krw <krw@nowhere.com> writes:> On Sat, 07 Nov 2015 13:10:43 +0000, John Devereux[...]>>I think if I build the whole SMPS section on a local ground "island" >>over the overall groundplane this will work OK. Any magnetic leakage >>through the island is only going to be at lower frequencies. As you >>pointed out my "Q" measurement indicates that efficiency should not be >>affected significantly. >> > The ground island my help here but it's not the reason to do a ground > island. I almost always use the powdered metal type of inductors > (similar to Vishay IHLP series)Nice, they don't seem to have the air gap problem (a bit like the multilayer chip inductors I mentioned, windings totally surrounded by ferrite). Rather more expensive than the "glued bobbin in a ferrite sleeve" type but I will get some in.> but also ground islands to keep the loop currents local to the SMPS.I appreciate this is the usual reason, but I think the "ground island" should also work nicely both to stop any residual field and to eliminate the effect of circulating induced currents from same.> Only the input and output capacitors and low-side switch/inductor get > connected to this local plane, though. The feedback, compensation, > and any "analog" ground pins connect to the over-all ground. > The ground connections of the input and output capacitors should be > close to each other on this island, too. Most SMPS recommended > layouts follow this strategy.Thank you (and that matches the datasheet very well). -- John Devereux
Groundplane under SMPS power inductor
Started by ●November 4, 2015
Reply by ●November 8, 20152015-11-08
Reply by ●November 8, 20152015-11-08
On Saturday, November 7, 2015 at 8:10:18 AM UTC-5, John Devereux wrote:> dagmargoodboat@yahoo.com writes: > > > On Friday, November 6, 2015 at 8:34:36 AM UTC-5, John Devereux wrote: > >> RBlack <news@rblack01.plus.com> writes: > >> > >> > On Thu, 5 Nov 2015 19:38:24 -0800 (PST), (dagmargoodboat@yahoo.com) > >> > said: > >> >> On Wednesday, November 4, 2015 at 11:33:34 AM UTC-5, John Devereux wrote: > >> >> > I have seen designs where there is a cutout in the groundplane under the > >> >> > power inductor in e.g. a buck converter. Inductor is nominally a > >> >> > "shielded" one. > >> >> > > >> >> > I assume it is to prevent some kind of "shorted turn" effect? What do > >> >> > you think? > >> >> > > >> >> > Might a continuous plane be better? It could help to shield any field > >> >> > leakage and reduce emissions and circuit noise. > >> >> > > >> >> > I am asking generally, but say 1A, 500kHz. > > [...] > > > I pulled up the old artwork for a look and thought it worth sharing > > some more details of my experience. > > > > It was a buck SMPS, 12V input, 5V output @2.4A, ~460KHz. > > > > As received, > > Layer 1 was needed to connect the inductor, > > Layer 2 had non-critical signals, > > Layer 3 was solid ground plane. > > > > > > Layer 1 > > .---------------------------// > > .------/-----------. > > | |-----. | .-(+)-. +5V output > > | | | | | C1 | > > | '-----'-----|-|- - -|-------// > > | .--|-|- - -|----------> > > | | .| '-(-)-'. . . . . > > | L1 |. | . . . . ..-------> > > | | .|. . . . . | > > | .-----. |. | . GND . .| > > | | | | .|. . . . . | > > | '-----' |. | . . . . .| > > '-------\----/--|--'. .TP1. . | > > | | /. . . (O) . .| > > > > > > TP1 is a solid through-hole metal post, connected to the GND trace and > > to the ground plane on layer 3. > > > > You can see that the original ground run ran under L1, parallel > > to L1's winding progression, creating a transformer-coupling in the topside > > GND above TP1 (a solid metal grounded test post at the lower bound) and > > prior to the bulk filter cap C1's negative terminal. > > > > So, no doubt there was significant coupling into that trace despite a > > solid plane two layers deeper. > > > > That rendered C1 (and additional bypasses) ineffective at r.f. The > > top-right portion of the GND trace supplied a ribbon connector, providing > > a path for wicked radiation and conduction of the induced GND signal. > > I guess the very low impedance is what makes the cap ineffective.My interpretation was that passing C1's ground lead under L1 created a parasitic transformer winding coupled to L1. L1 ---. . . .---------+-------+--->+5V '-'-'-' | | =======~~~=== --- C1 --- C2..n .-. --- --- GND trace ~>| | | | under L1 | '---+-------+---> GND' to | connector (o) TP1 | === GND plane, layer 3 Since C1's ground now carries the same signal meant to be filtered from +5V, it (and bypasses [C2..n]) cannot function as intended to attenuate +5V ripple and spikes. +5V and GND', which are cabled off-board, are then laden with said spikes, hence massive RFI.