Custom transformer for LM2577 multiple output flyback converter - Page 1
Forum section: electronics / general

author: bmxsesh
author_rank: Regular Contributor
timestamp: September 30, 2024, 03:38:43 pm
content: Hi everyone,I am wondering if someone has experience winding own transformers. The transformer I need to make is for creating 5 Dc voltage levels from a single +9V battery supplyCAD magnetic designer program advised me to use such core as:Ferroxcube E13/7/4 material 3C90and it has a 20-pin SMD coil formerI have no clue how these things are arranged, how the primary and secondary windings are wired and so don't know what pins should be used for primary and secondaryI am trying to use this Pulse PE-68422 similar 20-pin arrangementThey always seem to have primary winding connections on the opposite sides, i.e. pins 1 and 16But mine has a common tap for all secondary windings so this could be possible on a 20 pin coil former?Some stackexchange guy says tappings must be on the primary side to avoid creating an arcIs there some rule how these primary vs secondary winding pins should be arranged on a coil former or it is absolutely random?Edit: seems like winding connections are always on the opposite sides so I need a 12Pin coil former for this type of assembly?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 30, 2024, 04:45:58 pm
content: If your circuit is common-ground (as the simulation suggests, but this may be merely a simulation convenience, we don't know), then isolation voltage is irrelevant, creepage distance is not required, and pins can be assigned freely.Note that individual windings aren't required, and it's probably worse that way. Instead, you can tap the +15V winding into thirds (approximately), making 5 and 10 from one single route. Same goes for the -15 and -7.5V.Note further that, since isolation isn't required, either flyback isn't strictly required, or some coupling may be expedient (allowing a much simpler wound transformer to suffice, or even independent inductors). That is, you can put coupling capacitors on any 1:1 windings (i.e. that have the same AC voltages on respective terminals), thus countering leakage inductance. The circuit becomes a SEPIC of sorts, but realize you can still have multiple outputs, and even inverting outputs (Ćuk style) all in the same circuit.Depending on loads and regulation requirements, several regulators might also be attractive, perhaps a SEPIC/Ćuk, buck, buck with "flybuck" (as TI calls it) winding, etc. Light loads might well be better obtained by plain linear regulator from higher supplies, even if the drop is relatively high (e.g. if +10 or -7.5V are low current, they can be obtained from +/-15, etc.).Don't expect great regulation with independent windings, and tapped windings are only somewhat better. There are winding techniques to improve regulation (leakage inductance is the problem, interleaving is the solution), but only so much is practical.Tim

author: nctnico
author_rank: Super Contributor
timestamp: September 30, 2024, 04:57:32 pm
content: Using a readily made transformer is the easiest by far.Where it comes to winding your own transformer: you'll need to determine the number of windings to get to a certain inductance while taking the air-gap into account. Where is becomes iffy is to determine core losses. I'd start with a low loss core and IIRC 3C90 is not very low loss.I think I used this appnote in the past to design a custom transformer:https://www.ti.com/lit/ml/slup127/slup127.pdfHowever, when winding a transformer for a flyback converter, the leakage inductance is extremely important as this is what is causing the spike at the switch transistor. It can cause quite a bit of losses. It requires a special winding technique to minimise leakage inductance.However, since you don't need isolated outputs, I'd go for several DC-DC converters instead of winding a custom transformer. Maybe even consider switched capacitor converters. Given the device is battery operated, I guess the currents a low anyway. And isn't there a way to optimise some supply voltages away?

