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I'm trying to design an AC/DC converter to implement into a project I'm working on. The project needs to be isolated from mains and shouldn't require more than 1A at 5V. My question for you is what is my cheapest option for converting 120AC mains to 5V without blowing anything up? Keep in mind this needs to be implemented on a PCB so simply buying an AC/DC converter won't work for me.

I'm looking for recommendations on different circuit types and especially component recommendations.

(NOTE: I do know the dangers of mains AC. I have a pretty good knowledge of circuit design but I'm a little lacking in the area of power conversion techniques.)

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closed as off-topic by Nick Alexeev, Daniel Grillo, Leon Heller, Dave Tweed, Chetan Bhargava Feb 11 '14 at 23:02

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    \$\begingroup\$ Why the requirement for implementing on a PCB? Are you aware there are AC/DC converter modules that can perfectly be mounted on a PCB? \$\endgroup\$ – jippie Feb 11 '14 at 20:10
  • \$\begingroup\$ Flyback converter is the most cost effective topology for the low power output like your 5W. Other than that, there's no royal road to power electronics [with apologies to Euclid]. \$\endgroup\$ – Nick Alexeev Feb 11 '14 at 20:40
  • \$\begingroup\$ You say 'not more than 1A', but how sure are you of the lower bound? Other things (like size of your production run) equal your cost depends a lot on the power required, so aim for 1A only if you need 1A. It makes no sense to do a cost-optimized 1A supply when optimizing the rest of the circuit brings its needs down to 100mA. \$\endgroup\$ – Wouter van Ooijen Feb 12 '14 at 8:23
  • \$\begingroup\$ -NickAlexeev Thanks for the advice, i wasn't familiar with a flyback converter as being the most cost efficient method I will look into various components to minimize my cost -WoutervanOoijen I hadn't thought about worrying about a minimum power output, thanks for that \$\endgroup\$ – TrapLevel Feb 14 '14 at 13:38
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The cheapest option is to copy exactly what is used in the cheapest 2.1A cell phone charger and buy the same garbage parts from the same garbage sources and have them put together in the same illegal sweatshops. They're almost always flyback converter designs, because it's cheapest at low power levels. I've seen such things for < $1 US shipped from China.

For bonus points, save money by ignoring UL/CSA/IEC rules on clearances, ignore flammability concerns, avoid using X and Y mains rated safety capacitors, use insulation materials and PCB material from the lowest bidder (preferably returns), don't bother with any EMI considerations, and, since you'll fail anyway, don't bother with FCC and safety agency approvals. Hint: You can buy rolls of "CE" (Can't Enforce) stickers quite inexpensively.

If you actually have needs beyond "cheapest" you could look at reference designs from Power Integrations, some of which are usable by a neophyte.

enter image description here

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    \$\begingroup\$ While an attack focusing on "cheapest," this is a good answer that demonstrates what not to do... \$\endgroup\$ – JYelton Feb 11 '14 at 20:03
  • \$\begingroup\$ And don't even think of running a high-pot test test. That'll just cause more production failures. Also, you bother designing to 1 A. Most people probaly won't push it to the limit, so you get away with 750 mA maybe. By the time someone does notice this, you'll have sold 100,000 and skedaddled off to some other corner of the world using a different name. \$\endgroup\$ – Olin Lathrop Feb 11 '14 at 20:06
  • \$\begingroup\$ @JYelton Hopefully enough over-the-top that nobody mistakes the first part for real advice. \$\endgroup\$ – Spehro Pefhany Feb 11 '14 at 20:11
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    \$\begingroup\$ @Spehro At first I thought this was Olin's answer, because, well, he likes to do that. :) I see now he's added a comment in the same spirit, so I am happy. Related video about a fake Apple USB charger that violates safety rules: youtube.com/watch?v=wi-b9k-0KfE \$\endgroup\$ – JYelton Feb 11 '14 at 20:14
  • \$\begingroup\$ @SpehroPefhany Thanks for the advice! I wanted the focus to be on cheapest so that I could add all the necessary components to filter out noise and be safe as possible later. \$\endgroup\$ – TrapLevel Feb 14 '14 at 13:47
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You're going to need a transformer in there somewhere to push 5 W accross a isolation barrier. Rectifying the AC and chopping the result at 100 kHz or more to drive the transformer will save more on the transformer than the extra electronics will cost.

Since cost is the primary objective, do a full wave bridge followed by a small cap. Somehow make 100-300 kHz and use that to drive the primary of the transformer. Full wave rectify the output and use a simple opto-isolator to kill the oscillations when the output gets above the regulation threshold. At 5 W out you can afford some inefficiency without it adding lots of cost, like a heat sink.

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Power Integrations do a series of cheapish off-line switching regulator chips and Premier Magnetics seem to have the transformers to suit (or you could wind your own).

Here is the link for the transformer supplier and the list of transformers they do. The data sheets look good and they also include the typical circuit using the power integrations device: -

enter image description here

Here is the device that looks about right for the voltage and power level you require: -

enter image description here

In quantity the TNY255G is about $1 (according to digipart). I've used a slightly more higher powered device from these guys on a unit that was shipping 5k per annum and only remember 1 failure in two years.

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  • \$\begingroup\$ @TrapLevel OK dude glad to be of help and pity it got closed down because I bet there may have been a few more answers but that's the way it is. Good luck. \$\endgroup\$ – Andy aka Feb 14 '14 at 16:52

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