> > The revised layout routed layer 1's GND well around L1 rather than under, > > and nailed it to layer 3's ground plane at both ends of the run, and in- > > between. > > > > The replacement inductor(*) was also a drum cemented into a ferrite box > > like the first unit, however, the new unit's down-facing gap was tighter, > > and filled with what appeared to be a ferrite-loaded cement. (The original > > unit had a larger downward-facing gap, air-filled, IIRC.) > > I suppose I need to buy a few types. > > > * Bourns SRR1240-150M > > > > The improved gap configuration wasn't obvious on the datasheet--I had to > > get a sample in hand. > > > > So, I can't say for sure that an inductor directly over a solid ground plane > > would be problematic from this experience. > > Thank you very much for the detailed write-up James. > > > I can say there was significant transformer coupling due to fringe flux > > from the downward-facing gap. > > I think if I build the whole SMPS section on a local ground "island" > over the overall groundplane this will work OK. Any magnetic leakage > through the island is only going to be at lower frequencies. As you > pointed out my "Q" measurement indicates that efficiency should not be > affected significantly. > > Thanks,Sure. Even if it's possibly ultimately okay I'm still not 100% comfortable with the prospects of inducing large currents in a ground plane, both for signal integrity and loss (efficiency) reasons. --quote-->Hey you beat me to it, thanks. I found ~3% with a SRU1048 22uH. If I >drill a 10mm hole under it the change is about 1.5%. So it makes a bit >of a difference but does not eliminate the effect. > >This was all at 500kHz. > >The indicated Q went from 24 to 22 as it was brought up to the plane.--/quote-- I read that too quickly, mistook 3 *per cent* reduction in L for 3uH out of 22uH. d(L)=3% is quite a bit smaller than Q dropping from 24 to 22 (a drop of 9%), so there may indeed be significant added loss, I'm not sure. Inductor loss is just a fraction of total SMPS loss, but big enough that I wouldn't want to increase it unnecessarily. Cheers, James Arthur
Reply by ●November 8, 20152015-11-08
On Sun, 08 Nov 2015 09:17:18 +0000, John Devereux <john@devereux.me.uk> wrote:>krw <krw@nowhere.com> writes: > >> On Sat, 07 Nov 2015 13:10:43 +0000, John Devereux > >[...] > >>>I think if I build the whole SMPS section on a local ground "island" >>>over the overall groundplane this will work OK. Any magnetic leakage >>>through the island is only going to be at lower frequencies. As you >>>pointed out my "Q" measurement indicates that efficiency should not be >>>affected significantly. >>> >> The ground island my help here but it's not the reason to do a ground >> island. I almost always use the powdered metal type of inductors >> (similar to Vishay IHLP series) > >Nice, they don't seem to have the air gap problem (a bit like the >multilayer chip inductors I mentioned, windings totally surrounded by >ferrite). Rather more expensive than the "glued bobbin in a ferrite >sleeve" type but I will get some in.Our volumes are typically in the millions, so the cost delta isn't a lot, if any. We usually get the top-tier volume price on any value in the series because they're used so much. Vishay is pricey but there are others in the market now. For smaller devices (under .5" or so) Toko/Murata is much cheaper. Delta/Cyntec is also in this market. There are probably several others now. These parts have soft saturation characteristics, too. One doesn't have to worry as much about saturating the core in an overload, for instance. They're really easy parts to work with. Toko/Murata also makes MLC inductors (2012 and 2520) in powdered metal. Nice parts!> >> but also ground islands to keep the loop currents local to the SMPS. > >I appreciate this is the usual reason, but I think the "ground island" >should also work nicely both to stop any residual field and to eliminate >the effect of circulating induced currents from same.Sure. The idea is to keep the loop area small but the island also keeps any voltage induced in the ground to a minimum.> >> Only the input and output capacitors and low-side switch/inductor get >> connected to this local plane, though. The feedback, compensation, >> and any "analog" ground pins connect to the over-all ground. >> The ground connections of the input and output capacitors should be >> close to each other on this island, too. Most SMPS recommended >> layouts follow this strategy.I forgot to mention that the island and the larger plane usually get connected under the chip, particularly if there is a power pad. This helps power dissipation, too.> >Thank you (and that matches the datasheet very well).I had to go out to lunch, on our TI FAE, many times to learn why things are done the way they are. ;-)
