Multiple output isolated supply - Page 1
Forum section: electronics / projects

author: Buriedcode
author_rank: Super Contributor
timestamp: August 16, 2023, 08:13:31 pm
content: Hi,Yet another thing that's been on my list of things to prototype has been a power supply for my pedal board - I realise you can buy pretty good ones already cheap, but I haven't designed many SMPS from scratch, so I thought it would be a good opportunity to get hands on with development and debugging.So, as all the outputs will be roughly the same voltage, max 100mA each, and regulated with linear regulators I decided on a open loop half bridge converter. I'm hoping that one driver can drive one or two transformers - which each has 4 isolated outputs. Open loop because it's easier, input voltage won't vary that much and I plan on regulating the outputs with linear reg's anyway (which will also provide a minimum load).To make winding transformers easier I thought I'd have just one winding per output, then a full bridge rectifier, inductor, smoothing can then regulator (perhaps with built in current limiting).My questions are:1) I assume each output will need its own output inductor - how is the value of this decided? Do I have to use lower value inductor as theres 4 outputs - each being L/4?2) How does one decide on primary turns of the transformer? The ratio will have to be 1:2 (12V in, ~ 11.4V out after rectifier for a half bridge). I've found some conflicting equations given the frequency, input voltage, output voltage etc.. Some say as little as 2 turns for primary.3) I plan on using some ED20/6 cores I have - but these are gapped with 0.3mm (Al = 150nH), which I assume will have to be removed or greatly reduced. How does one calculate the minimum inductance required? I have the specs of the cores (Bmax, core loss Ae etc..) but in order to know how much I have to reduce the gap, I'll need to know the minimum inductance for the turns required.4) I've read a bit about flux walking, and it seems adding a series capacitor to the primary can help. How is this value calculated? The only info I've found on that is from microchips app note:page 21 of app note AN1114https://ww1.microchip.com/downloads/en/AppNotes/01114A.pdf5) Given the relatively low power, can I get away without current measurement on the primary side? I can add simple current liming on the outputs with resistors or active current limtingbuilt-into the regulators.Cheers!

author: Benta
author_rank: Super Contributor
timestamp: August 16, 2023, 09:37:54 pm
content: Nice lil project.1: As you'll be running open loop with 50% DC, no need for output inductor. A smoothing cap is enough.2: Get your calculator out. This is totally dependent on switching frequency, supply voltage and desired primary magnetizing current.3: the gap might be a blessing, hard to say. It relates to your point:4: yes, this can be an issue. How do you plan to make the V/2 voltage for the other side of the primary?5: completely open. If you protect the primary side, all outputs will go down if one is shorted. If this is undesirable, you need protection on each output.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 16, 2023, 09:42:58 pm
content: This can be a nice project to learn about those things. And I don't want to discourage you if that is your goal, but are you aware that low power isolated SMPS modules are a standard part, are available in many input / output (single dual) voltages and different power levels and cost around EUR2?

author: Terry Bites
author_rank: Super Contributor
timestamp: August 17, 2023, 09:40:17 am
content: The transformer doesn't operate like a conventional AC step up or step down. Sepic or flyback? The inductor requirements differ.The primary inductance is used to store energy on the "switch on" state and transfered to the seondary during the "switch off" state.The ratio is determined by the energy transfer requriements per cycle and switching waveform. Transformer design needs to manage heat core saturation interwinding capacitances.You'd treat the secondary side as the sum of the mutiple outputs.www.monolithicpower.com/how-to-design-a-flyback-converter-in-seven-stepsSimple. You mightfind the TI note helpfulYou might want to put some thought into the input filter. A common mode choke at the converter input will redcuce the supply loop (transmitting antenna) area down.

author: Buriedcode
author_rank: Super Contributor
timestamp: August 17, 2023, 03:44:21 pm
content: Quote from: Benta on August 16, 2023, 09:37:54 pmNice. I've been running simulations in LTspice with a simple half bridge driver, and it looks like without an output inductor, the input current spikes are much larger. But this is something that a real circuit and scope should illuminate.Quote from: Benta on August 16, 2023, 09:37:54 pmYeah, I was hoping for quick and dirty equations, but I realise now I'm going to have to remember my second year module on power electornics (many years ago..). This is a learning exercise too, so kind of the point.Quote from: Benta on August 16, 2023, 09:37:54 pmCapacitive divider. Again, LTspice with ceramic 4.7uF, with 100kHz Fsw shows they sit fairly pretty at V/2 (12V in, so 6V) albeit with 1v p-p triangle variation in voltage. Would it be wise to add resistors across each cap too to balance out variations in the caps? Given the lower power/current I'm hoping to get away with X5R ceramic caps for these - as well as the DC blocking cap in series with the primary.Quote from: Doctorandus_P on August 16, 2023, 09:42:58 pmYes, as a half measure I could take the hassle out of the switching part and use prebuilt modules - but I have a fair number of transformer cores that are sitting in storage being lazy - both new (NOS, with full datasheets) and salvaged (a hassle to measure parameters for, but with Al, core loss and Isat I can measure them well enough for the odd flyback/boost/buck prototype, woudn't trust them for anything more).Many of the off-the-shelf ones can be quite noisy - Of course I am throwing a fair bit of filtering at the outputs anyway but designing from scratch means I have complete control over switching frequency. I do tend to make life difficult for myself sometimes, but I'm not trying to reinvent the wheel here, just trying to DIY something, but choosing the easier route (no resonant converters, low power, slack efficiency requirements etc..).Quote from: Terry Bites on August 17, 2023, 09:40:17 amI avoided going for flyback since they tend to radiate more EMI, although I'll admit when I've previously required a few rails its been my goto topology, as theres so many controllers/designs about, and they tend to "just work". Also, it'll require some kind of feedback for regulation, and I could just use feedback from one winding, but then I'll still be getting into optoisolators, compensatione etc..The main requirement here is isolation, I never had issues with ground loops using 2 pedals, but once I got to 5-6 it started to hum using a single 2000mA 9V supply. Plus I despise 9v batteries (and their cost). This is a first prototype, but if it works well, the next version I can go for better efficiency for a portable pedal supply (Lithium, 8 outputs, smaller, lighter more portable etc..).I have an awful habit of going 0-60 on a new project, then getting sidetracked and forgetting about it for months, then coming back to it - but I'll try to post here if I work on it and my successes/failures.Thanks for your replies!

