Pulse Transformer Design Resources - Page 1
Forum section: electronics / projects

author: forrestc
author_rank: Unknown
timestamp: February 03, 2017, 10:44:01 am
content: I have something new for me, and strangely not having much luck finding the right resources on the web.I'm needing a multi-winding signal transformer for a project I'm working on. Every transformer I have used in the past I've been able to just use a purpose built transformer and I haven't had to get into the details of the actual properties of the transformer. In this case, I'm doing something a bit odd so I need to understand a lot more about pulse transformers in particular. The issues I need to deal with is understanding core saturation in the presence of DC current, determining the pulse behavior of a transformer (i.e. will a pulse of a specific shape go through the transformer), and impedance matching (although this seems straightforward).I've dug through AoE 3rd edition and there isn't much information in there. Practical Electronics for Inventors talks at a fairly high level, but doesn't seem to go into the depth I need - in fact pulse transformers are only mentioned in two paragraphs. Various other documents I've looked at seem to assume way more knowledge than I have. "Here's the formula you need, good luck interpreting it".What I'm hoping to find is some document reasonably accessible to a mostly-digital designer which discusses how best to select and/or do basic design of a pulse transformer. I.E. what all of the parameters a pulse transformer datasheet mean. If I want to have a go at winding my own (and eventually getting someone to wind a couple thousand for me), how do I select a core, and what else do I need to consider. And so on. Any pointers would be helpful (maybe I've overlooked something obvious).Alternatively, if there's a manufacturer out there that people have had good luck with this type of work with, let me know. I'm more than willing to just let someone else do this and sell me suitable magnetics.

author: cat87
author_rank: Regular Contributor
timestamp: February 03, 2017, 11:51:16 am
content: When you say "Pulse transformer" do you mean the kind that is used in telecommunications? Or the kind that is used in Gate drives?If it's the telecommunications type, I had a look through my book collection. The only mentions I have found was in:- William Flanagan - Handbook Transformer Design and Applications Second edition - Chapter 18.2 and 18.3Maybe if you could give some more details about your application, if possible, someone more knowledgeable than me might step in and give you some more information.

author: MagicSmoker
author_rank: Super Contributor
timestamp: February 03, 2017, 01:05:06 pm
content: Quote from: forrestc on February 03, 2017, 10:44:01 amA pulse transformer is just a transformer in which the winding and core are optimized for preserving pulse fidelity across an isolation barrier; this usually means you want the highest mutual inductance and lowest leakage inductance and distributed capacitance possible, and those requirements distill down to a low number of turns, with primary and secondary windings interleaved, on a core with a very high permeability. No transformer likes DC, and small pulse transformers tend to be especially intolerant (even a few mA can cause saturation) so AC coupling or DC restoration tricks are a must.Commercially available pulse transformers are often specified by a simple metric which is especially useful for their intended application: the V*us rating. As might be inferred, this is simply the product of the voltage and the time it can be applied across the primary winding. For example, a rating of 15 V*us means you could apply 15V for 1us, or 5V for 3us, etc. Attempting to apply more V*us results in excessive voltage droop on the secondary windings and, eventually, saturation. V*us rating is proportional to core area andinversely proportionall to the # of primary turns (see the conflict with the aforementioned desirability to keep the # of turns as low as possible? Welcome to the wonderful world of magnetics design, where every parameter is in conflict with another!).I don't know of any good design resources for transformers on the web, but that's because I haven't looked for such. I will warn you that magnetics design is an area in engineering where there is astonishing abuse of units; even when the units are in metric there is indiscriminate mixing of CGS and MKS. So, make sure anything you read spells out very clearly what the units are, and be on the lookout for wild scaling factors in the equations like 10^7.EDIT: strike out incorrect phrase above

author: oldway
author_rank: Super Contributor
timestamp: February 03, 2017, 01:45:47 pm
content: http://www.vacuumschmelze.com/fileadmin/Medienbiliothek_2010/Produkte/Kerne_und_Bauelemente/Anwendungen/Uebertrager/Ansteueruebertrager/Ansteueruebertrager_en_2012.pdf

author: David Hess
author_rank: Super Contributor
timestamp: February 03, 2017, 08:15:20 pm
content: Quote from: MagicSmoker on February 03, 2017, 01:05:06 pmDon't you mean V*t rating is proportional to the number of primary turns? Doubling the number of turns doubles the number of amp*turns so saturation occurs at half the magnetizing current but quadruples the inductance so it takes twice as long to get to that point.Pulse transformers have the same requirements as small inverter transformers so the same cores are used.Books on the subject of off-line switching power supply design often have a chapter on pulse transformer design. The usual problem is keeping the core balanced with different duty cycles.