Reply by ●November 10, 20152015-11-10
On Fri, 6 Nov 2015 11:19:49 -0800 (PST), (dagmargoodboat@yahoo.com) said:> On Friday, November 6, 2015 at 8:34:36 AM UTC-5, John Devereux wrote: > > RBlack <news@rblack01.plus.com> writes: > > > > > On Thu, 5 Nov 2015 19:38:24 -0800 (PST), (dagmargoodboat@yahoo.com) > > > said: > > >> On Wednesday, November 4, 2015 at 11:33:34 AM UTC-5, John Devereux wrote: > > >> > I have seen designs where there is a cutout in the groundplane under the > > >> > power inductor in e.g. a buck converter. Inductor is nominally a > > >> > "shielded" one. > > >> > > > >> > I assume it is to prevent some kind of "shorted turn" effect? What do > > >> > you think? > > >> > > > >> > Might a continuous plane be better? It could help to shield any field > > >> > leakage and reduce emissions and circuit noise. > > >> > > > >> > I am asking generally, but say 1A, 500kHz. > > >> > > >> I inherited a SMPS a few years ago that was blanking out nearly > > >> the entire F.M. band. > > >> > > >> Part of the problem was the main inductor's gap, which faced down > > >> into a ground plane and impressed a very impressive signal across > > >> same. > > >> > > >> This inductor was "shielded," a drum core cemented inside a 5-sided > > >> ferrite box. > > >> > > >> I substituted a gap-up inductor, and (that part of) the problem was > > >> solved. > > >> > > >> Cheers, > > >> James Arthur > > >> > > > > > > Interesting. I have a very similar problem at the moment, a board with > > > two synchronised 90W boost converters, using the type of inductors you > > > describe. They're made by W�rth, who are pretty good IME, but I've > > > never tried this inductor construction before. > > > > > > The board is 4 layers with a solid ground plane under the inductor, and > > > no traces or components on the opposite side to the inductor. > > > Originally I thought any currents induced in the ground plane would have > > > a small enough loop area, and the plane itself would be a low enough > > > impedance, that this wouldn't be a problem. Clearly this analysis > > > doesn't hold up at this sort of power level... > > > > Looking at it pessimistically, you could think of it as a voltage > > transformer. 12V on the "primary" and xxV around the "secondary" which > > is a circular region of ground plane under the inductor. > > > > If the inductor has 6 turns say that would be 2V induced with perfect > > coupling. If it only couples 5% that is still 100mV which is not nothing > > when it appears at RF on a cable or in a sensitive circuit. > > > > The rest of the ground plane tries to short this out but there could be > > a lot of current available, the same ratio that steps down the voltage > > steps up the available current. > > > > (Sorry for the above travesty of transformer theory and I know it's all > > a bit handwavey but I can see how it could be an issue!) > > > > > I will try and find a gap-up inductor with the same specs which fits the > > > PCB pads. I suspect the problem I will then have, is that there is an > > > array of right-angle connectors on an adjacent board, whose PCB pins > > > pass within a few mm of the top of one of the inductors. The boards are > > > in a metal housing and the connectors and cabling are shielded, but > > > still not ideal. > > > > Unfortunately I never see the leakage mentioned anywhere as an inductor > > specification. Except that it might be "low". > > > > The dirt-cheap multilayer inductors are quite good here, I guess the > > turns are fully embedded in ferrite. But they only seem to go to low > > power levels and are more lossy than other types. > > > > > > -- > > > > John Devereux > > > I pulled up the old artwork for a look and thought it worth sharing > some more details of my experience. > > It was a buck SMPS, 12V input, 5V output @2.4A, ~460KHz. > > As received, > Layer 1 was needed to connect the inductor, > Layer 2 had non-critical signals, > Layer 3 was solid ground plane. > > > Layer 1 > .---------------------------// > .------/-----------. > | |-----. | .