Transformer design question - Page 1
Forum section: electronics / projects

author: applicanon
author_rank: Contributor
timestamp: January 06, 2022, 10:37:39 am
content: Alright.. So I am designing a high frequency transformer for use in a SMPS utilizing a Full H-Bridge using IGBTs as the switching modules. The aim is to have the operating frequency of the h-bridge something like 50khz to keep the core size reasonably small.The input voltage will be 360VDC and the output voltage will be 60VDC. The peak output current will be 200 Amps. (making for a peak power rating of something like 12,000 VA)I found this ferrite core and I think that it looks to be something which should do the job for me. Here is a document that comes with it, although it doesn't come with a BH curve. Can anyone help me out with a BH curve that would probably match this type of core?The main question that I have is how to calculate just how many turns to put on the core. Obviously I will need to make the ratio of turns from Primary to Secondary 6:1 to give the proper voltage output, however, it's also obvious that simply using 6 turns on the primary and 1 turn on the secondary most likely will not work properly.What is the calculation to be used in order to find out the most effective number of turns? Is it simply to calculate the maximum number of turns which can physically fit within the core window area?Hope to hear back soon with positive assistance!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 06, 2022, 01:25:56 pm
content: Look at the figure popping up all over the page: tesla (T), flux density. This has units of Vs/m^2. That area is Ae. Vs is flux, as in the area under the curve, the curve being the applied voltage. This will be the per-turn figure, so you can apply more voltage or lower frequency (longer pulse width = more flux) with more turns, for the same flux density.It seems they don't give a material designation / mix, on this part of the sheet anyway, or other material properties like density (aside from weight being given), or saturation flux, mechanical strength, etc. (Permeability is given in a roundabout way: AL = mu_r mu_0 Ae / le, so evidently mu_r ~ 2070. Which varies with temperature, generally, so that's another material property that's missing.)(Or it's in the Chinese characters, which I just can't read.)So, 360V at 50kHz is:N = V / (4 Bmax Ae F)(The 4 comes from the bipolar square wave; you would use 2 for a unipolar (half wave) forward/flyback converter, or 4.44 for a symmetrical sine wave.)= (360V) / ( 4 (100mT) (1225 mm^2) (50kHz) )Note that using T == uVs/mm^2 and frequency in MHz (0.05MHz) gives the same scaling, so, 360 / 4 / 0.1 / 1225 / 0.05 = 14.69. The nearest multiple of 6, is 12 or 18; 12 would give Bmax = 122mT, which is probably not unreasonable for this core. (Judging by the core loss plots, much over 150mT is probably not a great idea, at least short of gluing water-cooling plates to it.)With 360V/33A = 10.9 ohms on the primary side, performance will be limited by leakage inductance. Suggest using copper tape/strap for secondary, and using several layers worth in parallel, interleaved with layers of primary. Primary must be Litz cable, for low AC resistance. This arrangement reduces leakage to acceptable levels, say with a PSPSPSP build. (Turns per layer don't matter so much; probably, three turns per secondary would work out well enough, then connect them in parallel. Or for FWCT output, stack two strips and wind them together, then connect opposite ends in series to make the CT winding. And connect all three sets in parallel. Doing a single turn per layer is probably harder, because of how much strip width you need to carry that current; not easy to get out of the way of the next primary layer.)Note two things about your system:1. You don't have transistors hard-switching into capacitors. You use inductors.2. You need some PWM anyway, to regulate the output voltage (presumably). Or current, as the case may be.Which, these numbers sound suspiciously welder-ly..?Oh right, you posted this before:https://www.eevblog.com/forum/projects/igbt-brick-based-inverter-design-question/Context always pays off!Right. I was going to say, you need extra output voltage (slightly lower turns ratio), so that PWM isn't saturating at nominal output and it can achieve regulation. But since this isn't just any DC power supply, that probably won't be important.3. You'll most likely be using a full-wave forward converter, into a FWCT or FWB rectifier, into a choke-input filter. Whether there's a filter capacitor or not, and what value, depends; it certainly can't be so large that the electrode just sparks as soon as it touches anything and never starts a smooth arc. Below that value, there is room for adjustment, and it should be feasible to have good filtering (smooth arc, even at high frequency, and stable down to fairly low currents) without taking up a ton of space/cost (e.g., film capacitors, or extra filter inductors?).4. You'll be using a current mode controller, which adjusts PWM to set the desired output (rectifier into filter inductor) current; if voltage regulation is required (e.g. for MIG I suppose), then that can be added with an outer loop (voltage error amplifier). Current mode control protects the IGBTs first and foremost, and is easy to control (output current is simply proportional to control input voltage).5. In addition, you can have desat protection, as a more crude current-limiting mechanism; something of a last resort. This is highly recommended to avoid burning through too many transistors during development.Desat protection is normally integrated with the gate drivers, which will be isolated, and you simply use as many isolated channels as you have transistors (so, H-bridge, 4 needed). The control circuit can then sit at SELV or earth potential, no need to contact the live AC side of things. (You will still need an isolation transformer or differential probe to check operation of the bridge itself.)6. I STRONGLY recommend you build multiple scale models of this, first. The same controller can be used on everything, even the same gate drivers (if you design them for final scale, or purchase ready-made ones of the same rating -- a very convenient option, I might add). You can fully debug your controls against a small scale inverter, then scale up the voltage, then current, then both voltage and current. You will see effects, at scale, that were negligible at smaller scales: especially the effects of stray inductance, its effects on the bridge itself (peak voltage, limited dI/dt), and on surrounding circuitry (induced voltages, transients, interference). Naive wiring of these things absolutely will inject enough voltage/current into nearby circuits to destroy them -- and then, on top of replacing your grenaded IGBTs*, you have to replace whatever components burned in the gate drivers and control circuitry too! (All the more reason to isolate the controls!)*Depending on how much supply capacitance you have, IGBTs will literally explode, emitting shrapnel. Use protective covers, wear eye protection -- the usual. The mechanical effects can be lessened by using a semiconductor fuse**, but IGBTs can only be saved through quick control action.**So called because they operate fast enough to savesomesemiconductors, namely the most robust kinds: diodes and SCRs. These fail in some ~ms of mains fault current. IGBTs fail in some 10s of µs, in which time the fuse hasn't even begun to warm up, let alone melt. Depends on supply capacitance, because electrolytics are more than capable of delivering mains-fault-magnitude currents, albeit only briefly. Since you'll be using an SCR input stage I believe?, it will need generous filter capacitance, so this should hazard will be present.Tim

