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
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.
-- John Devereux
Interesting, thanks.
-- John Devereux
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Interesting. I have a very similar problem at the moment, a board with two synchronised 90W boost converters, using the type of inductors you
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...
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.
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!)
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
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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.)
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 plan e 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
Cool. That might matter if you need a super-efficient switcher.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Since the Q fell by roughly the same factor as the inductance, I take that as indicating minimal added dissipation. Q = xL/R, etc.
That's encouraging, actually.
Cheers, James Arthur
[...]
I guess the very low impedance is what makes the cap ineffective.
I suppose I need to buy a few types.
Thank you very much for the detailed write-up James.
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,
-- John Devereux
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) but also ground islands to keep the loop currents local to the SMPS. 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.
[...]
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.
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.
Thank you (and that matches the datasheet very well).
-- John Devereux
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.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--
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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
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!
Sure. The idea is to keep the loop area small but the island also keeps any voltage induced in the ground to a minimum.
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.
I had to go out to lunch, on our TI FAE, many times to learn why things are done the way they are. ;-)
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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...
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.
If you posted a part number, the intrepid s.e.d. crowd-sourcing crew might conjure up a substitute...
Cheers, James Arthur
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milliohms, in a cubic SMD package roughly 12mm on a side.
the net a bit wider this time I think.
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If you need a tighter spec on leakage inductance, get somebody to wind some thing for you. The people who make ferrites also make formers onto which yo u can wind what you want.
There aren't as many coil-winding shops around as there used to be - in par t because somebody in China will always under-cut them - but it's not a par ticularly expensive option.
EPCOS has particularly nice manuals and application notes.
-- Bill Sloman, Sydney
Is leakage inductance the same thing as the magnetic leakage we are discussing?
Maybe it's possible, but we are talking about rather small, surface mount parts here.
-- John Devereux
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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 c an 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 sam e idea.
I haven't been playing close attention to what EPCOS and FerroxCube are sel ling 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
At least he'd know exactly where his windings were.
-- Bill Sloman, Sydney
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Yow! (But two of those on opposite sides of a PCB might make a nice isolati on transformer. :-)
Close, but close enough?
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
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