-+++-. +5V output > | | | | | C1 | > | '-----'-----|-|- - -|-------// > | .--|-|- - -|----------> > | | .| '-xxx-'. . . . . > | L1 |. | . . . . ..-------> > | | .|. . . . . | > | .-----. |. | . GND . .| > | | | | .|. . . . . | > | '-----' |. | . . . . .| > '-------\----/--|--'. .TP1. . | > | | /. . . (). . .| > > > TP1 is a solid through-hole metal post, connected to the GND trace and > to the ground plane on layer 3. > > You can see that the original ground run ran under L1, parallel > to L1's winding progression, creating a transformer-coupling in the topside > GND above TP1 (a solid metal grounded test post at the lower bound) and > prior to the bulk filter cap C1's negative terminal. > > So, no doubt there was significant coupling into that trace despite a > solid plane two layers deeper. > > That rendered C1 (and additional bypasses) ineffective at r.f. The > top-right portion of the GND trace supplied a ribbon connector, providing > a path for wicked radiation and conduction of the induced GND signal. > > The revised layout routed layer 1's GND well around L1 rather than under, > and nailed it to layer 3's ground plane at both ends of the run, and in- > between. > > The replacement inductor(*) was also a drum cemented into a ferrite box > like the first unit, however, the new unit's down-facing gap was tighter, > and filled with what appeared to be a ferrite-loaded cement. (The original > unit had a larger downward-facing gap, air-filled, IIRC.) > > * Bourns SRR1240-150M > > The improved gap configuration wasn't obvious on the datasheet--I had to > get a sample in hand. > > So, I can't say for sure that an inductor directly over a solid ground plane > would be problematic from this experience. > > I can say there was significant transformer coupling due to fringe flux > from the downward-facing gap. > > Cheers, > James Arthur >Thanks for the write-up. The inductor I am using is a different construction again from the two you have mentioned, the windings are on a core which goes parallel to the PCB surface, which is then cemented into a ferrite box. The cross section looks a bit like: +----------------+ | | | +------------+ | | | | | | | oooooooooo | | | +------------+ | | | | +------------+ | | | oooooooooo | | | | windings | | | | | | +-+ +-+ ------------------------- PCB ------------------------- So the gap, such as it is, is the (presumably) lower-mu material which joins the core rod to the inside of the box. I'd expect this construction to have a significant downwards fringe field as well, due to the assymmetry. Another poster has mentioned the Vishay IHLP series, I will check these out. I'm hoping a different inductor design will help here, having to re-spin this PCB will not make me popular...
Reply by ●November 10, 20152015-11-10
On Tuesday, November 10, 2015 at 4:08:48 AM UTC-5, RBlack wrote:> On Fri, 6 Nov 2015 11:19:49 -0800 (PST), (dagmargoo...@yahoo.com) > said:> > I pulled up the old artwork for a look and thought it worth sharing > > some more details of my experience. > > > > It was a buck SMPS, 12V input, 5V output @2.4A, ~460KHz. > > > > As received, > > Layer 1 was needed to connect the inductor, > > Layer 2 had non-critical signals, > > Layer 3 was solid ground plane. > > > > > > Layer 1 > > .---------------------------// > > .------/-----------. > > | |-----. | .-+++-. +5V output > > | | | | | C1 | > > | '-----'-----|-|- - -|-------// > > | .--|-|- - -|----------> > > | | .| '-xxx-'. . . . . > > | L1 |. | . . . . ..-------> > > | | .|. . . . . | > > | .-----. |. | . GND . .| > > | | | | .|. . . . . | > > | '-----' |. | . . . . .| > > '-------\----/--|--'. .TP1. . | > > | | /. . . (). . .| > > > > > > TP1 is a solid through-hole metal post, connected to the GND trace and > > to the ground plane on layer 3. > > > > You can see that the original ground run ran under L1, parallel > > to L1's winding progression, creating a transformer-coupling in the topside > > GND above TP1 (a solid metal grounded test post at the lower bound) and > > prior to the bulk filter cap C1's negative terminal. > > > > So, no doubt there was significant coupling into that trace despite a > > solid plane two layers deeper. > > > > That rendered C1 (and additional bypasses) ineffective at r.f. The > > top-right portion of the GND trace supplied a ribbon connector, providing > > a path for wicked radiation and conduction of the induced GND signal. > > > > The revised layout routed layer 1's GND well around L1 rather than under, > > and nailed it to layer 3's ground plane at both