Help needed with small transformer design - Page 1
Forum section: electronics / projects

author: akis
author_rank: Frequent Contributor
timestamp: June 08, 2012, 07:13:24 pm
content: I need to design a transformer but my knowlegde is quite limited. I have read about transformers but still am unsure about a few points. Here is the spec I require:1) primary voltage perfect sinusoidal +/- 4V AC @ 200 KHz2) secondary voltage +/- 50 V AC @ 200 KHz3) secondary load nominal 1K Ohms4) no DC at allFrom the above I have deduced that I need a ratio of 1:12.5. I do not need an air gap because there is no DC.I have constructed 3 transformers, two on an RM10 and one on RM12 core. I have the ferrite specs somewhere and I have used an average of 10 turns on the primary.I am not sure about the choice of:(a) ferrite cores(b) number of turns on primary (and therefore secondary)(c) wire gauges (only rough ideas using Ohm's law)(d) size of core to be able to transfer the energy required(e) primary or secondary wound first (innermost)(f) direction of windingsIf anyone could shed some light I would appreciate it.

author: amspire
author_rank: Super Contributor
timestamp: June 09, 2012, 04:53:55 am
content: It is hard to know if the cores can handle the power, but around 1A peak into the primary, you will need gapped cores - or alternatively go to powdered cores.Without looking at the data sheets, 10A amp-turns will easily saturate a gapless ferrite RM12 core. What a gapped core does for you is that most of the energy is stored in the gap, and the gap does not saturate. The core is used more for a magnetic short circuit from one side of the gap to the other rather then energy storage. The result is you can end up handling much more energy, and the inductance of the transformer is far more constant. Gapless cores are usually restricted to low level signal transformer applications.So you have to start off with a value for the primary inductance - perhaps something like 50uH. You might end up with more like 30-50 turns on the primary, and even if the core can take it, you probably cannot fit that many turns of 1A wire on half a RM12 core.What voltage isolation do you need?Richard.

author: nctnico
author_rank: Super Contributor
timestamp: June 09, 2012, 12:39:31 pm
content: Sorry, but gapped cores are only necessary for fly-back, buck or boost SMPS.There are 2 things to check for: maximum core flux density and core losses. At 200kHz there will be some core losses and you may want stranded wire to reduce skin effect losses. A transformer is basically a current device. The current that goes in, goes out multiplied by the turns ratio between in and out.Look what Google came up with:http://ecee.colorado.edu/copec/book/slides/Ch15slides.pdf

author: ejeffrey
author_rank: Super Contributor
timestamp: June 09, 2012, 01:29:20 pm
content: QuoteProper transformers do not (intentionally) store energy. A transformer transfers energy nearly instantaneously from primary to secondary. At any point in time the primary and secondary have equal and opposite current waveforms so their H fields cancel and the core is not magnetized. In practice, due to imperfect magnetic coupling and losses, the fields do not cancel and there is some magnetization of the core. This is the 'magnetizing current' which is the current that flows in the primary even when the secondary is open-circuit. In any well designed transformer this is much smaller than the load current. A traditional transformer should generally not have a gap. For some core geometries this is not possible, but the core is made as small as possible.In some forms of isolated switch-mode converters -- notably flyback converters -- the transformer also fulfills the role of an inductor, and is required to store energy. Only in this case are gaps required. Other topologies, like forward converters, use separate inductors for energy storage and the transformer only performs its conventional function.

author: akis
author_rank: Frequent Contributor
timestamp: June 09, 2012, 01:32:55 pm
content: Some more calcs and practical measurements to help you to help meFrequency = 200 KHzPrimary voltage = +/- 4VSecondary voltage = +/- 50VSecondary Load = 1K OhmPrimary Load = 1000 / 12.5 / 12.5 = 6.4 OhmPrimary current = 0.44 A RMS / 0.625 A peakI have made 3 transformers so far:T1 : RM10 ungapped material 3C90 - primary = 0.228mH, sec = 55.4 mH, 1:14T2 : RM10 ungapped material 3C90 - primary = 0.426mH, sec = 51.7 mH, 1:10T3 : RM12 ungapped material N97 - primary = 0.751mH, sec = 65.5 mH, 1:10All three transformers produce a nice looking sinusoidal on a load of 1.1K Ohms. I am not sure how else to test whether the transformer is doing what it is meant to be doing (hense I asked here).

