How do they make inductors so small and cheap? - Page 1
Forum section: electronics / projects

author: hkBattousai
author_rank: Regular Contributor
timestamp: April 04, 2015, 10:21:20 am
content: I have to design a 4.7uH inductor to be used with the buck convertor contoller ICZSPM4012AA1W33, which will switch up to 1A current at 1MHz switching frequency. I can't make a decision whether should I design it myself or buy a finished product.If I choose to wind it myself, the cheapest ferrite core I can find is the toroidalB64290L0044X087.AL = 1.12 µHur = 2200l = 30.09 mmSo if make N=2, the inductance will beL=4.48uHwhich is in the acceptable range.The core flux density is thenB = [u . I . N] / l = [(2200 . 4 . pi . 10^(-7) H/m) . (1 A) . (2 turns)] / (0.03009 m) = 183.755503 mT,which means that the core will not saturate at 1A load current.The price of the core is $0.32 and the space it consumes is 14mm x 14mm x 6mm = 1176 mm^3.If I decide to buy a finished product, I preferNR6028T4R7M, which saturetes at 2.7A.It is only $0.1242 and it consumes only 6mm x 6mm x 2.8mm = 100.8 mm^3.When I compare them:| Volume | Saturation | Price------------------------------------------Homemade | 1176mm^3 | 1A | $0.32Finished | 101mm^3 | 2.8A | $0.1242Q1) How do they make inductors so small, with high saturation current, and so cheap?Q2) When do I make my own inductor and when do I buy a finished product?

author: Wolfram
author_rank: Frequent Contributor
timestamp: April 04, 2015, 11:33:19 am
content: Because your first case is far from an optimal design. Ungapped ferrite toroids are not ideal for power inductors. What you have achieved in that first design is an inductor with extremely low DC resistance (if you dimension the wire to fill the winding window), with every other parameter compromised. The NR6028T4R7M is a balanced tradeoff between volume, winding resistance, saturation current and many other parameters, probably optimized for the application.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 04, 2015, 11:39:36 am
content: Only make your own if there is nothing suitable off-the-shelf. Coil manufacturers are much better at it than you are.1A at 1MHz is mainstream, there is no reason to use anything other than a ready-made part.Unless you're really tight on space, inductors for swithing regs are pretty straightforward.What I normally do is buy a range of parts that fit the space and are in the right ballpark, and test to see which gives the best efficiency.There are some other factors - some are more fragile than others, some may squeal, some can have issues with pick/place vision (rough solder on pads),

author: nctnico
author_rank: Super Contributor
timestamp: April 04, 2015, 11:45:58 am
content: One of the things I run into is that datasheets don't provide information on the core material. If the ripple current is high (=large core losses) then it takes testing various brands and types to find the inducator with the least core losses.

author: hkBattousai
author_rank: Regular Contributor
timestamp: April 04, 2015, 11:54:00 am
content: Quote from: Wolfram on April 04, 2015, 11:33:19 amPlease correct me if I'm wrong:We leave gap in the core in either of the following cases.1) When the peak current is enough to saturate the core even if there is only one turns.2) We need a little turns but non of them precisely reach the inductance value we demand. So we leave some gap and try for a different number of turns.3) Because of what I explained below.L = B.A.N / IB: flux densityA: effective core cross sectional areaN: turnsI: currentWe can lower B, if we increase N without changing L, A and I. We need to leave gap for this.Does your statement depend on the item (3)?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 04, 2015, 05:50:01 pm
content: Such an inductor would also have high core loss (relatively high Bpk) and very poor stability (mu varies with amplitude and bias, probably by +/-20%, below saturation).The fundamental insight about inductors is, air gap stores energy. Core conducts and concentrates the magnetic field around the air gap.The energy density of a magnetic field is given by: e = B^2 / (2 mu_r mu_0)So you can see, when mu_r is smaller, the energy density is higher. The energy is almost entirely stored in the air gap, because it's the portion of the magnetic circuit with the lowest mu.e has units of energy density (J/m^3, say), which are also units of pressure (Pa, say). Indeed, the two are always physically equivalent, and the force exerted on a magnet is equal to the difference in magnetic pressures on its ends. This pressure (generalized as a tensor, I believe?) is called the Maxwell stress.When mu_r is very small, efficiency goes back down. The reason for this is, we do not have any better conductors than copper and silver to make inductors with (except for the rare times when superconductors are available; but, it's noteworthy they are not ideal for alternating currents, either!). The need for a core (to concentrate the field into a smaller region of air gap) arises from the resistivity of the winding wire.So, the best results are typically had when the core's average permeability (i.e., including the air gap) is in the range of 10-100.Traditional "bobbin" style cores are easily the most popular, having an optimal to wide air gap. Enclosed and "low profile" styles have an optimal to small air gap. Molded types are, probably increasingly popular, using some proprietary formulation of resin-bonded powdered metal; they typically perform very well (far better than conventional powdered iron cores), and achieve small package sizes because the molded powder fills the gaps between wires.Tim