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Like a parrot, I'll repeat ad infinitum: Read a lot, research the subject, that's the best start, then you (or I) will understand some things, but not others, but be able to formulate more specific and therefore practical forum questions.
You seem to know Vout and Iout. You may also have decided whether your SMPS will be mains- or battery-powered. These two points, "A" and "Z" will be a good guide as to what to study to have an idea of what "B" to "Y" may need to be.
I suggest a way to begin designing the power supply is comtemplating whether the circuit being powered by the SMPS is analog or digital, will it function erratically with switching noise, etc? Will it therefore need for the SMPS to be followed by an LDO and/or appropriate filtering?
Another staring point is asking oneself: Do I trust my ability and knowledge to safely implement a non-isolated PS, or should I leave that kind of project for a future occasion when I feel certain I will not a) hurt myself and others, and less importantly b) keep destroying components with ill-designed circuits?
Looking at different SMPS topologies - and only you can do that, laziness or expecting others to fill in your learning gaps in a quickie answer isn't learning, my friend - will give insight into what you, at first, think will be the best-fit solution, never hurts to understand which are harder to provide compensation for if you're a beginner.
To summarise, my limited knowledge suggests: check topologies, choose one, do the calculations, simulate, if all goes well, prototype or make the finished PS, otherwise, back to the top of the design stage flow diagram
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Hi again,
Another first step question is: What's "better" - designing the whole thing myself, or getting some fantastic $0.70 multi-purpose SMPS IC that solves maybe 50 - 80% of the design procedure?
Along with: Have I thought the battery thing through yet...Maybe a 1.5V AA battery won't power this design, and a car battery won't make for a portable device, is there something inbetween that will be adequate, or must I accept that I need to plug this thing into a wall, or adjust my current requirements?
IC manufacturers have a host of design tools and reference designs that should be useful. Here are links to SMPS/PSU design tools, if you're interested in looking at them, and there are many others...
PowerEsim "Free SMPS Switching Power Supply / Transformer Design Software"
Design Simulation and Device Models "a variety of custom design simulation tools and device models to allow even novice designers to quickly and easily evaluate circuits using high performance switching regulators, amplifiers, data converters, filters and more"
I'm not pretending any of these tools make anything easier, some may be hard to figure out and use effectively, but there are a whole range of free tools which theoretically simplify the design stage, and usually lead to the appropriate choice of IC, saving hours of cross-comparison work.
there are many SMPS IC manufacturers.
They all want to sell their parts and thus they give you all information you need to choose their parts.
They have interactice selection tools to find the optimal IC. Every IC has it´s datasheet with circuit descriptions.
Often there are additional informations like application notes and so on.
Back in the 1980s, designing a SMPS was a major endeavor.
Nowadays like d123 mentions, all of the major semiconductor manufacturers have free, very sophisticated design software. They have full fledged online selection tools. They have full characterization of all the ancillary components, some of which, like the inductors, hold a lot design secrets. They sell evaluation boards, to test and review performance on a proven board. They give you the Gerber files for you to copy those boards. They have lots and lots of app notes.
With as little knowledge as you have, I'd recommend trying
to source a module rather than (re-)invent a supply at the
blank-board level. There are many vendors in this space and
you'd find them easily enough by browsing the back pages
of any electronics design periodical.
Right, 12V and 5V outputs are incredibly common and competitive. It's unlikely you're going to benefit by designing this yourself versus finding and buying something.
For 220AC to 12V/5V dc at about 100 watts you're talking about something like a 2"x4" open frame which, again, is very common and easy to come by.
It would be not you who are designing the SMPS but the tool, you are just entering specifications as input, anyway it is allways required to have a technical backgrond in order to be able to examine the solution presented, as well as to be able make minor adjusts when necessary. Based on the history of your issues throughout this forum, you seem to have a very limited level of knowledge in several areas of electronics, so you should firstly to deeply LEARN tutorials, wiki, etc..., instead of repeatedly ask for untied questions into several topics.
Here is the datasheet for the LM2675, page 14 shows the list of suitable inductors and their part numbers, the link in post #13 is a link to the list of suitable parts made by the manufacturer recommended in the datasheet.
It is only AN EXAMPLE of what inductor manufacturers offer as turnkey solutions for popular switchmode ICs.
I know that it is almost always far easier to purchase ready made units from EBAY.....but the OP wants to learn SMPS circuits. The best way in my opinion are low power devices which have been charectarized.
You may actually spend 3X of what would have costed you to purchase a ready made module on EBAY, but the learning experience is -to use the credit card commercial- priceless.
There is a list of diode part numbers on page 16 of the datasheet. 9.2.1.2.3 Catch Diode Selection (D1) on page 15 explains how to choose the right one for your circuit.
Two points mentioned are:
"For normal operation, the catch diode current rating must be at least 1.3 times greater than its
maximum average current. However, if the power supply design must withstand a continuous output short, the diode must have a current rating equal to the maximum current limit of the LM2675."
and
"The reverse voltage rating of the diode must be at least 1.25 times the maximum input voltage."
So, depending on Vin, and your output current/load, you can choose a Schottky diode from that list, it won't hurt to choose one of the 3A ones, so you just need to know Vin and make sure its Vr is rated at least 1.25 times higher.
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