Pulse transformer leakage inductance is amazingly low? - Page 1
Forum section: electronics / general

author: opampsmoker
author_rank: Regular Contributor
timestamp: October 01, 2020, 09:22:29 am
content: The Murata 78602/1C is a 1:1:1 pulse transformer. It has L(pri) of 2mH.The leakage inductance is stated as 600nH.Thats a K factor of 0.99985Surely it cant be that good?78602C/1C pulse transformer datasheethttps://www.mouser.co.uk/datasheet/2/281/kmp_786-41420.pdf

author: capt bullshot
author_rank: Super Contributor
timestamp: October 01, 2020, 10:03:41 am
content: It's all about the toroidal transformer is wound. This one most probably uses a trifilar winding at the price of a higher interwinding capacitance. For comparison, check e.g. the model 78615/1C.

author: HighVoltage
author_rank: Super Contributor
timestamp: October 01, 2020, 10:13:28 am
content: 600nH seems alright for this type of pulse transformer.But it is so low, its probably difficult to measure reliably.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 01, 2020, 01:23:11 pm
content: Suspicious of the leakage being that low, while the capacitance is that high (for some parts). If they aren't lying, it suggests multifilar windings with alternating wires, and a total winding length of a meter or two.Would be very nice gate drive transformers.Tim

author: David Hess
author_rank: Super Contributor
timestamp: October 01, 2020, 04:05:13 pm
content: Quote from: T3sl4co1l on October 01, 2020, 01:23:11 pmOr small inverter transformer for floating supplies.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 01, 2020, 08:40:12 pm
content: Quote from: T3sl4co1l on October 01, 2020, 01:23:11 pmConfirmed. I've got a few 78602/J1C floating in my heap of assorted junk, it's a small (maybe 7mm outer diameter) toroid and a multifilar winding. As the datasheet suggests by the specified interwinding capacitance and leakage inductance. They also make variants with segmented winding for lower capacitance but more leakage inductance.

author: chris_leyson
author_rank: Super Contributor
timestamp: October 01, 2020, 09:05:16 pm
content: No wonder they are expensive, it can't be easy to wind small toroids like that. The wire might be double enamelled as well to guarantee 1kV isolation voltage. Tim is right, they do make good gate drive transformers but I can't remember which one I used it was a while ago.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 02, 2020, 01:00:13 am
content: Well, 1kV is pretty ordinary for enamel, if you don't mind it being sketchy AF. They're not rated "reinforced", for good reason...I recall a line of Murata DC-DCs rated for 3kV, but still not reinforced -- same thing, with better enamel I suppose but still the single layer of it.Toroids are quite cheap to wind, as far as I know. Whether that's because of automation, or outsourced labor, I don't know...Protip: if you don't need much flux, Ethernet transformers are rated 1.5kV, have excellent bandwidth, and come with CT windings and a CMC on the secondary. (The flux, and LF cutoff, are much too low for direct gate drive purposes, but can be used as coupling for modulated signaling.)Related note, I'm fond of these for certain purposes:https://www.digikey.com/en/products/detail/pulse-electronics-power/PH9185-011NLT/3503430Leakage is impressively low, between halves of each winding and between windings. Reinforced insulation, can use for gate drives (at highish frequencies), small converters/isolators, and signals.Tim

author: jonpaul
author_rank: Super Contributor
timestamp: October 07, 2020, 06:26:06 am
content: Hello all: Have been designing and making transformers since 1970s.Measurement of Llkg is easy with a good HF bridge, short the sec, zero the bridge on lowest L range and highest Fmeas, read Lpri.The specs and numbes quoted seem reasonable.One tradeoff in pulse and HF transformer design is between Llkg and C p-s.The lowest leakage design is always a 1:1 ratio.Best Llkg is with parallel wide foil conductors. We used this technique in 4 kV radar pulse transformers for 80 nS pulses, decades ago.The trifilar toroid in the photo is machine wound and built at very low cost. Toroid winding machines are available for all sizes.The capacitance is high due to the trifilar winding and epoxy impregnation. The isolation HyPot V is thus improved.For wire insulation, ('enamel") most production parts use a poly nylon, soldereaze, nyleaze or other solderable insulation, class 155C. Double is the most popular to prevent shorts.All magnet wire is available in single, double (heavy) or even triple and quad insulation.It is possible to optimize a design for lowest Llkg and minimum C p-s but the winding is complex and expensive to manufacture. It is NOT a toroid!Just the ramblings of an old retried EE!Jon