author: PChi
author_rank: Frequent Contributor
timestamp: June 09, 2012, 02:01:34 pm
content: A good source of information is the EPCOS website.(a) The choice of ferrite core is a compromise depending on what parameters are important. Size / Cost / Efficiency / load regulation / Ease of manufacture etc.(b) The number of turns on the primary determines the magnetizing inductance = The number of primary turns N x AL for the core you have chosen.Also is the flux density, B, is well below the saturation level. B = V x t / ( N x A ) where V = Voltage, t = time, A = effective core cross sectional area. There is a simpler formula for sine waves but I can't remember it.(c) Wire gauge, what fits the winding gap and can be physically wound on the former. Also it's a long time since I used RM coil formers but the plastic was the most brittle ever made. Looking at the pins used to be enough to make them fall off! So only thin wire could be wrapped around the pins.Also what is the acceptable resistive loss. Can be calculated using the resistivity of copper and geometry. Sometimes the core manufacturer will give an effective turn length. I'm not sure how to calculate for Skin effect increasing the power loss. It's probably time to build and test.(e) Primary or secondary first? Probably not much difference. It depends what fits best on the former.(f) The winding direction only effects the signal polarity. What ever is easiest.Core loss can be calculated using the maximum flux density, frequency and core dimensions (for the volume of ferrite).I guess the tests are how hot does it get.What is the magnetizing current, set by the primary inductance.Do you care about load regulation? Limited by resistance and leakage inductance.

author: amspire
author_rank: Super Contributor
timestamp: June 09, 2012, 02:06:17 pm
content: Quote from: nctnico on June 09, 2012, 12:39:31 pmYes. I probably did underestimate the power handling of RM10 and 12 cores in a transformer mode rather then inductor mode.I should look at the data sheets next time.It should be able to handle 1A primary, as long as the power losses are within the core ratings. Doesn't mean you cannot make a better transformer with a gap, but the gap is not essential.Richard.

author: G7PSK
author_rank: Super Contributor
timestamp: June 09, 2012, 03:51:08 pm
content: There is a lot of useful information on this link.http://ludens.cl/Electron/Magnet.html

author: eevblogfan
author_rank: Frequent Contributor
timestamp: June 09, 2012, 06:20:45 pm
content: good stuff indeed !thank you G7psk !!!have a nice day !

author: akis
author_rank: Frequent Contributor
timestamp: June 09, 2012, 07:26:33 pm
content: "First, measure the section of iron through which the magnetic flux will flow"The area mentioned in the RM10 specs is 90mm^2 - is this the right value to use in the formula?

author: PChi
author_rank: Frequent Contributor
timestamp: June 09, 2012, 08:28:38 pm
content: It looks about right. It might depend on the manufacturer. I don't know how 'standardised' the RM cores are.The EPCOS data sheet give Ae as 83 mm2 with center hole and 98 mm2 without center hole.The Amin is specified as 90 mm2 without center hole.I would use Ae which EPCOS define as 'Effective magnetic cross section'.The EPCOS Ferrite and accessories Application notes are useful,http://www.epcos.com/web/generator/Web/Sections/ProductCatalog/Ferrites/RMCores/PDF/PDF__Application,property=Data__en.pdf;/PDF_Application.pdf

author: akis
author_rank: Frequent Contributor
timestamp: June 09, 2012, 10:24:13 pm
content: I would like to look at the problem from the bottom up.Supposing I have a made a transformer, without any regard for the rules specified above, but only caring about the turns ratio, choosing a "convenient" number of turns for the primary (eg 10) so that I can get a "practical" number of turns for the secondary (eg about 120 turns).Under the oscilloscope this transformer produces the required voltage at the output on the expected load.What can we say about this "randomly" made transformer? Is it fit for purpose? How will I know if for example my primary turns are all wrong, or if the ferrite core is not suitable? I also do not know if I can wind the primary over or under the secondary or side by side.