ends of the run, and in- > > between. > > > > The replacement inductor(*) was also a drum cemented into a ferrite box > > like the first unit, however, the new unit's down-facing gap was tighter, > > and filled with what appeared to be a ferrite-loaded cement. (The original > > unit had a larger downward-facing gap, air-filled, IIRC.) > > > > * Bourns SRR1240-150M > > > > The improved gap configuration wasn't obvious on the datasheet--I had to > > get a sample in hand. > > > > So, I can't say for sure that an inductor directly over a solid ground plane > > would be problematic from this experience. > > > > I can say there was significant transformer coupling due to fringe flux > > from the downward-facing gap. > > > > > > Thanks for the write-up. > The inductor I am using is a different construction again from the two > you have mentioned, the windings are on a core which goes parallel to > the PCB surface, which is then cemented into a ferrite box. The cross > section looks a bit like: > > +----------------+ > | | > | +------------+ | > | | | | > | | oooooooooo | | > | +------------+ | > | | > | +------------+ | > | | oooooooooo | | > | | windings | | > | | | | > +-+ +-+ > ------------------------- > PCB > ------------------------- > > So the gap, such as it is, is the (presumably) lower-mu material which > joins the core rod to the inside of the box. I'd expect this > construction to have a significant downwards fringe field as well, due > to the assymmetry.Man, that's ugly. That's not an inductor, that's a rail gun / field projector! Here's a cross section of the units I used: .--..-------..--. |.-'|_ _|'-.| || oo| |oo || || oo| |oo || || oo| |oo || || oo| |oo || || oo| |oo || |'-..-' '-..-'| '--''-------''--' I guess they can't do this... .--..-------..--. |.-'|_ _|'-.| || oo| |oo || || oo| |oo || || oo| |oo || || oo| |oo || || oo| |oo || || .-' '-. || || |_______| || |'-------------'| '---------------' ....because they need some way to get the leads out the bottom.> Another poster has mentioned the Vishay IHLP series, I will check these > out. I'm hoping a different inductor design will help here, having to > re-spin this PCB will not make me popular...If you posted a part number, the intrepid s.e.d. crowd-sourcing crew might conjure up a substitute... Cheers, James Arthur
Reply by ●November 11, 20152015-11-11
On Tue, 10 Nov 2015 09:04:44 -0800 (PST), (dagmargoodboat@yahoo.com) said:> On Tuesday, November 10, 2015 at 4:08:48 AM UTC-5, RBlack wrote: > > On Fri, 6 Nov 2015 11:19:49 -0800 (PST), (dagmargoo...@yahoo.com) > > said: >[snip]> > > > Thanks for the write-up. > > The inductor I am using is a different construction again from the two > > you have mentioned, the windings are on a core which goes parallel to > > the PCB surface, which is then cemented into a ferrite box. The cross > > section looks a bit like: > > > > +----------------+ > > | | > > | +------------+ | > > | | | | > > | | oooooooooo | | > > | +------------+ | > > | | > > | +------------+ | > > | | oooooooooo | | > > | | windings | | > > | | | | > > +-+ +-+ > > ------------------------- > > PCB > > ------------------------- > > > > So the gap, such as it is, is the (presumably) lower-mu material which > > joins the core rod to the inside of the box. I'd expect this > > construction to have a significant downwards fringe field as well, due > > to the assymmetry. > > Man, that's ugly. That's not an inductor, that's a rail gun / field > projector! > > Here's a cross section of the units I used: > .--..-------..--. > |.-'|_ _|'-.| > || oo| |oo || > || oo| |oo || > || oo| |oo || > || oo| |oo || > || oo| |oo || > |'-..-' '-..-'| > '--''-------''--' > > I guess they can't do this... > .--..-------..--. > |.-'|_ _|'-.| > || oo| |oo || > || oo| |oo || > || oo| |oo || > || oo| |oo || > || oo| |oo || > || .-' '-. || > || |_______| || > |'-------------'| > '---------------' > ....because they need some way to get the leads out the bottom. > > > > Another poster has mentioned the Vishay IHLP series, I will check these > > out. I'm hoping a different inductor design will help here, having to > > re-spin this PCB will not make me popular... > > If you posted a part number, the intrepid s.e.d. crowd-sourcing crew might > conjure up a substitute... > > Cheers, > James Arthur >The part is W�rth Elektronik 7443320330 - 3.3 uH, 17 amps, 4.4 milliohms, in a cubic SMD package roughly 12mm on a side. W�rth are normally my go-to supplier of magnetics, I will have to cast the net a bit wider this time I think.