author: bmxsesh
author_rank: Regular Contributor
timestamp: September 30, 2024, 05:51:23 pm
content: Quote from: nctnico on September 30, 2024, 04:57:32 pmI thought that these have pre-defined winding ratios, and the output voltage depends on 1) duty cycle 2) winding ratioMost off-the-shelf converters that I see are either all channels step-up or all channels step-downBattery discharges slowly from 9V to 3.5V which is the minimum Vin for LM2577From 3.5V-9V to 5V that's a step-down and after some point is a step-upFrom 3.5V-9V to 7.5V likewiseFrom 3.5V-9V to 10V step-upFrom 3.5V-9V to +-15V step-upAll these voltages are needed by the CCD (ICX415AQ) and +-15 is for the FET transimpedance op ampNone of these are high current, each channel should draw 100mA (CCD only consumes 5mA), most of the power will go to filtering op amps that's +-15V railsThere already are inductor-based DC-DC converters supplying three lower voltages 3.3V 1.8V and 0.95V to the logic part of the design, and they were all step-down anywayCharge pump was on board but that's quite a few DC-DC converters at this point and generating all voltages from one flyback converter seems to be convenient and even probably cheaperQuote from: T3sl4co1l on September 30, 2024, 04:45:58 pmIs this even doable by hand or do I have to make a custom one at the factory?Everything is referred to one common GND a metal sheet on one of the PCB layersI referred to this appnote:https://www.onsemi.cn/pub/Collateral/AN-4140JP.pdfthe only thing that's hard is understanding how to transfer the 2D drawing of the winding onto the real device and do it by hand, the circuit isn't difficult by itselfAlso, the CAD program also seems to do a lot of computations for methere is a design error but it may be due to the wire gauge I have not picked right yet for the 13mm x 3.7 mm coreNow the 66% tap of +15V winding is 10V, 33% tap of +15V winding is 5V and the 50% tap of the -15V winding is -7.5V

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 30, 2024, 06:23:06 pm
content: Transformer winding materials are readily available. Typical set is: core (halves), bobbin, enameled copper (magnet) wire, and PET or PI tape (example: 3M #56, #92). Varnish is not required for low power prototypes. Isolation is not required either, and the enamel alone may suffice, no tape needed.Tim

author: nctnico
author_rank: Super Contributor
timestamp: September 30, 2024, 06:27:42 pm
content: Yes. Biggest problem with transformer winding is keeping count of the number of turns. If you need to wind identical windings, I highly recommend to calculate the total length and cut several wires and wind the identical windings in one go.

author: bmxsesh
author_rank: Regular Contributor
timestamp: September 30, 2024, 06:53:57 pm
content: This is the part itself that I will be trying to do all this withI guess the bobbin part the CAD does not know and these are the numbers I need to adjust

author: bmxsesh
author_rank: Regular Contributor
timestamp: September 30, 2024, 07:47:39 pm
content: Quote from: nctnico on September 30, 2024, 06:27:42 pmIs this how the EE core should be connected? Some numbers don't match upCAD tells me window is 7mm x 1.6 mm and it seems to be fixedBut the from the datasheet it's 9mm x 2.6mm if I combine two of these E shaped cores if I understand the picture of the product correctly(8.9 mm - 3.7mm)/2 = 2.6 mmAnd the height is just 2x4.5 mm = 9mmAm I calculating this wrong?

author: nctnico
author_rank: Super Contributor
timestamp: September 30, 2024, 08:01:58 pm
content: The thickness of the bobbin eats into the usefull width / height of the windings.

author: bmxsesh
author_rank: Regular Contributor
timestamp: September 30, 2024, 11:27:42 pm
content: Quote from: nctnico on September 30, 2024, 08:01:58 pmThis is what I got so far. Litz wire actually gives worse results than the normal single wire. Thinner wire also takes the efficiency down to 90% and raises the temperature from 30*C to 55*C and higherQuote from: T3sl4co1l on September 30, 2024, 04:45:58 pmOnly would be difficult with 1/3 and 2/3 winding taps where I have 14 turns that doesn't divide well by 3

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 30, 2024, 11:30:32 pm
content: Why a duty cycle so high?Tim