author: HighVoltage
author_rank: Super Contributor
timestamp: October 07, 2020, 08:47:13 am
content: Quote from: jonpaul on October 07, 2020, 06:26:06 amWhat do you consider a good HF bridge for this pourpose?Which one would you recommend?Thanks

author: jonpaul
author_rank: Super Contributor
timestamp: October 07, 2020, 10:48:21 am
content: Hello again: For measurement of Cp-s (down to 0.3 pF) and Llkg (down to ~ 100 pH)we had used many bridges and a HP network analyzer with impedance adapter and fixtures:Here is a list, in approximate order of cost descending:100 kHz...500 MHz HP4195A test fixt 41951Ahttps://www.keysight.com/en/pd-334101-pn-4195A/network-spectrum-analyzerWayne Kerr 6425 to 300 kHz Precision Component AnalyzerHP 4275A 10 MHz LCRHP 4277A LCZHP 4262AHP 4332A LCR MeterAll are available in various conditions and prices on epay, but best source is Ham Radio flea markets.Using this equipment and in circuit tests with special test fixtures, we have got designs reliably at 0.3 pF Cp-s and BW to 200 MHz.Warnings:1/ The leads and test fixtures are critical at these frequencies and residual L/C levels!2/ Expect to spend time in design and debugging the test fixtures and setup, if you want reliable data! We built special PCBs to do this. Overall It took us years!3/ The older bridges like Wayne-Kerr 6425 are apt to be ornery as they are circa 1970s!4/ HP has some excellent app notes on test fixtures and measurement of LCR at high frequencies.5/ Network analyzers with correct test T/R sets can measure the displacement current from primary to secondary, and infer Cps with a sweep eg 100 kHz-500 MHz.6/ Suggest to Avoid Chinese knockoffs or small DVM handhelds, found they can be misleading and useless with zero support.Bon courage!Jon

author: voltsandjolts
author_rank: Unknown
timestamp: October 07, 2020, 11:13:29 am
content: Quote from: jonpaul on October 07, 2020, 06:26:06 amHmmm, intriguing!Any more clues?!

author: jonpaul
author_rank: Super Contributor
timestamp: October 07, 2020, 05:50:47 pm
content: Hello again: More tips.....Every part of the setup needs to be rigid, no wires or cables or test leads can move. Hand and body capacitance will affect measurements. Stay away!Keep loop inductance as low as possible, expect ~ 1nH per sq foot of enclosed area.Use shields and guards as appropriate, many (but not all) bridges have guarding.Lower Cps is in a shielded transformer, but Llkg will increase vs unshielded.Start with a 1:1 design, optimize it first, then proceed to desired ratio.Avoid impregnated designs.Just the ramblings of an old retired EEEnjoy!Jon