author: SeanB
author_rank: Super Contributor
timestamp: June 10, 2012, 06:27:27 am
content: Normally with a pot core you will be winding the primary ( or the coil with less turns at least) on the inside of the bobbin, then provide an insulating tape layer ( depending on how much isolation you need determines the thickness and amount of tape, but these cores are not meant to provide mains rated isolation, mostly just voltage isolation between parts of a circuit) and wind the secondary over that. Tape up the windings, assemble the core together again and clip it together and test. If the core does not get hot ( more than about 20c above the ambient, measured by a thermocouple placed between the windings and the core outer) and delivers the output load correctly then it can be declared a working design. May not be optimal cost wise and overload wise, but for a one or two off it will do.

author: akis
author_rank: Frequent Contributor
timestamp: June 10, 2012, 09:20:08 am
content: Quote from: SeanB on June 10, 2012, 06:27:27 amThanks.I have used a variety of assemblies, but none with insulating layer. Isn't the wire enamel insulation enough ?

author: SeanB
author_rank: Super Contributor
timestamp: June 10, 2012, 09:39:24 am
content: Depends on voltage. Tape is good between primary and secondary, especially if the voltage difference is over 50V, as the insulation can fail on the wire, normally it will have a few volts across it in most cases. You definitely insulate the wires to inner windings, as they have both been bent repeatedly so will have cracks in the insulation, and have the highest voltage between then and other wires. If you need more than 100V isolation between windings then you need to use tape.BTW the tape is not PVC insulation tape, but a Kapton tape designed for transformer winding, or a paper that will be varnished after assembly. Often the paper tape will be varnished before the second winding is would over it.The leadout will normally have a woven fibreglass sleeve or a silicone or PTFE sleeve that is just larger than the wire for protection from the windings wound over or next to it.

author: akis
author_rank: Frequent Contributor
timestamp: June 10, 2012, 06:16:21 pm
content: Quote from: PChi on June 09, 2012, 02:01:34 pmI am having terrible regulation, 50% in one transformer, 66% on another and 12.5% on another. The transformers with the highest turns ratio have the worst regulation it seems.

author: SeanB
author_rank: Super Contributor
timestamp: June 10, 2012, 06:40:17 pm
content: You need a bigger core then, or one with a different material.

author: amspire
author_rank: Super Contributor
timestamp: June 11, 2012, 02:32:06 am
content: Quote from: akis on June 10, 2012, 06:16:21 pmYou will want the minumum number of turns in the primary as possible. 50uH inductance in the primary is probably fine, and that may be as little as 3 turns. Make sure the wire is a decent thickness for handling 1A, or use multiple strands of thinner wire in parallel so you fill the width of the bobbin. Then one layer of Kapton tape. Then abut 40 turns on the secondary. If you can get the secondary on one layer, the transformer will work better. If you need two layers, put one half the secondary windings on. Then a layer of tape. Then the primary winding. Then a tape layer. Then the other half of the secondary winding. keep the windings very neat. Do not go for a scrambled winding.It should perform much better.Richard.

author: akis
author_rank: Frequent Contributor
timestamp: June 11, 2012, 01:00:05 pm
content: I tried today with 4 primary turns of 1mm enamelled copper as neatly as possible on the full width of the bobbin. I then put electrician's tape to hold it down. Then I wound lots and lots of 0.5mm for the secondary and it was not as neat, as some of it went in the crevices created by the thick primary windings. Anyway I think I must have put like 50 turns of the secondary. More electrician's tape to hold it down. Then I measured = +/-4 Volts in and +/- 24V out. Disaster. No idea what went wrong there. I will unwind the secondary and try again.

author: sorin
author_rank: Frequent Contributor
timestamp: June 11, 2012, 01:09:47 pm
content: with 200kHz you need to consider the capacitance of windings

author: nctnico
author_rank: Super Contributor
timestamp: June 11, 2012, 02:44:59 pm
content: Quote from: akis on June 11, 2012, 01:00:05 pmYou really must do the math first! Just winding something won't get you anywhere. I recall something odd about the number of windings; more is better to reduce the core losses.