author: bmxsesh
author_rank: Regular Contributor
timestamp: September 30, 2024, 11:57:46 pm
content: Quote from: T3sl4co1l on September 30, 2024, 11:30:32 pmFrom LM2577 datasheet 0 <= D <= 0.9 and for stepping 3.5V (edge of operation) to 15V in a boost configuration gave 0.805 duty cycleAnd obviously it depends which of the positive outputs is wired to the feedback resistor dividerI was thinking connecting the lower amplitude output to the feedback as in LM2587 flyback multiple output example but that's a different kind of transformer used there, with a separate windings for +15V and +5VUsing this CAD it seems that my only option is to have +15V wired to the feedback resistor network since the program doesn't do taps at allIn fact, when the battery goes down to 3.5V and the chip is about to shut down, duty cycle is 0.89 (almost at maximum) using the flyback formula that takes into account N windingsIt asks for a Average primary voltage which I don't know how to tell it's just a battery discharging at a uniform rate so I guess the average is just between 3.5V and 9V that is 6.25V?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 01, 2024, 12:20:44 am
content: Right, that's a boost configuration... but you're using flyback? You can have any turns ratio you want!Tim

author: bmxsesh
author_rank: Regular Contributor
timestamp: October 01, 2024, 12:54:29 am
content: It wouldn't let me type in how many turns I want on primary/secondary but I noticed that changing GAP* which I guess is the air gap between two E-shaped cores, does change the number of windingsDatasheet isn't helpful at all, only says at 15N+-5N clamping force air gap is ~0m which I guess isn't physically possible or very hard to achieveAir gap of 0 gives 1 turn on each winding (nonsense)

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 01, 2024, 01:32:34 pm
content: Quote from: T3sl4co1l on September 30, 2024, 04:45:58 pmThis is NOT true in general.In SOME cases protective insulation in the transformer (i.e. solid insulation between windings, creepage/clearance on the PCB) is not necessary if safety is ensured in other ways (i.e. protective earthing of the circuit and chassis, protective insulation to prevent access to live circuitry, etc).If the primary is supplied by a 9V battery, then it's unlikely that any of this is a concern though.

author: bmxsesh
author_rank: Regular Contributor
timestamp: October 01, 2024, 02:11:15 pm
content: Quote from: T3sl4co1l on October 01, 2024, 12:20:44 amFor some reason, positive rail is quite accurate but negative rail amplitude is lower than it should be

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 01, 2024, 05:19:55 pm
content: Quote from: mtwieg on October 01, 2024, 01:32:34 pm"protective earthing of the circuit and chassis" yes in other words, common ground ("BASIC" type insulation requirement)?To be fair, enamel doesn't (usually?) count as more than "FUNCTIONAL" type, and at least one layer of tape would be required to meet "BASIC".Or conversely, tape might nonetheless be required when the operating voltage is high, despite the common ground.Obviously I meant in context of this thread -- low operating voltage and such. Both sides are SELV, isolation is irrelevant for safety purposes.Tim

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 02, 2024, 12:04:27 pm
content: Quote from: T3sl4co1l on October 01, 2024, 05:19:55 pm"Common ground" is, AFAIK, not a recognized term in any safety standard (unlike "protective earth" or "functional earth" which have distinct meanings in IEC 60601-1). Here I interpret "common ground" as meaning the primary and secondary circuits share the same ground (which may or may not be a protective earth).QuoteIt's definitely possible to get protective insulation out of enamel wire alone (i.e. double and triple insulated wire).In IEC 60601-1, having your secondary circuits connected to a protectively earth chassis (i.e. class I medical equipment) counts for one means of protection (MOP). You need at least two MOP to be compliant. The other MOP can come from the transformer's insulation. This is as opposed to if the secondary circuits were not protectively earthed (i.e. class II medical equipment), in which case the transformer would need to have reinforced/double insulation to provide two MOPs by itself.Quote

