AC Switching Supply Design - Page 1
Forum section: electronics / projects

author: ShatteredRAM
author_rank: Contributor
timestamp: November 04, 2015, 06:20:17 pm
content: Hi All.Currently I am working on the design and manufacture of a Desk Lamp for my A2 (Pre-Uni) Product Design Coursework.I built a working prototype for the electronics making use of a boost converter to up 12V to ~30V in order to drive two independent strings of LEDs, original design would of put out about 1000 lumen, I have toned down the number of LEDs to only output 600 lumen.Lighting my room when lights off:I want to further the design (Even though I don't get marks for the electronics.) by building the power supply into the unit, both for small gains in efficiency, reducing the drop from the converted DC from the base of the lamp to the lights, but also the number of wires, passing AC up the lamp would reduce the number of wires to two. (Fed through the body using custom made hinges.)While I understand the basics of how to convert AC to DC, I wanted to find out if there are any traps that I should concern myself with, and anything specific that I should look at, what protection should I build into the power supply? My constraints are pretty tight, I want to build it into the unit, the base, will be 25x75mm Steel tube, its a challenge but I want to get a 230V to < 20V transformer capable of 1A peak current (For 100us Max) in there, the rest of the lamp is 40x40mm Aluminium Square tube.Currently I know that MOVs are an option for AC protection, although I am more than a little confused by the selection and proper use, would a PTC also be a good option?Current Board:Any help is appreciated!Byron Theobald.

author: mariush
author_rank: Super Contributor
timestamp: November 04, 2015, 07:12:08 pm
content: You don't want a dc-dc converter, you want a LED DRIVER.LEDs work with current, not voltage, so as long as the voltage is higher than the forward voltage of a LED (or the sum of forward voltages of leds connected in series), the LEDs will light up. From that point, it's a question of limiting the current going into them.Since as you say you're not graded on the electrical work, leave the AC to DC power supply to professionals, just get a reasonable power adapter capable of delivering the power you need. For example, if you have 2 strips with 12 leds each with a forward voltage of about 3v and a current rating of 20mA and they're connected in series, it means you need a led driver or two led drivers capable of delivering 12x3v = 36v at 20mA or 0.72w Considering a led driver's efficiency of about 85% and led driver's own consumption and taking other things in consideration, it's safe to round up this fictional example to 1w, or 2w for both strips. That means you could power this fictional lamp with a 5v at 2w/5 = 0.4A or more , or a 12v @ 170mA or more and so on.Of course, your leds are probably for more current but you get the idea.I see a box from Farnell in your picture, so I'm going to give you Farnell links... they have a whole section dedicated just to led driver ICs ... here's the link :http://ro.farnell.com/led-drivers?searchRef=SearchLookAheadIf your mains power adapter will output less than forward voltage of the leds you connect in series, choose a step-up (boost) led driver.. use the parametric search to choose one that can do a wide input voltage and a high enough output voltage and high enough current to work for you.For example, this one will drive up to 36v and up to 20mA (configurable) and it's super cheap :http://ro.farnell.com/taiwan-semiconductor/ts1937cx5-rf/led-driver-white-boost-5sot25/dp/1824778

author: ShatteredRAM
author_rank: Contributor
timestamp: November 04, 2015, 07:53:05 pm
content: That is what I have, My current board makes use of an LT3466 LED driver IC, it was a bit pricey but it had what I needed, I wanted to be able to control the drivers individually using PWM from a micro, which this chip allows for. Preferably I want to be able to keep this topology and integrate the power supply into the lamp as well. (Currently the idea was to use a wall wart, but I wanted to try building the PSU myself.)I am doing the electronics for this purely as an exercise to better my understanding and for my portfolio.Planning to study EEE at degree level (starting next year) as it is something of a passion for me.But I do see what you mean, and I just wanted to get a feel for the necessary components and safety considerations, cost is also a factor here, and a remade module would be far cheaper and is more likely to work.Testing the circuit properly, peak startup current is about ~600-700mA, drops to bang on 400mA when operating. 6W input at 12V then? I assume the start-up current can be counted as inrush?

author: ShatteredRAM
author_rank: Contributor
timestamp: November 04, 2015, 10:10:09 pm
content: From experience I know that the LEDs do survive being hot plugged.Regardless, the CC and LEDs are meant to be directly connected, this was also one of the reasons for moving the converter to the top of the lamp as well, so it can be self contained in one unit, the design of the lamp is somewhat modular, with idea being that the long sections could be user replaceable to allow them to customise the lamp to their liking, in the development this would be something that I will address and state as one of the reasoning for combining the electronics, as it could cause an issue.These LEDs are pretty hardy, they can take 300mA pulse current.

author: ShatteredRAM
author_rank: Contributor
timestamp: November 05, 2015, 12:33:02 am
content: What should I be looking at for hot swap protection? The chip (LT3466) itself is specified as having soft start and puts itself into low power mode (pulse skipping), reducing output current.A brief look brings up TVS Diodes to lower the voltage, preventing overvoltage, unsure how this would effect normal operation, warrants further investigation, thanks!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 05, 2015, 11:56:20 am
content: Regarding the layout... why are the SMT pads so gratuitously long? Not even wave solder needs that much extra.At a glance, the placement looks reasonable though.Tim

author: DanielS
author_rank: Frequent Contributor
timestamp: November 05, 2015, 01:32:03 pm
content: Quote from: T3sl4co1l on November 05, 2015, 11:56:20 amI would guess:1- ease of hand-soldering2- accommodate some alternate footprintsFrom that point of view, only the 1206 footprint looks oversized beyond reason.

author: ShatteredRAM
author_rank: Contributor
timestamp: November 05, 2015, 04:08:53 pm
content: Quote from: T3sl4co1l on November 05, 2015, 11:56:20 amThat was the footprint that I picked using KiCAD, at the time I wasn't sure if I would have access to a hot air gun, so I put in footprints that would be easier to hand solder, going to change these to much smaller footprints when I redesign the PCB.Quote from: DanielS on November 05, 2015, 01:32:03 pmYeah, I must admit it is definitely oversized, that is the included hand-soldering footprints in KiCAD.I am mainly doing a re-spin because there are some issues with the current layout, the linear regulator is flipped and, MOSI and MISO are swapped, not critical, but something I want to clean up, I also want to clean up the mounting and pin out the inputs. (Currently they are mounted on the rear of the board.)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 05, 2015, 06:32:53 pm
content: Wowww, those are defaults in KiCAD?! I would've hoped the people in charge of those libraries would know their head from a hole in a PCB...Tim

author: ShatteredRAM
author_rank: Contributor
timestamp: November 08, 2015, 09:14:45 pm
content: Defaults isn't the right word, more like the one you find, for some parts, there are handsoldering that have longer pads, which is what I used here, there are also non handsoldering footprints as well.Library / Footprint management is a mess in KiCAD, schematic is completely detached, I could link a resistor to a QFP package if I really felt the need.

author: Marco
author_rank: Super Contributor
timestamp: November 09, 2015, 01:28:34 am
content: How realistic has the design have to be from a product standpoint? Does it need to have PFC? Does it need to be very efficient? Can't you just orderthis? If PFC&efficiency are required you're going to have to ditch your existing CC driver and build something around a single mains->LED IC, every extra stage is extra loss.If they don't matter and you really want to make something yourself I'd design around easily available flyback transformers, look at what Coilcraft and Wurth have which is in range, generally they recommend an IC to go with it which will have suggested circuits in it's datasheet.