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 07, 2020, 09:47:32 pm
content: The general underlying theory is that a transformer is a transmission line turned sideways (or not, depending on how you use it).The transmission lines may not always be simple: multilayer windings, with odd turns ratios, will have wires overlaying wires from the same winding, wires overlaying wires from the other winding at different lengths, etc.This may be easier to see in comparison to a simpler setup:1. Consider plain old twin lead, or twisted pair. The wires have equal length, together they form a transmission line with consistent Zo of modest magnitude (twin lead can be quite high, but let's say it's closely spaced, so gives around 100 ohms).Say you drive both ends of one wire in the TL. You'll get induction in the other wire -- you have an isolation transformer. Not very much (in terms of bandwidth), but it's there. If we pass the TL through a core, the core's impedance adds in series between ends. This is the magnetizing impedance. Notice the leakage (LL and Cp) hasn't changed: the TL is still a TL in mostly-free space, with the same length and Zo. In fact, LL and Cp are proportional to TL length, and LL is proportional to Zo while Cp is inverse. We might define a cutoff frequency 1 / (2 pi sqrt(LL Cp)), which is simply the 1/4 wave electrical length of the TL.If we have the LF equivalent properties of the TL, we can calculate these directly: LL is inductance per length, times length. Cp is capacitance per length, times length. Zo is sqrt(Lo / Co).2. Consider a twin-lead TL, wound edgewise onto a cylindrical bobbin, with a core through the center (which might be a U, E or other style core shape). Then one wire occupies the whole inner layer, and the other, the outer. We simply have a transformer of two single-layer windings. Now we can have insulating tape between whole layers, rather than depending on insulation of the wires themselves.Waves travel along the TL in this configuration, in the space between wires, following them around the path to either end. (Some also couples between adjacent turns, when they're closely spaced (small winding pitch); this mostly acts on the impedance at high frequencies (above cutoff), so we aren't too worried.) The wires have similar lengths (not quite equal anymore, the outer winding is longer), so we might expect some dispersion (difference in propagation velocity with respect to frequency), but that's still mostly a HF (above cutoff) thing. Neat.Now suppose we increase the number of turns in one winding (using proportionally finer wire). Its length goes up, and even stranger, turns no longer line up with same-numbered turns on the other winding, they've moved position entirely. Multiple turns in the one winding, overlay a given turn in the other. Waves can no longer track between a given pair of wires, but are spread out over several. One path is much slower and longer (the waves, in part, have to loop completely around, taking extra turns, to catch up). The average path length is much longer, so we expect LL to increase. Meanwhile, the spacing remains constant (insulating tape and whatnot), and the area is the same, so Cp hasn't changed. Zo rises.But this isn't at all unexpected, because yeah, we're increasing the length of a winding. If we had the same Zo and increased length, we'd expect LL to increase proportionally. Simple as that. But if the insulation thickness remains the same, the ratio between wire diameter and spacing goes up, so Zo goes up, and LL goes up faster -- quadratically, more or less.What if we do multilayer block windings, rather than interleaving per layer? Well now the waves travel not only with respect to the opposite winding, but with respect to other layers of the same winding. The wave jumps forward in part, travelling through even more length -- LL goes up even faster in this configuration!A good example of this construction is a typical common mode choke for power line use. These are wound on split bobbins, each winding bunched up separately. This gives high isolation voltage for safety, and high LL which is kind of useful as it gives differential mode filtering. LL is high and Cp is low, so the Zo is quite high -- typically 600 ohms. And the bandwidth quite low, 10MHz or even lower. They don't really have high order transmission line behavior, waves just get too "lost" inside a winding like that. The 2nd order transformer equivalent circuit really does work well here:So, if you want arbitrarily low LL and don't mind paying Cp to get it, just make a low-Zo structure. Use lots of twisted pairs in parallel for example, or use wide parallel plates (foil). Loop the bundle around a core as many times as needed, and you've got a TL transformer with the properties desired.Design can be approached even more fully from the TL side, by supposing that we apply a wave to some series-parallel combination of TLs at one port, and rearrange them into another series-parallel combination at the other port. If the TLs each have cores (or are wound on however many cores with enough turns to account for the respective CM voltage drops), we don't much care about their common mode voltages, and their two ends (ports) can be wired in series-parallel with others, pretty much arbitrarily. This is a Guanella TLT. If the TLs are all equal length, then the waves are delay-matched, and dis/reassembled perfectly at each port. LL is still there, but it has to be measured from one port to the other; the through-bandwidth isnot limited by transmission line length!The other basic type is the Ruthroff TLT, which uses unmatched delays, so has a cutoff frequency inverse with TL delay. Simply put: say you wire one end of a TL in parallel with the source; wire its other end in series with the same source. Now you have a 1:2 ratioBoth types are only autoformers -- ratios, but no isolation. This isn't so important to RF purposes, but is a bit annoying for wideband applications!For wideband, the best strategy is probably a 1:1 isolation transformer. This uses the minimum length (citation needed?) so offers the most bandwidth. You can't harness the propagating mode any obvious way, it always takes a full electrical length to propagate. (If you're okay with strong attenuation, you can use a very lossy transmission line, absorbing the propagating mode and giving a flat -6dB above the HF cutoff.) To get other ratios, use an autoformer type TLT, and to improve isolation (CMRR), use CMCs.Ethernet transformers are an okay example of this: their windings are typically 1+1:1+1 (i.e. both windings CT'd), which can be done without too much extra trouble. They have a CMC on the line side, to reject noise and improve balance. The line side CT is used to terminate ESD and other stray signals (or for PoE, to carry DC load current), while the PHY side CT is used to supply bias to the output drivers. (The drivers for 10/100 are usually push-pull, so the CT is mandatory; 1G is usually driven H-bridge (and some low-power 100M drivers) so doesn't need the CT, or it's bypassed with a cap, no DC bias needed.)Tim