author: jonpaul
author_rank: Super Contributor
timestamp: January 07, 2022, 09:24:55 am
content: Bonjour applicanon, bravo for the design challenge.We have designed power electronics and magnetics since 1970s.The power electronics converter can be viewed as a system with many design choices in topology, switches, magnetics and passive components.The filed of application, regulatory compliance(safety, EMI) and environment (temperature, vibration, etc) are important factor s a very different design for a medical device vs flying avionics vs industrial.The choice of core may depend on the quantity and leadtime to production.We never used Chines cores. The best data is for the TDK and EPCOS/Philips/Siemens cores.Power ferrites are characterized for losses vs B and temperature, and BH curves, etc.The winding design may include a bobbin which allows easy fabrication and is essential for insulation and safety.The above mentioned vendors have a selection of bobbins to match the cores.Modern power ferrite transformers and inductors avoid square leg EE/EI and use a round cross section center leg.Core shapes suggested: ER, EER, PQ, etc.In case a particular primary inductance is needed, a gap in the core is used.Interwindng Faraday Shields reduce Ccp-s and interference.There are many fine websites, technical papers and texts on power converters and magnetic design.You may save time and cost by referring to some of these before you proceed.Perhaps these notes from an old retired EE can benefit your work.Bon Chance, amicalement,Jon

