Unfortunately, at a quick glance, it doesn't look like any of the three options you chose are really intended for SMPS applications but rather EMI suppression. Depending on additional characteristics which are not in the quick descriptions I saw, they may also be acceptable for SMPS. To answer your other questions and (I hope) point you in the right direction: volume resistivity is the normal resistivity figure, usually represented by the Greek letter rho which looks like a Roman letter "p". It can be a strong function of frequency for these kinds of materials. It is important for SMPS design because it factors in to the eddy current losses which happen because your changing magnetic field within the core will tend to induce current in the core material which ends up generating heat. This is usually more of a problem for SMPS transformers and flyback designs than for simple traditional inductor designs like buck and boost. This is because the magnetic field is usually changing more in a transformer core than an inductor which is primarily carrying DC current with some small ripple current on top of it. Each material has a characteristic B-H curve, where B is the "flux density" and H is the "magnetic field", although I do not like those terms because really B is the true magnetic field and H is what the magnetic field of the wire coil would be in the absence of the core material. The slope of the B-H curve at low to moderate field strength is usually fairly constant and that is the permeability. There will be some B field value where the core saturates and will no longer add to the magnetic field produced by the current in the wire. This can be a sharp transition or a gradual one. In either case, the curve has become nonlinear and permeability is decreasing for increasing current. The level of H field (magnetic field from the current in the coil) where this saturation B value is reached will be higher when permeability is lower, which means that for a fixed number of turns and core geometry, and the same maximum B field value, lower permeability will allow higher current. However, the core geometry makes a big difference here, especially when a gap in the material is created so that there is no closed magnetic field loop which is contained entirely inside the material. In this case, the slope of the B-H curve becomes mostly dependent on just the dimensions of the gap and almost independent of the permeability of the material itself. In an inductor or a flyback coil, gaps are commonly used to control the saturation point so that you can stay below it despite the DC current flowing through them. Transformers are designed to have zero DC current in both primary and secondary and if the windings are done well so that the leakage inductance (portion of the magnetic field which is not shared by both primary and secondary) is minimized, then the secondary current provides a field which nearly cancels the primary field. The permeability of the material, then, merely serves to aid in the coupling between the two coils and the permeability can be rather high without saturation being a problem. Sean On Tue, Jul 30, 2013 at 1:51 PM, veegee wrote: > The local electronics surplus store sells a few standard ferrite toroid > cores made of either #43, #61, or #75 material. I looked at the > following randomly-googled chart for remarks about each of those > materials: 193.6.189.120/letoltes/HG0MAT/toroid_datasheet.pdf > > I want to buy a few toroid cores for "general purpose" use. Forgive my > ignorance; I understand that the correct material must be chosen for a > given purpose, but I'm still learning the math. Most of my use will be > for power applications and I would like to make my designs as efficient > as possible. Example uses: hand wound toroid inductor for various high > current SMPS from 30kHz to 3MHz, small power inverter at 60Hz, > experimenting with multiple windings, etc. Looking to wind them in an > inductance range of 1-100uH or possibly up to 500uH. > > A few questions: > > 1. What core material would be good for my purpose(s)? I will probably > get a couple of each. I understand this may be a stupid question, but if > I had to pick only one or two materials, what would be a good core > material for my "general purpose" use above? > > 2. What is volume resistivity and what role does it play for things like > a toroid inductor in a ~600kHz boost converter? > > 3. I have read somewhere (quite possibly an unreliable source) that > lower permeability somehow correlates to higher current capability. Why > would someone state that? > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .