2W Regulated Power Supply - Page 1
Forum section: electronics / projects

author: washer
author_rank: Newbie
timestamp: June 22, 2012, 05:07:31 am
content: Hi all,I'm pretty new to the forum here but have been loving the EEVblog videos for a few years now. Sorry if this is in the wrong place or duplicated, I found quite a few posts on bench power supplies and some big honkers, but nothing on smaller supplies so here it goes!I'm working on a 2W power supply with a 277Vrms (line-to-neutral) source and a 3.3Vdc output with a max current draw of around 200mA.Since we have pretty meager requirements, I've been looking at a resistive or capacitive supply similar to the ones discussed in thisMicrochip app note. I simulated the rectified resistive supply in LTspice and found out (more from trial and error with load resistors than the equations in the note) that it can't support a load of more than 50mA. I've attached two pictures of my simulation with a 16? load failing the ckt, and a 70? load showing proper operation.I can follow the math in the note, but am a little confused as to which equations apply to which topologies and how I can find out what the capacity of the supply is.Can anyone help explain what is the bottle neck in this supply? Is there a way to pull 200mA from a resistive or capacitive supply? If not, what would be the next easiest/cheapest/fastest design?I haven't found many step down transformers that I could use for a linear supply, but maybe I haven't been searching well enough. I've also started to look into some Flyback and other SMPS designs, but was getting pretty confused and would really like to get a "good enough" design working while I try to learn about a more efficient SMPS design. If anyone has any advice/references for a SMPS, I'd love to hear that as well!So is there any way to pull 200mA from this type of cheap supply?Thanks for any help!

author: T4P
author_rank: Super Contributor
timestamp: June 22, 2012, 06:52:26 am
content: Not that you have any wayThese sort of power supplies are extremely dangerous as they are not isolated but rather one touch even on the output and you can be sure of quite a fantastic firework for freeEither way both methods are pretty scary, if you really want to avoid transformers i'm afraid what you are looking for is quite hard to accomplish1)Capacitive Supplies depend on reactance ( I learned that the hard way ... ) and can't supply much2)Resisitve Supplies involves a huge resistor and very limited currentNo i don't think you ever have a option for a transformerless power supply, use a cheapy switching wall wart at 5V and then filter out with a LD1117AV3.3

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 22, 2012, 01:07:42 pm
content: Apart from being dangerous you are fighting a lost battle here. If you remove the LT and everything to the right of it and just put your load parallel to the capacitor you will see exactly the same output.

author: Flávio V
author_rank: Regular Contributor
timestamp: June 22, 2012, 07:12:40 pm
content: Transformers bellow 10VA are small,light and cheap...so there is no need to be suicidal and cheap...

author: washer
author_rank: Newbie
timestamp: June 22, 2012, 09:05:12 pm
content: Thanks for the advice guys. I'll look to source a few transformers sometime tomorrow and ping this thread again to get some sanity checks on the options and new design.

author: mukymuk
author_rank: Contributor
timestamp: June 22, 2012, 10:51:27 pm
content: Try decreasing the value of R3--but just for simulation. The circuit you have is impractical to build due to the power dissipation required of R3. Look at the capacitive drop techniques described in the app note. That's the way to go. Even so, 2W is pushing the limits of practicality with this type of design.Look at Power Integrations website for low-power mains switchers. They have software tools that will allow you to design something that works without knowing a whole lot about smps. 2W designs can be done with standard inductors--no custom transformers required. BOM cost for low volumes will be very close to 4.4VA transformer designs though (maybe even higher).As far a safety--use a fuse and don't touch the sparky parts.

author: Zero999
author_rank: Super Contributor
timestamp: June 24, 2012, 08:43:52 am
content: Why not buy an SMPS?If you don't want to have an ugly plug top PSU, buy one, rip out the guts and put them in your project. Avoid the really cheap plug top PSUs which are often dangerous.Another option is to design your own SMPS but you need to know what you're doing, otherwise it could result in death and smoke; it's only really worth it for educational purposes.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 26, 2012, 04:04:54 am
content: A 277V input power supply is not going to be a common off the shelf device, but maybe you can use a transformer to buck it down to a more reasonable voltage and then use a more common power supply?For low power, it's possible to use a capacitive dropper followed by an isolating converter. Just think of the capacitive dropper as approximating a current source when the output voltage is more than an order of magnitude lower than the input voltage.