author: jonpaul
author_rank: Super Contributor
timestamp: October 08, 2020, 05:24:53 am
content: Bonjour Tim at Seven Transistor lab: BRAVO pour your excellent note on transmission line theory applied to transformers.I would add that wavelength of the applied signal should be compared to the winding length, and velocity factor of the dielectric taken into account.In applications mentioned above, winding length is a small fraction of the wavelength, thus lumped modeling is reasonably accurate.Examples of Transmission line theory in transformers:1/ Nikola Tesla's 1890s megavolt HV transformers (20m dim), a 1/4 wave length down the winding to maximize the V at top and maximize I at bottom, f = 20..60 kHz.2/ Ham Radio Amateurs use "choke baluns" or 'current baluns" for Zo matching, coupling antennas, etcWe have fine results with lumped parameter modeling to 200 MHz with up to 10 cm winding length.A few text books for our Readers, to explore pulse magnetics design:Magnetic Components: Design and Applications, Steve Smith (lots of pulse design eg radar)Static Electromagnetic Devices, Hunt and Stein (1970, basics of magnetics)Handbook of Transformer Design and Applications, William Flanagan (no information, have not reviewed)Bon courage!Jon

author: JohnG
author_rank: Frequent Contributor
timestamp: October 09, 2020, 01:29:19 pm
content: Quote from: jonpaul on October 07, 2020, 05:50:47 pmShould this be 1 uH per square foot?John

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 09, 2020, 08:22:57 pm
content: Technically it's square foot per foot, the cross-dimensions of a volume the same way resistance is. For a known cross-section, it reduces to a proportional linear measure. Hm, the unit is wrong either way, the inductance of free space is ~1.257 uH/m.Tim

author: JohnG
author_rank: Frequent Contributor
timestamp: October 09, 2020, 10:23:55 pm
content: Ok, for a loop area of 1 square foot, to be specific, which I thought was the intent. To be more specific, a square loop with sides of 1 foot length, since it assumes negligible mutual inductance between segments of the loop, and also implicitly assumes conductor forming the loop has a maximum dimension much smaller than the smallest dimension of the loop.This should be close to 1 uH, not 1 nH, and I assume the latter was just a typo. I always liked the 1 uH per 1 meter of circumference of a circular loop, which is strange since I grew up on feet and inches. Anything more exact needed, and you can pull out Grover for a more exact number, or do an actual analysis via your favorite method.Fun fact: both Frederick Grover and Charles Steinmetz served as faculty at Union College in Schenectady, NY.Cheers,John

author: jonpaul
author_rank: Super Contributor
timestamp: October 11, 2020, 03:00:31 pm
content: To John G and Tim: Mia culpa! I meant 1 uH for 1' sqor 1 nH for 1" sq. !Sorry that this old EE (started 1968) is loosing it.......Upon checking the formulas for a square 1 t loop, aircore flat loop, the numbers are1" sw 1 mil wire = 124.6 nH (more like it!)for 12" sq 1 mil wire 2.102 uHIt varies with wire diameter and permeabilityMy original point was: In measuring leakage L and other small L, Minimize test leads or test fixture loop area and hold everything ridgidly!Enjoy,Jon

author: JohnG
author_rank: Frequent Contributor
timestamp: October 11, 2020, 07:43:59 pm
content: I hardly think that one typo qualifies as losing itJohn

author: jonpaul
author_rank: Super Contributor
timestamp: October 12, 2020, 06:09:29 am
content: Additional Note re Pulse Response...The "acid" test of a pulse transformer performance in circuit with transmission of required pulses !A special part was tested on SMD PCB, carefully matched Zgen/Zload, eg 50 or 75 Ohms, using fast rise low aberration pulse generators, Yokogawa and TEK scopes and HP4195A network analyzer.Results: ~1 nS risetime and 1-2% pulse abberation.I Highly suggest such an in-circuit test on any "pulse" transformer.Best,JonPS: To John G, you are too kind! At 73 I am still active...

author: f4eru
author_rank: Super Contributor
timestamp: October 12, 2020, 08:41:15 am
content: why still use pulse transformers to drive mosfets or IGBTs ?There are good driver ICs for half bridges to avoid that need completely...

author: jonpaul
author_rank: Super Contributor
timestamp: October 12, 2020, 10:34:26 am
content: ......why still use pulse transformers to drive mosfets or IGBTs ?The choice of isolation and driver for floating switches eg H bridge and half bridge, is one of the design decisions.For high voltages and powers and depending on the duty cycle, transformers can be a better choice.For widest duty cycle the IC drivers and optos are better.Jon

author: JohnG
author_rank: Frequent Contributor
timestamp: October 12, 2020, 12:57:36 pm
content: Interestingly, the On-Semi NCP51820 gets surprising close to $1.00 in smallish quantity (a few thousand). It's targeted at GaN, so 5V outputs on the gate drive, but otherwise looks nice. 5V drive is pretty low for 600V MOSFETs, though, probably not usable.John