Reply by ●November 11, 20152015-11-11
On Wednesday, 11 November 2015 21:27:46 UTC+11, RBlack wrote:> On Tue, 10 Nov 2015 09:04:44 -0800 (PST), (dagmargoodboat@yahoo.com) > said: > > On Tuesday, November 10, 2015 at 4:08:48 AM UTC-5, RBlack wrote: > > > On Fri, 6 Nov 2015 11:19:49 -0800 (PST), (dagmargoo...@yahoo.com) > > > said: > > > [snip] > > > > > > > Thanks for the write-up. > > > The inductor I am using is a different construction again from the two > > > you have mentioned, the windings are on a core which goes parallel to > > > the PCB surface, which is then cemented into a ferrite box. The cross > > > section looks a bit like: > > > > > > +----------------+ > > > | | > > > | +------------+ | > > > | | | | > > > | | oooooooooo | | > > > | +------------+ | > > > | | > > > | +------------+ | > > > | | oooooooooo | | > > > | | windings | | > > > | | | | > > > +-+ +-+ > > > ------------------------- > > > PCB > > > ------------------------- > > > > > > So the gap, such as it is, is the (presumably) lower-mu material which > > > joins the core rod to the inside of the box. I'd expect this > > > construction to have a significant downwards fringe field as well, due > > > to the assymmetry. > > > > Man, that's ugly. That's not an inductor, that's a rail gun / field > > projector! > > > > Here's a cross section of the units I used: > > .--..-------..--. > > |.-'|_ _|'-.| > > || oo| |oo || > > || oo| |oo || > > || oo| |oo || > > || oo| |oo || > > || oo| |oo || > > |'-..-' '-..-'| > > '--''-------''--' > > > > I guess they can't do this... > > .--..-------..--. > > |.-'|_ _|'-.| > > || oo| |oo || > > || oo| |oo || > > || oo| |oo || > > || oo| |oo || > > || oo| |oo || > > || .-' '-. || > > || |_______| || > > |'-------------'| > > '---------------' > > ....because they need some way to get the leads out the bottom. > > > > > > > Another poster has mentioned the Vishay IHLP series, I will check these > > > out. I'm hoping a different inductor design will help here, having to > > > re-spin this PCB will not make me popular... > > > > If you posted a part number, the intrepid s.e.d. crowd-sourcing crew might > > conjure up a substitute... > > > > Cheers, > > James Arthur > > > > The part is W�rth Elektronik 7443320330 - 3.3 uH, 17 amps, 4.4 > milliohms, in a cubic SMD package roughly 12mm on a side. > > W�rth are normally my go-to supplier of magnetics, I will have to cast > the net a bit wider this time I think.If you need a tighter spec on leakage inductance, get somebody to wind something for you. The people who make ferrites also make formers onto which you can wind what you want. There aren't as many coil-winding shops around as there used to be - in part because somebody in China will always under-cut them - but it's not a particularly expensive option. EPCOS has particularly nice manuals and application notes. http://en.tdk.eu/tdk-en/1190522/products/product-catalog/ferrites-and-accessories/epcos-ferrites-and-accessories -- Bill Sloman, Sydney
Reply by ●November 11, 20152015-11-11
Bill Sloman <bill.sloman@gmail.com> writes:> On Wednesday, 11 November 2015 21:27:46 UTC+11, RBlack wrote: >> On Tue, 10 Nov 2015 09:04:44 -0800 (PST), (dagmargoodboat@yahoo.com) >> said: >> > On Tuesday, November 10, 2015 at 4:08:48 AM UTC-5, RBlack wrote: >> > > On Fri, 6 Nov 2015 11:19:49 -0800 (PST), (dagmargoo...@yahoo.com) >> > > said: >> > >> [snip] >> >> > > >> > > Thanks for the write-up. >> > > The inductor I am using is a different construction again from the two >> > > you have mentioned, the windings are on a core which goes parallel to >> > > the PCB surface, which is then cemented into a ferrite box. The cross >> > > section looks a bit like: >> > > >> > > +----------------+ >> > > | | >> > > | +------------+ | >> > > | | | | >> > > | | oooooooooo | | >> > > | +------------+ | >> > > | | >> > > | +------------+ | >> > > | | oooooooooo | | >> > > | | windings | | >> > > | | | | >> > > +-+ +-+ >> > > ------------------------- >> > > PCB >> > > ------------------------- >> > > >> > > So the gap, such as it is, is the (presumably) lower-mu material which >> > > joins the core rod to the inside of the box. I'd expect this >> > > construction to have a significant downwards fringe field as well, due >> > > to the assymmetry. >> > >> > Man, that's ugly. That's not an inductor, that's a rail gun / field >> > projector! >> > >> > Here's a cross section of the units I used: >> > .--..-------..--. >> > |.-'|_ _|'-.| >> > || oo| |oo || >> > || oo| |oo || >> > || oo| |oo || >> > || oo| |oo || >> > || oo| |oo || >> > |'-..