author: washer
author_rank: Newbie
timestamp: June 28, 2012, 04:21:12 am
content: Since we were having some trouble getting enough power out of the capactive supply and it also wasn't as safe as an isolated option, we're going back to the old tried-and-true linear transformer for our current design while we try to buff up on SMPS for the next rev when size is more of an issue.I've found two options from Triad Magnetics that should work for our supply, but I wanted to run the specs by y'all to see if there is a better choice.Option 1 is theFP16-150, 2.5VA, 16V@0.15A (series), 8V@0.3A (parallel). Option 2 is theFS10-250, 2.5VA, 10V@0.25A (series), 5.0V@0.5A (parallel).Nothing fancy here, just the transformer, diode bridge, smoothing cap, and LM317. I was able to talk to an engineer from Triad today and these models rated for 230VAC should be fine for 277VAC as well, we just need to account for a ~20% increase on the secondaries.So if our load will peak at 200mA on a 3.3V power rail, but most of the time it will be drawing much less, is there a better choice of transformer? Will either/both work fine? I'm assuming that the voltage will peak on the transformer with less of a load, but the input range of the LM317 should be able to handle it (up to 40V, but hopefully much less due to heat).Critiques?

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 29, 2012, 04:00:32 am
content: Add a cap on the output and one of the diodes is backwards. You'll also want to fuse the input.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: June 29, 2012, 10:24:53 am
content: And since your running at 277V ,personally I would go with your option 2 (the 5v@.5 using center tapped windings) so a full wave rectifier rather than a bridge . this will give you a V of10*1.41/2 +20%= 8.4 - 1 (typical drop across 1 diode) =7.4V at your regulator input.

author: washer
author_rank: Newbie
timestamp: June 30, 2012, 03:20:02 am
content: Are pins 6 and 7 a center tap? The way I was reading the schematic was that they are treated like separate transformers. Can you treat them like a center tap for a full wave circuit instead, shorting out pins 6 and 7?

author: Kevin.D
author_rank: Frequent Contributor
timestamp: June 30, 2012, 09:35:07 am
content: Pins 6 and 7.? I dont know. If it's a transformer with 2 separate secondaries then you can wire them in series or parallel . If you have the windings wired in series , you then use the common connection of the 2 windings as the centre tap and use 2 diodes on the ends to make your full wave rectifier.

author: Zero999
author_rank: Super Contributor
timestamp: June 30, 2012, 09:18:41 pm
content: Quote from: NiHaoMike on June 26, 2012, 04:04:54 amLots of ready made quality SMPS units, designed for 90V to 250V operation will work hapilly up to 277VAC, as long as the voltage doesn't rise any higher, it should be fine. If reliability may be an issue, a better alternative could be an industrial unit designed to work from 250V to 500VAC.

author: T4P
author_rank: Super Contributor
timestamp: July 01, 2012, 02:14:26 am
content: NOTE: The quality ones, not the POS OHLJthe ones with >400V bridge rectifiers and >400V caps and 400V parts

author: szhighstar
author_rank: Contributor
timestamp: July 01, 2012, 06:11:57 am
content: please advise input voltage and current and output voltage and current and work frequency and dimension requirement, I can design transformer for you, thanks!

author: Zero999
author_rank: Super Contributor
timestamp: July 15, 2012, 01:25:01 am
content: I've just realised it's obvious.Use a small transformer as an auto transformer to reduce the voltage by 30V.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 15, 2012, 03:05:17 am
content: Quote from: washer on June 28, 2012, 04:21:12 am*I'm a bit concerned about this bit--it sounds a bit like this is not just a "fun" or Uni project,& that it is going to be sold,or installed in an Industrial situation.The non standard "mains" supply also gives me that impression,as I am not aware of anywhere in the world which uses a domestic 277V 60 Hz supply.If it is to be used in Industry,I would suggest to try to buy a ready made DC supply ,just add the 3.3 v stuff on after it.

author: szhighstar
author_rank: Contributor
timestamp: July 15, 2012, 03:48:31 am
content: first, please advise work frequency, you select line transformer, which is applied on 50Hz or 60Hz circuit.Addition, seconday and primary windings are connected from your drawing, primary winding and secondary winding should be isolation.