SMPS Revision  - Page 1
Forum section: electronics / projects

author: Els_Chaos
author_rank: Regular Contributor
timestamp: April 11, 2025, 03:25:09 am
content: Hi everyone, this is my very first smps design using to power an OTL amplifier for my assignments project. The professor asked me that the input voltage must be lowest possible and the output power is highest as possible. So that i come out with this SMPS to fulfill both requirements of him. But honesty i have no experience in before. So could you revise it for me and suggest some change to make it more stable?My intention is using 3525 as it is cheap and convenient, and a low side mosfet driver to make the circuit works in half-birdge mode. My frequency is around 87khz.The schematic is attached below.

author: bostonman
author_rank: Super Contributor
timestamp: April 11, 2025, 03:39:33 am
content: Unless I'm not seeing something, this circuit doesn't make sense.You have MOSFETs being driven by outputs, I don't see where the 35v comes from, and what is the purpose of the bottom right circuit where U4 is?

author: Els_Chaos
author_rank: Regular Contributor
timestamp: April 11, 2025, 03:47:43 am
content: TL431 and PC817 is used for output voltage feedback. The +35V rail is boosted from 12V battery through transformer sir

author: bostonman
author_rank: Super Contributor
timestamp: April 11, 2025, 02:23:46 pm
content: Maybe I'm wrong, but this circuit looks confusing.Did you design this or use AI?

author: xavier60
author_rank: Super Contributor
timestamp: April 11, 2025, 03:05:40 pm
content: Most of all, it needs an output coupled inductor and bridge rectifier to regulate properly.https://www.diyaudio.com/community/threads/smps-output-coupled-inductors.290711/

author: temperance
author_rank: Super Contributor
timestamp: April 11, 2025, 03:25:12 pm
content: Quote from: xavier60 on April 11, 2025, 03:05:40 pmThis is push pull converter and not some half or full bridge. The rectifiers are fine as they are.I also don't see outputs driven by inputs. Only the labels are of the wrong type.

author: Els_Chaos
author_rank: Regular Contributor
timestamp: April 11, 2025, 04:47:01 pm
content: I just made some updates to make it more clear.

author: Els_Chaos
author_rank: Regular Contributor
timestamp: April 11, 2025, 04:59:12 pm
content: actually no, i just based on its datasheet, using provided formular to calculate the frequency. and i know that if mosfets run at high frequency range it should have a driver so i choose 1.2A driver. the feedback i have no idea to deal with it. i just using the internal opamp of sg3525 as comparator mode. With 2.5V fixed at the non-inverting and an optocoupler work as a switch, as the output voltage higher than 35V the TL431 will conduct, there is a current run through PC817, trigger its transistor to conduct and then pull the inverting pin up to Vref = 5V. So that the PWM will be regulated.

author: temperance
author_rank: Super Contributor
timestamp: April 11, 2025, 05:34:41 pm
content: If this is for an audio application, you are better of using a soft switching half bridge converter running in open loop. The efficiency will be much higher, less components (the push converter as shown will explode within a few seconds and requires more components to solve those issues) and less common mode noise.Edit: while it seems modern to draw boxes around each section and give those section a name you should actually avoid net labels whenever possible. Your schematic looks like a bunch of net labels and because of all the disconnections the schematic is difficult to read. Avoid net labels and connect things as they should be connected. This makes it easier to spot mistakes.

author: Els_Chaos
author_rank: Regular Contributor
timestamp: April 11, 2025, 05:44:06 pm
content: Soft switch open loop? What is that sir, i have never heard about that. Does it seem like ZVS.I'm so sorry that i am really bad at networking, wiring management so that i use labels 😥

author: temperance
author_rank: Super Contributor
timestamp: April 11, 2025, 09:59:26 pm
content: Indeed, ZVS switching. Here is a reference:https://www.ti.com/lit/ml/slup089/slup089.pdfIt starts with a resonant buck regulator. The for you relevant converter is Fig. 16. Lr in the schematic can be part of the transformer.Those can be regulated as well if you need that.

author: jonpaul
author_rank: Super Contributor
timestamp: April 11, 2025, 10:58:08 pm
content: read any text on SMPS topology , design and basics before you try this project.J

author: wraper
author_rank: Unknown
timestamp: April 11, 2025, 11:12:28 pm
content: R10 does nothing other than create power loss for nothing as there is no current feedback in this circuit.

author: Els_Chaos
author_rank: Regular Contributor
timestamp: April 12, 2025, 06:00:12 am
content: actually i have intention to use a differential amplifier to create a shutdown signal for sg3525 to protect the circuit against over current crisis

author: Els_Chaos
author_rank: Regular Contributor
timestamp: April 12, 2025, 06:09:06 am
content: thank you very much, i will try my best to learn the new things. There absolutely may be some silly mistake that i can make.