author: Benta
author_rank: Super Contributor
timestamp: August 17, 2023, 06:26:56 pm
content: Quote from: Terry Bites on August 17, 2023, 09:40:17 amYou're thinking of flyback.This is different, it's an open loop half bridge, and it operates exactly as a 50/60 Hz mains transformer, but at much higher frequency and with square- instead of sine voltage input.

author: Benta
author_rank: Super Contributor
timestamp: August 17, 2023, 06:55:55 pm
content: Quote from: Buriedcode on August 17, 2023, 03:44:21 pmYou shouldn't be seeing large current spikes on the input. If you do, it's most likely power stage shoot-through. You need to have a controlled dead-time between the two power switches.No need for resistors. No need for a DC blocking cap either, the cap divider does that function by itself. The blocking cap is often used on high power converters (>200 W) but unnecessary here.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 17, 2023, 11:03:56 pm
content: Quote from: Buriedcode on August 17, 2023, 03:44:21 pmAs a sanity check, there are isolated modules at 1W~3W 12V - search '1212' finds ~ 47 stocked, 0.75W to 3W, priced from 81c/10+Or search 12S12 or 1209 or 12S09 ...https://www.lcsc.com/products/Power-Modules_634.html?keyword=power%20moduleThis one is not cheapest $2.33/10https://www.lcsc.com/product-detail/Power-Modules_TDPOWER-TWRB1212S-3WR2L_C5119727.htmlbut it is 3W and assuming the image is correct, it is 'open frame' so you can scope it and modify bits if you want to.Appears to have a standard TL431+opto feedback, for regulatedThen, you can focus on simple LC filtering and design a PCB with power jacks, added LDO's etc.

author: Benta
author_rank: Super Contributor
timestamp: August 17, 2023, 11:23:58 pm
content: Yeah, I'd prolly do the same.Not as much fun, though, and no learning experience.

author: Buriedcode
author_rank: Super Contributor
timestamp: August 21, 2023, 06:36:37 pm
content: Ok, whilst I wait for some half-bridge drivers to arrive, I was looking at what wire I have. Luckily I have several reels over various enamelled copper (solderable enamel too).Whilst the actual inductance of the transformer isn't critical, since its for a half bridge, I realised that the actual gauge of wire and bobbin (its a 20/6 ED size) winding size have a pretty big influence on deciding the primary turns. But it got me thinking - for 100kHz (might increase this to 200kHz max, but 100 is a nice round figure for switching losses).. and for low power (10W) do I need litz wire?The winding length is 10.6mm, so for say 21 turns =0.5mm diameter wire. I'd like to have the primary with an odd number of layers so each connection is at the opposite side of the xfmr. And with 4 secondaries I thought having each winding connection on the same side to make layout easier - so thats an even number of layers. This works out nicely with Np = 21, and Ns ~ 44 (ratio of 1:2.1).I have a huge reel of 30AWG (0.25mm) wire, four strands of that will give a maximum diameter of 0.5mm. Am I over thinking this? I do have some 0.5mm solid wire, but its effectively the same area/amount of copper? The only real advantage I can see of using multistrand is that it'll be easier to wind around the bobbin, but even 24AWG/0.5mm is pretty easier to work with. I suppose I wouldn't even need to twist it, so it'll jsut be 4 parallel windings to fill the width of the bobbin.

author: Benta
author_rank: Super Contributor
timestamp: August 21, 2023, 08:03:58 pm
content: No need at all for Litz wire, neither at 100 or 200 kHz.There thing you need to get into is power dissipation when designing transformers.There are two main parts:1: iron/ferrite losses. These correspond to frequency and how "hard" you magnetize the core. Can be tricky to calculate, but most core data sheers have appropriate graphs.2: copper losses. Relatively simple to calculate (resistance and current).Designing switching transformers is an iterative process where you'll have to go back and forth several times until you have a reasonable result. There are no closed-loop equations. Lots of "rules-of-thumb" though.I'd start by finding/defining primary inductance. You get this from frequency and peak magnetizing current. For low power like this, a reasonable figure is 10%...20% of full-load primary current, but can be higher. This is a design tradeoff left to you.Take it from there.

author: uer166
author_rank: Super Contributor
timestamp: August 21, 2023, 08:14:19 pm
content: Quote from: Benta on August 21, 2023, 08:03:58 pmYou sometimes need Litz even at 70kHz.. It's a case-by-case basis. It depends on proximity effect which can be orders of magnitude worse than skin effect alone due to multiple layers of windings going the same direction. Sometimes there is no choice but to use Litz, particularly for inductors (or large coupled inductors, i.e. flyback). Same for LLC and such where you may prefer to non-interleave primary/secondary windings.That said, OP case specifically is low-power, so you are correct in thisparticularcase.

author: Benta
author_rank: Super Contributor
timestamp: August 21, 2023, 09:17:46 pm
content: Well, I answered in thisparticular case.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: August 22, 2023, 12:09:48 am
content: Why winding your own transformer? Wurth, Colicraft, etc will have plenty to choose from and ship you even samples.