-' '-..-'| >> > '--''-------''--' >> > >> > I guess they can't do this... >> > .--..-------..--. >> > |.-'|_ _|'-.| >> > || oo| |oo || >> > || oo| |oo || >> > || oo| |oo || >> > || oo| |oo || >> > || oo| |oo || >> > || .-' '-. || >> > || |_______| || >> > |'-------------'| >> > '---------------' >> > ....because they need some way to get the leads out the bottom. >> > >> > >> > > Another poster has mentioned the Vishay IHLP series, I will check these >> > > out. I'm hoping a different inductor design will help here, having to >> > > re-spin this PCB will not make me popular... >> > >> > If you posted a part number, the intrepid s.e.d. crowd-sourcing crew might >> > conjure up a substitute... >> > >> > Cheers, >> > James Arthur >> > >> >> The part is Würth Elektronik 7443320330 - 3.3 uH, 17 amps, 4.4 >> milliohms, in a cubic SMD package roughly 12mm on a side. >> >> Würth are normally my go-to supplier of magnetics, I will have to cast >> the net a bit wider this time I think. > > If you need a tighter spec on leakage inductance, get somebody to wind > something for you. The people who make ferrites also make formers onto > which you can wind what you want.Is leakage inductance the same thing as the magnetic leakage we are discussing?> There aren't as many coil-winding shops around as there used to be - > in part because somebody in China will always under-cut them - but > it's not a particularly expensive option.Maybe it's possible, but we are talking about rather small, surface mount parts here.> EPCOS has particularly nice manuals and application notes. > > http://en.tdk.eu/tdk-en/1190522/products/product-catalog/ferrites-and-accessories/epcos-ferrites-and-accessories-- John Devereux
Reply by ●November 11, 20152015-11-11
On Wednesday, 11 November 2015 23:05:47 UTC+11, John Devereux wrote:> Bill Sloman <bill.sloman@gmail.com> writes: > > > On Wednesday, 11 November 2015 21:27:46 UTC+11, RBlack wrote: > >> On Tue, 10 Nov 2015 09:04:44 -0800 (PST), (dagmargoodboat@yahoo.com) > >> said: > >> > On Tuesday, November 10, 2015 at 4:08:48 AM UTC-5, RBlack wrote: > >> > > On Fri, 6 Nov 2015 11:19:49 -0800 (PST), (dagmargoo...@yahoo.com) > >> > > said: > >> > > >> [snip] > >> > >> > > > >> > > Thanks for the write-up. > >> > > The inductor I am using is a different construction again from the two > >> > > you have mentioned, the windings are on a core which goes parallel to > >> > > the PCB surface, which is then cemented into a ferrite box. The cross > >> > > section looks a bit like: > >> > > > >> > > +----------------+ > >> > > | | > >> > > | +------------+ | > >> > > | | | | > >> > > | | oooooooooo | | > >> > > | +------------+ | > >> > > | | > >> > > | +------------+ | > >> > > | | oooooooooo | | > >> > > | | windings | | > >> > > | | | | > >> > > +-+ +-+ > >> > > ------------------------- > >> > > PCB > >> > > ------------------------- > >> > > > >> > > So the gap, such as it is, is the (presumably) lower-mu material which > >> > > joins the core rod to the inside of the box. I'd expect this > >> > > construction to have a significant downwards fringe field as well, due > >> > > to the assymmetry. > >> > > >> > Man, that's ugly. That's not an inductor, that's a rail gun / field > >> > projector! > >> > > >> > Here's a cross section of the units I used: > >> > .--..-------..--. > >> > |.-'|_ _|'-.| > >> > || oo| |oo || > >> > || oo| |oo || > >> > || oo| |oo || > >> > || oo| |oo || > >> > || oo| |oo || > >> > |'-..-' '-..-'| > >> > '--''-------''--' > >> > > >> > I guess they can't do this... > >> > .--..-------..--. > >> > |.-'|_ _|'-.| > >> > || oo| |oo || > >> > || oo| |oo || > >> > || oo| |oo || > >> > || oo| |oo || > >> > || oo| |oo || > >> > || .-' '-. || > >> > || |_______| || > >> > |'-------------'| > >> > '---------------' > >> > ....because they need some way to get the leads out the bottom. > >> > > >> > > >> > > Another poster has mentioned the Vishay IHLP series, I will check these > >> > > out. I'm hoping a different inductor design will help here, having to > >> > > re-spin this PCB will not make me popular... > >> > > >> > If you posted a part number, the intrepid s.e.d. crowd-sourcing crew > >> > might conjure up a substitute... > >> > >> The part is W�rth Elektronik 7443320330 - 3.3 uH, 17 amps, 4.4 > >> milliohms, in a cubic SMD package roughly 12mm on a side. > >> > >> W�rth are normally my go-to supplier of magnetics, I will have to cast > >> the net a bit wider this time I think. > > > > If you need a tighter spec on