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 02, 2024, 12:49:43 pm
content: Anyways, I should also help the OPIs this a one-off project, or is this for a product? How many do you intend to make?If this is a one-off, then I think it's fine to try winding your own flyback. As discussed above, you don't really need to bother with rigorous insulation, which makes constructing it by hand a lot easier.Regarding the bobbin pinout: at higher working voltages (i.e. AC-DC converters) a minimum creapage/clearance is required between mains and secondary circuits, and to meet that the primary and secondary windings will usually be terminated on opposite sides of the bobbin. For your low voltages, you don't need to follow such a rule.As for whether a multi-output flyback is a good topology to choose... that's a very difficult question to answer without knowing much more about your applications (i.e. how tight your regulation of each rail needs to be, required power efficiency, etc).As for using a separate secondary winding for each output vs one secondary winding with a tap for each output... I'm not sure how these compare in terms of sensitivity to leakage inductance and winding resistance. A single tapped winding certainly makes better use of winding are on the bobbin.The reason your negative rails have the wrong voltage in your second simulation is a simple misunderstanding of magnetic circuits. The three winding sections for the positive rails each have 1.32uH, and you reasoned that to get -15V on the other side the total inductance should be 3*1.32uH = 3.96uH, and to get -7.5V you should cut that into 3.96/2 = 1.98uH. But the actual inductance of the three 1.32uH sections is (3^2)*1.32uH = 11.88uH, because these inductances are all coupled together. So for the negative sections you also want the total inductance to be 11.88uH, so each of the two sections should have inductance 11.88uH/(2^2) = 2.97uH. That should give the desired voltages for the negative windings. Though keep in mind the rectifier diodes will reduce that voltage somewhat. You could try adjusting the turns ratios to compensate, but depending on how many turns each section has, this may not be feasibly (don't want to delve into fractional turns...).I highly recommend you continue to tinker with the LTspice simulation. Gradually add in more realistic transformer models. Try reducing coupling coefficient from 1.00 to 0.99, or even 0.95. Add series resistance to the windings. Add pulsed loads to each output rail to see the effects of cross-regulation. If you can't get the simulation to behave to your satisfaction, then don't even bother thinking about details like bobbin pinout and core material.Also, you mentioned 100mA on each output? I don't think a 9V battery can supply nearly that much power....

author: bmxsesh
author_rank: Regular Contributor
timestamp: October 02, 2024, 06:49:52 pm
content: Quote from: mtwieg on October 02, 2024, 12:49:43 pmWhere does the (3^2) come from? I calculated the total inductance of the secondary winding from the turns ratio according to Lp/Ls=(Np/Ns)^2 and then merely divided by 3 or by 2 since it is simply series inductors?49uH/Ls = (53/15)^2Ls = 49uH/((53/15)^2) = 3.92uH (the entire secondary winding)But I guess the series inductors analogy is not really applicable as we are "splitting" the inductor?Edit: oh, I see now.It's not simply a series inductor, it'sLp/Ls_tap = (Np/Ns_tap)^2Good point!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 02, 2024, 08:04:59 pm
content: Quote from: mtwieg on October 02, 2024, 12:04:27 pmMmm, a stretch to call it "enamel" (a dipped or sprayed liquid material, dried or cured). One reference says TIW is extruded, making it just a variant of common extruded-insulation wire, repeated and with appropriate materials for typical insulation systems. Makes sense, would be hard to do without it going blobby.Tim

author: dch53
author_rank: Contributor
timestamp: October 04, 2024, 11:49:22 am
content: I'm a hobbyist rather than a professional but I'm planning on winding my own flyback transformer.I found this video useful SDG #305 How to Wind a Flyback Transformer for SMPSU.There are lots of tutorials on designing flyback transformers but I also found it useful to use the Power Integrations PI Expert tool to design a flyback converter to my requirements, not because I wanted to use a PI device but because it gives you a complete design, including for the flyback transformer WITH winding diagrams and instructions. I've already purchased the different kinds and diameters of wire.I'm only be winding a limited number of transformers so rather than using a machine like the one shown in the video I'm going to use a hand drill clamped to my bench and count the turns myself.The only problem I've found is that both the PI Expert tool and the evaluation board datasheets I'm using (they include winding diagrams and instructions too!) refer to cores that I can't buy in small quantities and /or with a material that is no longer available like a Ferroxcube PQ26/20-3F3. I've had to try and find something similar from the likes of Mouser and DigiKey.

author: bmxsesh
author_rank: Regular Contributor
timestamp: October 04, 2024, 01:26:28 pm
content: Quote from: dch53 on October 04, 2024, 11:49:22 amI will give it a try, there is also 2000's software named Magnetics Designer which has some more manufacturer options and has a nice zoomed-in cross-section view of the winding (still only a 2D, however)