Reading material on feedback/control theory/error amplifiers for SMPS - Page 1
Forum section: electronics / projects

author: Phaedrus
author_rank: Frequent Contributor
timestamp: February 03, 2015, 03:00:51 am
content: I'm trying to learn more about SMPS by designing a PSU in Spice from the ground up with all discrete control circuitry (op-amps and comparators). I'm running into issues though designing the error amplifier, and I'm having trouble finding literature discussing it in any detail. Are there any good app notes on designing a PSU this way? I could also use advice on a good rail-to-rail single supply op amp to use; I've been using an LT1792 because it's pretty good all around, but I'm sure there's something better suited.Design parameters are a 12V -> 5V 3A buck converter, with <5% overshoot and <3% load regulation. Switching frequency 100-150KHz. I want to start with the bare minimum circuitry then add features like OVP/UVP, soft start, synchronous rectification, etc.

author: moffy
author_rank: Super Contributor
timestamp: February 03, 2015, 03:10:22 am
content: The error amp in a SMPS chip can be very different from an ordinary opamp. If designing a SMPS use the special simulation tools: LTPowerCAD II and TI has one too, for their products. The models are much better.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: February 03, 2015, 06:43:26 am
content: LTPowerCAD and Webench are useful, but I'm trying to understand and apply the underlying theory of switching regulators. I know it's not done now, but they used to control these things with discrete analog components, before TI and Linear controllers got really good. Building a PSU that way seems like a good learning experience. It's just a type of black magic I'd like to understand. And I'm not sure where to find the information I need, especially on the practical circuit elements.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: February 03, 2015, 07:06:23 am
content: I like Microchip's AN1114.http://ww1.microchip.com/downloads/en/AppNotes/01114A.pdf

author: rx8pilot
author_rank: Super Contributor
timestamp: February 03, 2015, 07:38:36 am
content: You may get some good info from:"Switching Power Supply Design" Abraham Pressman, Keith Billings, Talyor Morey"Switchmode Power Supply Handbook" Keith Billings, Taylor MoreyBoth are available in PDF with a quick Google search. They have helped me push a bit further in SMPS design. FWIW, I found the circuit design to be rather straightforward compared to the PCB layout. You will be punished for bad layouts and the problems may be disguised as a schematic problem. Add in extra pads for a snubber, gate resistors, etc... anything you may need. SMPS is not really a project that can be prototype without a real PCB layout. I have only been doing 4+ layer PCBs so I always have a solid, unbroken GND layer to protect the delicate signals. The SMPS learning curve has been quite a challenge for me although I am after very high performance. Not sure what your expectations are.Linear AN136 is a good app note on layout considerations.

author: rbola35618
author_rank: Frequent Contributor
timestamp: February 04, 2015, 12:59:14 am
content: Hi Phaedrus,This is the method that my mentor Jack Alexander taught me how to compensate the loop. You can use Jack's average model to simulate you supply. The same technique can be used to compensate forward and other typologyCurrent mode flyback, part 1Current mode flyback, part 2Volatege mode flyback, part 3

author: rx8pilot
author_rank: Super Contributor
timestamp: February 04, 2015, 03:30:19 am
content: I watched all those videos a few days ago, very helpful. Thank you for the time you put in.The work you did with Cleverscope to get an affordable FRA looks very promising for those of us scared to spend $15k ++ on loop analysis.

author: rbola35618
author_rank: Frequent Contributor
timestamp: February 04, 2015, 02:54:30 pm
content: Hi rx8pilot,Thanks for the comments. Bart, the designer of the Cleverscope, has made further improvements on the FRA. The automatic mode is now working much better. In addition, he has added LCR measurement features to the Cleverscope. I will be making an updated review of the Cleverscope in the near future.Robert