Assistance designing a switch mode power supply - Page 1
Forum section: electronics / projects

author: Syllith
author_rank: Regular Contributor
timestamp: November 14, 2019, 03:01:25 pm
content: So, for learning purposes, I'm trying to design and build a SMPS. I have a basic schematic drawn up but I'm concerned about my switching control circuit. Once I get passed the MOSFET and start feeding the transformer and feedback circuit, I'm not sure if what I'm doing is right. I know its very crude, but can you please let me know if I'm doing anything wrong before I try to wire this up? Also, with the FBR at the end, I'm not sure if I can just use a single diode or something. I'm kind of confused after that part. Any more info I can get would be appreciated.Oh, and just so you guys understand my thought process here, I've included 2 additional images I've been trying to model. One of them is simply a voltage regulator that includes the feedback circuit. I also have a basic block diagram that I'm modeling my power supply off. I just tried combining the 2 but I feel I'm missing something.

author: Dave
author_rank: Super Contributor
timestamp: November 15, 2019, 04:56:21 pm
content: Please do not attempt to actually build this circuit.Not only will the circuit shown in the schematic not work, it's also going to be very dangerous to handle, as rectified mains voltages are in play. One wrong move and you might be seriously injured or dead.Try to start with something simpler first. A buck converter would be a good starting point, IMO. Power it through a step-down transformer of maybe 12 volts. That way you can safely poke at it without risking your life in the process.Study up on switch-mode power supplies. Only when you know exactly what you're doing and have sufficient experience debugging switchers, should you attempt to build an offline flyback converter.

author: Syllith
author_rank: Regular Contributor
timestamp: November 15, 2019, 10:17:57 pm
content: I'm not going to attempt to build it because I know it wont work. I dont have any of my values calculated yet. I'm mostly focused on the logic of it. I appreciate your concern on safety but I'm very experienced with dealing with very high voltages. I work in a laboratory repairing att cell phone tower equipment. My job is to repair the rectifiers and converters and tower mount microwave radios. If I were to build a circuit similar to this, I'd be in the perfect environment for testing it. Things blow up here all the time and the lab and equipment is designed to be as safe as possible. But anyway, I do still need help making this a functional power supply. Do you have any suggestions on how to make this functional? It will be crude to start with but once I get the concept down I will be making a much better design.

author: station240
author_rank: Unknown
timestamp: November 17, 2019, 09:18:11 am
content: A few obvious flaws that would prevent this working.1. You need to use an actual PWM controller, they are more than just one opamp.2. With only a single switching transistor you are limited to Flyback mode, which is simpler but has design requirements.3. Replace the bridge rectifier with a single diode oriented so that when the primary side transistor is on, the diode isn't conducting.4. The voltage feedback isn't going to work as shown, needs to be on the secondary.I have found some PSUs with a fixed duty cycle, hence no actual feedback circuit, those are simpler still.