author: mag_therm
author_rank: Frequent Contributor
timestamp: January 15, 2022, 04:50:35 am
content: Hello Applicanon,For your "main question", you have to put enough turns on so that the core never saturates.Furthermore , the control needs to ensure that the shutdown pulse leaves the core in negative bias,while the start pulse starts in the positive bias. ( here bias == remanence)An airgap is a big help.I suggest you read about these effects, and those mentioned by jonpaul.Power ferrite cores have not changed much in 30 years or so. Usually can go to +/- 0.3 Tesla peak approximatelySo use the classical transformer equation to get initial turns estimation.For 100% duty, square:E = 4 * f * N * B * A -- all in ISO unit where B : 0.3 T approximatelyOver about 50 years I designed many power electronics ferrite transformers like you mention.One similar range I remember was 25 kHz 400Volt primary rated 30 ~ 50 kVA at 50% duty , being hand held for brazing power gen. alternator windings.Use of 2D FEM electromag. model is a big help in design,because it shows the distribution of core magnetic field strength (H) and conductor current density (J) .

author: jonpaul
author_rank: Super Contributor
timestamp: January 15, 2022, 09:36:28 am
content: Mag therm, excellent, we are both veterans of power electronics and magneticsWe used the Intusoft Magnetics Designer for 20 yrs, still available.Shines for bobbin design and build.http://www.intusoft.com/mag.htmWhat books or references did you learn from?I would modify a few notes:.....enough turns on so that the core never saturates....A few applications need sat like the Royer inverter. Saturation is not a binary, the B-H curves vary a lot depending on material.An airgap is a big help...Required i a definite Lpri needed as in resonant conv, and any topology with DC bias like a flyback or foward converter.Power ferrite cores have not changed much in 30 years or so.The main advances from TDK, Philips, EPCOS1/ much higher frequency capabilities for soft switching and resonant converters to 1..10 MHz range (much lower flux density)2/ Design for lowest losses at higher hotspot temp3/ Efficient shapes like PQ, ER, EER rather than old square leg EE, EI or pot cores.4/ SMD capable shapes like EER5/ Special long cores for HV apps like CFL drivers.6/ Amorphous and nanocrystaline tapewound cores for lowest losses in power distribution and mil/defense eg satellites.https://vacuumschmelze.com/products/Inductive-Components-and-Cores/Amorphous-and-Nanocrystalline-CoresThe latest offerings are at distributors of ferrites like MHW International (TDK, USA)https://mhw-intl.com/Allstar Magneticshttps://allstarmagnetics.com/ferrite/Kind Regards,Jon

author: mag_therm
author_rank: Frequent Contributor
timestamp: January 16, 2022, 05:08:02 pm
content: For Applicanon:You might consider to send RFQ for purchase of a custom transformer. There are manufacturers of high frequency higher power transformers. And they do one-off designs because it is a small volume specialized field.Two I know are Hunterdon in New Jersey and Jackson in Florida.Maybe a vendor local to you in Canada ? I don't know.For JonPaul:Yes I was in power electronics and magnetics design for almost all of my engineering career starting in 1972 and ending on Dec 31 2021 !I was interested in computer simulation and design right from the start, particularly for numerically solving ODEs and DEsIn 2004 I started independent contract development and the biggest project of my career was development of a electro-magnetic-thermal simulation system.I suppose it is somewhat like the Intusoft you indicate, where user can either create his own elements or select verified ones for construction of model involving coupled multi physics solvers. For this I did not use any commercial software, I wrote all the numerical ODE, PDE and analytical solvers from scratch and used parallel processing.Apart from that I also used QuickField from about 2007 onwards for electro-magnetic and thermal design of magnetics. API's (User Front ends) in some cases controlled the underlying QuickField solvers. But often I just used the QuickField GUI. Solving times for some models took overnight (!) some times, because QF did not use parallel cores. Maybe they do now.Building front ends for an underlying commercial simulation package is a good choice for users designing in specialized areas,like high power inverters, high frequency magnetics etc.Yes, I agree that the saturating ferrite inverter (Royer etc) is a good tool for understanding the BH curve.Last year I put up a post somewhere on eevblog with traces of the loop using a saturating ferrite inverter coupled to oscilloscope with XY inputs.We experimented with amorphous strip cores some years ago in power transformers at either 10 kHz or 30 kHz(I forgot)But we had catastrophic failures at inner corners in the cores after some time in service.I recall even at 0.5 T.At that time I concluded they were no better than ferrite, above 10 kHz.But maybe that can be revisited now that materials are further developed and we have better models.bonne chance! to you,