author: MagicSmoker
author_rank: Super Contributor
timestamp: February 03, 2017, 10:56:15 pm
content: Quote from: David Hess on February 03, 2017, 08:15:20 pmYes, as the number of turns goes up for a given primary voltage the induced flux goes down, so... it looks like I need to edit my post!

author: calexanian
author_rank: Super Contributor
timestamp: February 03, 2017, 11:07:41 pm
content: What frequency and power level? That will determine which design philosophy to run with.

author: Benta
author_rank: Super Contributor
timestamp: February 03, 2017, 11:44:22 pm
content: The search term for this is:"Soft Ferrites" (sounds strange, I know, but the term refers to the magnetic characteristics, not the mechanical)If you stumble over one of the old Siemens, Philips/Ferroxcube databooks/manuals on this, buy it immediately (or give me a hint).Cheers.

author: jbb
author_rank: Super Contributor
timestamp: February 04, 2017, 02:10:37 am
content: Hi ForrestQuote from: MagicSmoker on February 03, 2017, 01:05:06 pmQuote from: MagicSmoker on February 03, 2017, 01:05:06 pmMagicSmoker is dead right. The physics for a pulse transformer and a power transformer are the same, but the designer makes a lot of tradeoffs to get the desired performance / size / weight / price. (FYI, smaller tends to be lighter and cheaper.) If you need to pass a DC value through the transformer, you're going to have difficulty because pure DC won't go through a transformer. You'll need extra circuitry.However, if you need to just pass an AC component, and ignore the DC component, this can be done. You would take a core and add an air gap to decrease its permeability. This will then reduce the magnetic field strength below the saturation point. But will vastly decrease the magnetising inductance, which may distort your signal.Leakage inductance is the 'other' part of a transformer. It's determined by the geometry of the windings. As MagicSmoker says, interleaving offers excellent results here by minimising the area between the primary and secondary. You can also interleave many tertirary etc. windings. However, having wires in close contact leads to more stray capacitance between the windings, which may be a problem for you.A typical high-signal integrity design goal would be to select your design parameters such that one winding (e.g. the primary) is wound onto the bobbin such that it completely fills the winding width of the bobbin on one layer. This offers minimal intra-winding capacitance. A layer of tape is wrapped over it. The secondary is then wound on top such that it too fills the winding width of the bobbin on one layer. This means that:the area between the primary and secondary is small, so the leakage inductance is smallthe primary and secondary aren't in overly-intimate contact, hopefully keeping parasitic capacitance downIf high insulation is needed, there is only one border between primary and secondary.It's a complicated field, so if you can give more details about the transformer, we can be more helpful. If this is for a hush-hush work project, then I would suggest you flag this design challenge to the project leader immediately, and seek out a magnetics design house for help. Marque Magnetics in NZhttp://marquemagnetics.comhas been quite helpful to me in the past. (And no, I don't work for them.)

author: oldway
author_rank: Super Contributor
timestamp: February 04, 2017, 09:16:38 am
content: For transformer and inductance profissional design, I recomand:http://www.rale.ch/

author: forrestc
author_rank: Unknown
timestamp: February 04, 2017, 09:37:43 am
content: Quote from: MagicSmoker on February 03, 2017, 01:05:06 pmThank you very much for that paragraph. In particular the V*uS description.I've been reading these datasheets based on my past experience with opamps, where When you see Volts-uSecond, this is a measure of slope, i.e. V/uS.V*uS makes a lot more sense. And a lot simpler to design.The good news is that I've found some off the shelf pulse transformers (Wurth WE-FLEX) which seem to fit my needs. They're actually designed for DC-DC converters (not my application), but as always seems to be the case, yet again, I'm using something for an application which it wasn't originally intended for. V*uS is well over my need. They have a range of inductance/saturation currents available, and plugging the whole thing into spice seems to yield results which match my expectations and need. I've got some engineering samples on the way, and a couple of breakout boards as well.Hopefully these will work just perfectly.

author: forrestc
author_rank: Unknown
timestamp: February 04, 2017, 09:42:40 am
content: Quote from: Benta on February 03, 2017, 11:44:22 pmI'm assuming you're not talking about this document:http://www.magneticsgroup.com/pdf/mmpa%20SFG-98.pdfBut yes, this is definitely the correct search term to stick into google.

author: jbb
author_rank: Super Contributor
timestamp: February 04, 2017, 09:24:20 pm
content: Quote from: forrestc on February 04, 2017, 09:37:43 amGlad you found something.The nice thing about Wuerth is that they actually stock all the stuff, and will sell it to you from their website. Other companies come back and say "oh, those? Minimum order is 10000 and there's a 6 month lead time."