Specifications for obsolete push-pull transformers - Page 1
Forum section: electronics / projects

author: Strand17
author_rank: Contributor
timestamp: September 16, 2022, 08:56:07 am
content: I am looking for the inductance, resistance and turns ratios for two obsolete push-pull transformers recommended for use with the LT1533 low noise switching regulator:CTX02-13664-X1Midcom 31351...and in an extension, what would be suitable substitutes for these parts.Any help would be highly appreciated!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 16, 2022, 02:52:17 pm
content: The one datasheet is easy enough:https://www.mouser.com/catalog/specsheets/213664es.pdfThey don't give leakage unfortunately.Coilcraft CR7984-CL might be okay, though inductance is much lower. PA6548-AL also, though inductance is even lower still.There are plenty of 1CT:1CT type transformers that would get you halfway there, but then one or two 1:3 would be needed to furnish the final output. Alternately if you can change the output to a FWB or doubler or even tripler type rectifier, maybe a lower ratio would be easier to find.The line of configurable transformers is nice (Coilcraft Hexa-Path, etc.), though not enough to get 1:3 CT unfortunately....Oh, fucking no wonder it didn't show up, it's completely lacking specs in the listing. Give these a try: Pulse PH9185.013NLT, PH9385.013NL.Tim

author: Strand17
author_rank: Contributor
timestamp: September 17, 2022, 09:15:06 am
content: Thanks for the link and the suggestions!Please excuse a few stupid questions about the CTX02-13664 data sheet...- Can the actual turns ratio be deduced from the data sheet? The nominal voltage ratio is 5V input and 15V output, but many 5V/15V transformers seem to have turns ratios in the range 1:3.5 to 1:4.5.- Under "Electrical characteristics", the primary side inductance is listed as 878 - 1631 uH, but in the box on the bottom right the transformer is described as "Transformer, Push-Pull, non-shielded, 310 uH". What inductance does the 310 uH refer to?...and I may end up contacting ADI tech support as well, just to be sure.- Jonas

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 17, 2022, 10:30:45 am
content: Could be the title is referring to the single or half-winding value; both primaries in series is 4x, or, in the right ballpark for the two figures.Yes, ratio could be higher. If equal winding area was used between pri/sec, with the wire diameters perfectly in ratio to current ratings, then the DCR ratios should give the square of the turns ratio. Which is about 20:1, so sqrt(20) = 4.5 turns ratio. Also assuming the max(DCR) has the same ratio to nominal for both windings, which isn't a good assumption either, but I'd feel okay treating this as an upper limit, maybe?Oh also, the voltage ratio is 1:3, but if this is a forward converter i.e. there's a choke-input rectifier at the output and some dead time is expected between pulses, then the ratio indeed needs to be higher by that margin, plus losses of course (primary switch + output diode).Less inductance may actually be required, but maybe also that screws with performance of the controller, I don't know. Yes, contacting them might be illuminating.Tim

author: julian1
author_rank: Frequent Contributor
timestamp: September 17, 2022, 11:00:42 pm
content: lt1533 is 78USD at lcsc, with no availability...What would be the main characteristics for the transformer? I think in a push-pull, at switch turnoff, the opposing winding works a bit like a (forward-converter) reset winding, to suppress voltage excursions due to parasitic inductance? ie. So, even with signal dead time, the body diode of switch fet, or PN junctions of bjt will complete the circuit.So the two primaries should be wound bifilar on a toroid, or the same section on a split bobbin transformer. While, the arrangement of the secondaries shouldn't matter too much.Is there anything else?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 18, 2022, 01:59:17 pm
content: Not sure why you'd go shopping on LCSC first... The part does indeed show on Mouser for example, albeit at legacy / boutique prices (~$12). No surprise for such a specialized part. None in stock, but they are showing as on-order.Leakage shouldn't be a big deal in general, due to the slow switching. Reduction of harmonics and ringing is basically to say the slew rates (current and voltage) are controlled, i.e. the derivatives are limited, and probably the 2nd and maybe higher derivatives as well. Which means there's not really anything to start ringing, it's all very gradually transitioning between on and off.The main thing you can't control is the output rectifiers, which can have reverse recovery snappier than the applied waveforms, i.e. they can have a sharpening effect. Schottky helps reduce this.Especially for EMI purposes, the primary and secondary respectively should be bifilar, to keep their voltages matched, so that common-mode sees an average zero, except only transiently (and again, said transients are limited by design, so can be ignored to that extent). Primary and secondary interleave are less important, which is generally one of the advantages of PP or 1/2-bridge forward: any spare magnetizing or leakage inductance is simply clamped by the opposing side switch. The biggest disadvantage is, at turn-on, diode capacitance (and also reverse recovery in CCM) and P-S leakage cause voltage overshoot, so can require relatively high voltage ratings (like 100V diodes for a 12V output). Again, the slow switching helps reduce this effect.Normally, at switch turn-off, the output choke forces the secondary to zero volts, at least momentarily in DCM; all the time in CCM. After its current decays below the transformer's magnetizing current, the voltage can rebound some, and remaining charge may flow from Imag to output, or into primary (GND clamp diodes).But anyway, yeah, they give design equations in the datasheet, follow that and things should turn out fine. Perhaps tweaking the slew rate components would be required to accommodate less ideal transformers, that might be it?Tim