OK, so here's the story so far... Byron believes (with tongue firmly planted in cheek) that switching magnetics are a black hole for all knowledge and the source of all that is evil in electronics design. Russell has countered: "Do NOT be afraid of an inductor based circuit. With off the shelf and off the net circuits and advice from here you should have no excessibely major [tm] problems with the converter." So I'm here to put this hypothesis to the test. For my electric riding mower project I see two switching projects that would really be helpful. Both are based on the 48V 47Ahr Chevy Volt module that I bought to run the mower. The first is pretty simple. I'm also aware that it's available as a black box cheap enough that it isn't really worth the effort. Call it a test case. The main electrical switch is a Tyco 500A contactor: http://www.mouser.com/ds/lrg/418/NG_CS_EV200_R_TBD_KILOVAC_EV200_Ser_Contac= tors_080-256777.jpg My model has the 9-36VDC coil. Because there are numerous USB car chargers available a buck regulator down to 12 VDC could easily serve the threefold purpose of driving the contactor, powering USB, and providing power for auxillary electronics. Since the contactor is the primary user the specs for the power supply of 3.8A inrush peak current and 130 mA of holding current give the ranges of current usage of the regulator. Now remember this is COTS design. And for me specifically, that means designing the project around the inductor and the current requirements and not start with a fixed switching frequency and designing to a specific inductor. So the candidate is here (because I already have them in my parts box): http://www.bgmicro.com/512uh-coil.aspx This part was choosen for a lot of reasons. The large inductance generally means a lower switching frequency. Also larger inductances facilitate continuous current operation at low current draw. Also both the core material (#26) and the size (T-184) facilitate higher current designs.The fact that they are available and relatively cheap doesn't hurt either. So one excellent example of the black hole of information is this TI Buck Regulator Formula sheet: http://www.ti.com/lit/an/slva477b/slva477b.pdf Note like most others it uses the design parameters of inductor ripple current and switching frequency to do inductor selection. However, since the inductor is fixed. We need to rearrange the basic formula: I(ripple) =3D (Vin(max) - Vout) * (Vout/Vin) / (f * L)=20 by swapping I(ripple) and f. Typically I(ripple) is computed to be between 20 and 40 percent of maximum current. Just for some overhead let's put the max current a 5A and I(ripple) at 30% of that: 1.5A. So given Vin (Max) of 50V, Vout of 12V, L of 0.000512H, and the given I(ripple): f =3D (50V - 12V) * (12V/50V) / (1.5A * 0.000512) f =3D 11.85 Khz That's well within the spec of the inductor, so bumping it up to an even 12 Khz or so should be fine. But wait! In order for continuous current mode, the current draw must be at minimum half of I(ripple). So that means a continuous draw of 750 mA is required. But there's no guarantee of that from the contactor. These are the types of problems that drive me nuts in trying to design such projects. And let not even get started with trying to identify a flyback transformer that'll take a 170 VDC bus voltage and generate an 8A charger for the battery. So any insight on how to design workable switching projects without resorting to black magic sure would be welcome. BAJ --=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 --=20 Byron A. Jeff Associate Professor: Department of Computer Science and Information Technol= ogy College of Information and Mathematical Sciences Clayton State University http://faculty.clayton.edu/bjeff --=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 .