author: akis
author_rank: Frequent Contributor
timestamp: June 11, 2012, 02:57:15 pm
content: Sorry. Not a disaster. I just checked again and I somehow had forgotten to press the "invert" and "add" on the scope - so my readings were wrong. I am getting +/- 55V or more, it looks good. Regulation is not great we are still dropping to +/-40 Volts under load. I have ordered bigger cores from Farnell, RM14, to see how that fares.

author: akis
author_rank: Frequent Contributor
timestamp: June 11, 2012, 04:18:17 pm
content: Many thanks for all the helpful comments and now I am reading the links from the latest reply.To recap, I am trying to make a 200 KHz, pure sinusoidal AC, step-up transformer from 4V peak to 50V peak on a load of 1K Ohm.I have made 5 working transformers, but all suffer from bad regulation. I have not run them long enough to decide if there are heat issues. I have used the common RM10 and RM12 bobbins and cores, not toroidal cores. For core material I have used EPCOS N97 and N87 which is specified for these frequencies (100KHz-500KHz). I have ordered a bigger size, RM14 to see if it makes a difference (better regulation).

author: akis
author_rank: Frequent Contributor
timestamp: June 12, 2012, 06:29:03 am
content: I read now that when you only have one or two turns you may/must use copper foil and not copper wire because the foil occupies the whole window whereas it is very hard to wind one or two turns spread out over the window area. As I do not have copper foil at home, I will experiment with aluminium foil to see if it makes any difference.

author: amspire
author_rank: Super Contributor
timestamp: June 12, 2012, 01:48:50 pm
content: Quote from: akis on June 12, 2012, 06:29:03 amYou definitely want to get Kapton tape or Polymide tape from other manufacturers. Electrician's tape is bad news for transformers in general, and for you, its thickness will result in loss of winding coupling.For the secondary, you should be able to get a wire gauge that will allow just one layer of winding which will help with the coupling. Foil, or multiple finer gauge wires in parallel for the primary will help. With skin effect, about 28 AWG is the largest you want to use at 200KHz. Above that the center of the copper is doing nothing, and the thickness of the 1mm wire is detracting from good coupling. Aluminium foil may work, but you may be better off trying to use about 6 strands of the wire you are currently using for the secondary. The wires need to be flat in a single layer, and you really have to be precise to minimize losses at 200KHz.I think the primary is the most challenging, so here is what I would do. Do a test to set a performance benchline. Take your current secondary wire and wind a perfect single layer - how even many turns that is.One layer of Kapton tape. If you do not have Kapton tape, use a layer of sticky tape just for the test. Ban the electrician's tape.Another full layer of the secondary wire with the same number of turns as the first layer. You now have a 1:1 transformer. Check the regulation. that is probably about the best regulation you will do with that core. If it is good, take the last layer off, and try and get your 4 ohm primary to regulate as well.It could be that a large part of the regulation problems are losses in the ferrite at 200KHz, and if so, then you need to search for better ferrite or ferrite geometry.Richard.

author: nctnico
author_rank: Super Contributor
timestamp: June 12, 2012, 04:11:45 pm
content: When using stranded wire: make sure to put the strands together first and then wind all of them together. If you wind multiple windings which are intended to be connected in parallel one by one you may forget a turn and effectively have a shorted turn.

author: akis
author_rank: Frequent Contributor
timestamp: June 12, 2012, 08:07:50 pm
content: Quote from: amspire on June 12, 2012, 01:48:50 pmI had heard that electrician's tape cannot withstand the potentially high temperatures in transformers but had never thought about its thickness being a problem.My ferrite core is EPCOS N87/N97 sold specifically for these frequencies. Specs show 100mT for 100KHz. Supposing we use 50mT for 200KHz, Vin=2.828 V rms, F=200KHz, A=1.6cm2 -- this calculates as just 0.44 turns for the primary. So it seems that by using 10, 12 and 2.5 turns for the primary I have been using too many turns.For the secondary I use 0.5mm and for the primary 1mm wire thickness.Taking into serious consideration your advice and the calculations, I will aim for just 1 primary turn. The window size is 14mm.The plan is to wind two 0.5mm wires for the secondary in parallel to get 14 turns in total to fit precisely inside the bobbin. Then I will add the special tape. Then I will use multiple of the 0.5mm in parallel to effect just one turn.Will have to do this tomorrow morning - there must be something about winding transformers, I hate doing it, they are so fiddly.

author: akis
author_rank: Frequent Contributor
timestamp: June 12, 2012, 08:13:23 pm
content: Quote from: nctnico on June 12, 2012, 04:11:45 pmI really hate winding transformers...

author: akis
author_rank: Frequent Contributor
timestamp: June 12, 2012, 08:17:35 pm
content: Quote from: AcHmed99 on June 12, 2012, 04:59:02 pmI think using a bigger size core would result in better regulation? I have already ordered and expecting to arrive tomorrow an RM14 which is a size up from my RM12. Hoping to get better regulation this way.And I tried the aluminium foil and yes you are right it is too fiddly. I will use multiple strands of 0.5 mm wire.The transformer is to step up a 4V peak into a 50V peak. Cannot be done electronically (easily) since the power supply is +/-6V (batteries). I had thought the transformer would be a 5 minute job, but it seems it is a small science in itself.

author: akis
author_rank: Frequent Contributor
timestamp: June 21, 2012, 10:57:45 pm
content: I made lots of experiments and built at least 10 transformers using varieties of windings for the primary and the secondary. Even with using the large RM14 core, it seemed there was no way I could get better than 18% regulation. I never had problems with transformer getting hot, it was always a problem with regulation being too high.Today I took a different approach. I suspected that with a turns ratio of 1:13 it is impossible to get better regulation. So I changed the design and the requirements and now the Vp is higher at 9V pk (was 4 V pk) and to achieve 50-55V pk I only need a turns ratio of 1:6 rather than 1:13. I quickly wound a transformer like that, and tested it, and voila! regulation solved, I did not even detect the curve changing on the scope when off and on load. For primary I used just one turn, and for secondary there are 6 turns. The secondary is wound in first, covering all the available area, then there is PTFE tape (plumber's tape), then just one turn on the primary.Tomorrow I will try using the smaller core, the RM12, because I only need about 1.8W power.I have read a ton about transformers and I think I am getting a good idea by now, but I still have some missing points. I will make a new post for this, as it is a more general question.

author: T4P
author_rank: Super Contributor
timestamp: June 22, 2012, 12:45:59 am
content: Quote from: akis on June 12, 2012, 08:13:23 pmI think you would hate dewinding a toroid with >3000 turns ... i better get back onto dewinding a shot toroid i have (I realized why it's shot ... the primary leads disconnected from the wire)

author: SeanB
author_rank: Super Contributor
timestamp: June 22, 2012, 05:12:06 am
content: Try rewinding motors........

author: T4P
author_rank: Super Contributor
timestamp: June 22, 2012, 06:42:20 am
content: Quote from: SeanB on June 22, 2012, 05:12:06 amI did ... my hands were tired as ****

author: szhighstar
author_rank: Contributor
timestamp: July 01, 2012, 06:47:39 am
content: Hi friend, you do not hate winding transformer, I am sorry I kike, it is very interested if you deep study, I reply your question.(a) ferrite cores , becasue power is less than 2.4W and 200K frequency , so you only need RM4 if you like RM configuration,EF13 or T14 are also good.(b) number of turns on primary (and therefore secondary) , according to formula: V=L*di/dt, V is minimum DC voltage, I is Irms, t is time of flow, you have frequency and duty cycle, you can know t value.(c) wire gauges (only rough ideas using Ohm's law) for power transformer, current density is 3-5mm/A, you know Irms and current density, you can know wire diameter,which type will you select depend on your design standard,EN61558 or EN60065 or EN60950 or other.(d) size of core to be able to transfer the energy required , please check (a)(e) primary or secondary wound first (innermost) , not special requirement, however you should be consider low leak inductance and capacitance(f) direction of windings, all winding should be keep same direction normallyAddition, will transformer applied either flyback and forward, if it is flyback transformer, it must open air gap.If you have other question, I am glad to help you.

author: szhighstar
author_rank: Contributor
timestamp: July 01, 2012, 06:50:39 am
content: sorry, I just printed wrong in last post, right word is I like