especially true if you're developing a commercial product that would need agency approval, like UL or CE listing.

But for education and fun, there's no harm doing it yourself, as long as you take the right safety precautions. And it's great to figure out how these things work.

Almost all modern efficient supplies are switched mode, and you need to read up on them to compete in the area of efficiency. As you figured out, they use high freqency, which among other things allows their inductors to be much smaller than the equivalent 60Hz transformer would be.

Without changing things too much, one minor tweak to your current design would help its efficiency a lot and make it run cooler, even if it doesn't bring it up to SMPS standards. Your 5V regulator shouldn't be dropping 20V. Choose your transformer and filter capacitors such that the regulator gets the minimum input voltage it needs to stay in regulation. Depending on what regulator you're using, that's probably around 7V input, but read the data sheet.

[dreiercarr]

Thanks for the help guys!

Last semester we did look into an app sheet by Microchip for transformerless power supplies, but couldn't get it to work. I think we were just trying to pull too much current and the charge time for the cap just couldn't keep up. We were also pretty intimidated by that 1F cap and running 120V over quite a bit of the ckt. http://ww1.microchip.com/downloads/en/appnotes/00954A.pdf

I agree that getting something off the shelf would be the way to go (and what we were hoping for when we started the project), but we haven't been able to find anything that comes remotely close to the size constraint we're looking for. We're looking at USB wall chargers for phones and really think that is the sweet spot.

Can anyone help point us in the right direction for an off-the-shelf solution?

[lyndon]

This should get you started:
http://www.maxim-ic.com/app-notes/index.mvp/id/4087
ISTR that TI has some good online wizards where you can enter your requirements and it will suggest a chip and component values. Been quite a while since I looked at them, tho.

Be warned, though that designing a good switched mode power supply is not trivial. I have a friend who's been doing it for decades and sometimes even he scratches his head. However, for your small power requirements (3V @0.5A) you should be able to find a reference design that can handle that. Follow it exactly, down to PC board layout if supplied.

Have fun!