leakage inductance, get somebody to wind > > something for you. The people who make ferrites also make formers onto > > which you can wind what you want. > > Is leakage inductance the same thing as the magnetic leakage we are > discussing?No. I was using the wrong term. I really should have said magnetic leakage . The magnetic leakage of interest is that bit of the fringing field that can induce current in a convenient ground plane. Leakage inductance is what you've got left when you short one winding on a transformer - or put on a shorted turn. dV1/dt= L1.dI1/dt + M.dI2/dt dV2/dt= L2.dI2/dt + M.DI1/dt where M is the mutual inductance of L1 and L2. In and ideal transformer M=L1.L2^0.5 and the extent to which it falls short of that is the leakage inductance. It's flux which doesn't thread all of both coils so it really isn't the same idea.> > There aren't as many coil-winding shops around as there used to be - > > in part because somebody in China will always under-cut them - but > > it's not a particularly expensive option. > > Maybe it's possible, but we are talking about rather small, surface > mount parts here.I haven't been playing close attention to what EPCOS and FerroxCube are selling at the moment, but they certainly did have some surface mount formers when I last looked. I don't know whether they would be small enough for the OP's job. A quick look at the element 14 range of 324 cores reminded me that OP could go for a printed winding, in the PCB, and glue the core halves onto either side of the board http://www.farnell.com/datasheets/1719196.pdf At least he'd know exactly where his windings were. <snip> -- Bill Sloman, Sydney
Reply by ●November 11, 20152015-11-11
On Wednesday, November 11, 2015 at 5:27:46 AM UTC-5, RBlack wrote:> On Tue, 10 Nov 2015 09:04:44 -0800 (PST), (dagmargoodboat@yahoo.com) > said: > > On Tuesday, November 10, 2015 at 4:08:48 AM UTC-5, RBlack wrote: > > > On Fri, 6 Nov 2015 11:19:49 -0800 (PST), (dagmargoo...@yahoo.com) > > > said: > > > [snip] > > > > > > > Thanks for the write-up. > > > The inductor I am using is a different construction again from the two > > > you have mentioned, the windings are on a core which goes parallel to > > > the PCB surface, which is then cemented into a ferrite box. The cross > > > section looks a bit like: > > > > > > +----------------+ > > > | | > > > | +------------+ | > > > | | | | > > > | | oooooooooo | | > > > | +------------+ | > > > | | > > > | +------------+ | > > > | | oooooooooo | | > > > | | windings | | > > > | | | | > > > +-+ +-+ > > > ------------------------- > > > PCB > > > ------------------------- > > > > > > So the gap, such as it is, is the (presumably) lower-mu material which > > > joins the core rod to the inside of the box. I'd expect this > > > construction to have a significant downwards fringe field as well, due > > > to the assymmetry. > > > > Man, that's ugly. That's not an inductor, that's a rail gun / field > > projector! > > > > Here's a cross section of the units I used: > > .--..-------..--. > > |.-'|_ _|'-.| > > || oo| |oo || > > || oo| |oo || > > || oo| |oo || > > || oo| |oo || > > || oo| |oo || > > |'-..-' '-..-'| > > '--''-------''--' > > > > I guess they can't do this... > > .--..-------..--. > > |.-'|_ _|'-.| > > || oo| |oo || > > || oo| |oo || > > || oo| |oo || > > || oo| |oo || > > || oo| |oo || > > || .-' '-. || > > || |_______| || > > |'-------------'| > > '---------------' > > ....because they need some way to get the leads out the bottom. > > > > > > > Another poster has mentioned the Vishay IHLP series, I will check these > > > out. I'm hoping a different inductor design will help here, having to > > > re-spin this PCB will not make me popular... > > > > If you posted a part number, the intrepid s.e.d. crowd-sourcing crew might > > conjure up a substitute... > > > > Cheers, > > James Arthur > > > > The part is W�rth Elektronik 7443320330 - 3.3 uH, 17 amps, 4.4 > milliohms, in a cubic SMD package roughly 12mm on a side.http://www.digikey.com/product-detail/en/7443320330/732-2141-2-ND/2175557 Yow! (But two of those on opposite sides of a PCB might make a nice isolation transformer. :-)> W�rth are normally my go-to supplier of magnetics, I will have to cast > the net a bit wider this time I think.Close, but close enough? http://www.digikey.com/product-detail/en/7443551370/732-1127-1-ND/1639188 This sort of thing really bears measurement. Your original part's geometry looks terrifying, but measurement might show it's completely fine. Or not. Cheers, James Arthur
