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The problem:
I need to learn to build the simplest switching mode power supply. If I succeed, I hope I'll be able to work my way up and make more advanced SMPSs for other applications.

I did my research:
I've been searching everywhere for a tutorial on how to build a simple switching mode power supply, however, the tutorials and solutions I found look super complicated for me. So far I have looked at a few articles and have used the PI Expert and the Power Stage Designer Tool (from Texas Instruments) to get familiar with designing the circuit. I know how an SMPS works, but I'm afraid I need to learn how to set up the "switching" part (i.e. how to set up the IC that is in charge of controlling the MOSFETs).

My Stash:
Includes 5-PC-power-supplies worth of scavenged components, and I was wondering if I can actually build a smaller PCB that only outputs 12V 2A.

The applications:
The simplest one is that I need to supply constant 12V to two fans, mounted on the heatsinks of two 50W LEDs, which are powered by a 100W LED driver. Linear regulation isn't a good idea for obvious reasons.

I would appreciate any help.

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    \$\begingroup\$ Is this honestly easier than buying a ready-made off-the-shelf supply from eBay for $4? \$\endgroup\$ – Bryan Boettcher Jan 26 '18 at 23:25
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    \$\begingroup\$ And... what is your source? AC wall outlet? 24 VDC? - I'm fairly certain that it's much easier for you to just take apart some old computer and use its PSU, because there you have 12 V able to supply 2 A. \$\endgroup\$ – Harry Svensson Jan 26 '18 at 23:34
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    \$\begingroup\$ I'd like to recommend Abraham Pressman's book, "Switching and Linear Power Supply, Power Converter Design." It is one of the earlier books on the subject and because of this the book approaches the reader as more of an interested beginner. Modern books, I think, now tend to assume a more rigorous background as part of a curriculum. This makes them less accessible by a hobbyist (to my view, anyway.) This particular book was my own introduction and I found I was able to follow most of it when I first read it about 30 years ago and knew a lot less. See if you can find a cheap copy somewhere. \$\endgroup\$ – jonk Jan 27 '18 at 5:14
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    \$\begingroup\$ @jonk Pressman's latest book is "Switching Power Supply Design". There's also a nice book named as "Switchmode Power Supply Handbook" from Keith Billings. They are all good for both beginners and advanced designers. Highly recommended. \$\endgroup\$ – Rohat Kılıç Jan 27 '18 at 6:53
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    \$\begingroup\$ Ali can you get web.archive.org in Iran? All the books are there. Billings makes good comments and adds to Pressman's work on SMPS Design. Keith designed for us once at Hammond and was one of the best. But the best way to learn is from a service manual, scope and working unit rather than trial and failure. Even for a TV. But to drive 100W LED's use a working PC PSU. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 27 '18 at 16:44
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First things first - a SMPS is not a simple circuit, and they just get more and more sophisticated. So one option you have is to start with a toy implementation with an inductor, a BJT, a capacitor and a microcontroller to drive the BJT. You wont get 2A out of it on your first go, and you'll probably fry a few BJTs, but you'll learn a lot.

But if you're keen on building something useful, then you need to pick an IC. It's hard to find much simpler than TI's Simple Switcher series. The LMZ14202H would probably meet your needs.

Both the inductor and the switcher are integrated, so you only have to concentrate on the feedback circuit. Follow their application notes very carefully and you should have something running fairly quickly. The LMZ14202H only comes in an SMD package though, so you'll probably want to get a 0.05" breakout board too for prototyping.

Trying to re-use components from existing power supplies is unlikely to be a fruitful path - those components are likely to be quite specialised, chosen to suit a specific design, and reverse-engineering a commercial power supply design is harder than building one from scratch.

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If you want to learn about switching power supplies, don't make one that works off line voltage. The simplest topology to learn the essentials from would be a buck converter. Once you have a good idea of the calculations and pitfalls with that try making an isolated switching converter of some sort (flyback, forward, push-pull). Then you can think about the difficulties of high voltage.

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