toroid inductor / transformer question - Page 1
Forum section: electronics / projects

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: April 08, 2014, 11:07:23 pm
content: Hallo ladies and gents, here yet to drain your knowledge for a question of mine.I want to build a transformer for an isolated DC/DC supply. I have mini Ring Core Calculator for calculating inductances and stuff, so that won´t bean issue but the question comes here. I´ve bought today some old stock torroids from a local hardware shop. They knew that they were intended for aninverter but didn´t have a datasheet at the moment. Using my google skills and the program mentioned before I found out that with 2 - 3 turnsI get the inductance I want. Still 2 - 3 turns for a 40W DC/DC converter is a little bit bad. I mean in theory it should work but I don´t feel well with it.So what comes in mind then is to use multiple n turn windings all paralleled up, to make the whole thing seem and feel more sturdy, plus marginallyachieving better coupling between primary and secondary side. The question now:By paralleling coupled inductances of the same magnitude, the resulting inductance is the same as the inductance of a single winding or is it theparallel combination of all?Best guess is the first one, but I had to ask. You see my main problem is to run the whole thing above the audible frequencies but the toroidal corehas a very high permeability. Finally I should mention that the DC/DC converter will just be used for isolation with 1:1 ratios and accuracy is notreally a problem.Thanks for your answers in advance,Lefteris, Greece

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 09, 2014, 02:10:10 am
content: From a hardware store..??The kind of store with hammers and screws and paint? And it's not just a magnet?There are different types of cores, you'd have to do some testing to know what type of circuit it's best for. Just building any random circuit (you didn't mention which type you were intending on using) will likely result in fires...Tim

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: April 09, 2014, 08:36:12 am
content: Quote from: T3sl4co1l on April 09, 2014, 02:10:10 amAs you can understand english is not my mother tongue.It´s an ISKRA FERITI T22G core which according to the online catalog of Iskra,the core material is a "perfect match" to the Epcos T35 Siferrit, which, according to the datasheet of course, is made for broadband transformersfrom 0 - 200kHz with a preferred working frequency from 10 kHz to 200kHz. If I am correct about the datasheet or not, we will see today,when I get my hands on lab equipment.Anyhow, for obvious reasons I want to run the converter above the audible range, but on the other hand I want to squish in as much as 40W.Or I could run it very low in frequency, but it´s for "hand held equipment". In low freqs, it´s gonna get bulky.The topology that I´ll use is a push-pull topology with an external driving oscillator. And no I don´t want to use a self oscillating circuit simply foroutput filtering reasons. For the input, I don´t really care, it´s gonna be a 4S Lipo. I just need to isolate the noise sensitive electronics from thenoisy ones.As far as I understand the push pull topology is a double forward converter. Just for reference, I already designed an 4th order output filter for thesupply at 100kHz operation, when I was testing the design theoretically. Double LC circuit with damping at each LC, little to no overshoot and noisedown to 2mV, simulating the real components (ESR etc). So, yeah, in theory I got everything but a real life transformer.Quote from: T3sl4co1l on April 09, 2014, 02:10:10 amThank you for your consideration, but no worries. I´m designing the circuit myself, I´m not building one out of the "Internets"I´ve also learned some time ago what current limiting is for, not just for blinking a LED in the power supply along with cheap disco smoke.Anyhow, thanks for your reply, I hope I made clearer, what I need. In the end, the hardest part is explaining to otherswhat you have in your mind.Best Regards, LefterisGreece

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 09, 2014, 05:34:33 pm
content: Ok, so it's an ungapped, high-mu ferrite, which pairs nicely with a PP forward converter.I would be fine running that near saturation (0.2-0.3T) at 100kHz, or a bit less at 200kHz, because it will be lossy. It should get hot even at 100kHz, but not too bad. If you can spare the room (or weight) for more turns, it will help. Especially for something packed into a box.Does your circuit have active current limiting? In my opinion, there are two types of inductive switching converters: those that use active current limiting, and those that are electrical fire hazards!The usual solution for this type of situation is an automotive power amplifier, which has a DC-DC converter of the same type, driven by a TL494 (or similar), with no current limiting or voltage regulation whatsoever. I shake my head at these "designs".Since this is for isolation, I expect you have something else connected to the battery? If that is also low noise circuitry, don't forget to filter the supply input to this converter -- it will be noisier than its output!Tim

author: planet12
author_rank: Regular Contributor
timestamp: April 10, 2014, 12:33:19 am
content: Yeah, probably the main things to keep in mind will be flux density and core losses; the core material itself looks suitable, but you'll need to figure out the size of your toroids so you can figure out the effective area (Ae) for calculating flux density, and keeping in mind the need to balance core losses with copper losses you may need to go for a peak flux density considerably lower than the datasheets specified Bsat.Also remember that if you're aiming for 100KHz operation, you may well need to use multiple finer windings in parallel to counter the skin effect (since the multiple windings share the same core, providing they're exactly the same, multiple in parallel will have the same inductance as a single winding).I'll point you to my favourite site to explain further:http://ludens.cl/Electron/Magnet.html

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: April 10, 2014, 12:30:28 pm
content: Quote from: T3sl4co1l on April 09, 2014, 05:34:33 pmWell, I don´t really have something to limit the current but... Instead of connecting the center tap of the primary directly to the battery I use a P-channel MOSFET witha frequency detector circuit. Simply put, if the frequency is within a tolerable range (bandpass) the MOSFET is on, or else it´s off. The whole thing is based on theoscillator plus some battery comparators (they have hysteresis no worries here) so the battery is not drained to the point that it is destroyed. If a single conditionis not met then no battery connection to the center tap.All these above also give me a large margin for getting away with not using feedback. It makes things complicated, plus I´ve seen many designs not using it.And I mean serious designs from companies like Maxim, TI, etc. There are also controllers for push pull with feedback but it all get´s complicated mainly because I wantto have 3 outputs isolated from each other and keep it small like not using separate circuits for each. Also I should mention that there is going to be a lineal regulatorfor each isolated output, so even if the voltage varies from 7 - 9.5V there will always be a 5V or 3V3 regulator to fix thingsTo answer to your question also, the battery will be powering only this DC / DC transformer and 600W+ dc brushed motor through an H-bridge.A 600W motor and instrumentation op amps, generally don´t go well together so, I thought it would be better to separate everything. It alsomakes it easier to trouble shoot everything cause everything then becomes a module of its own.I´ll probably have more questions later, I guess...Thanks for the replies,Best Regards, LefterisGreece