DIY 12V 30A SMPS  - Page 1
Forum section: electronics / general

author: andyB2022
author_rank: Regular Contributor
timestamp: March 22, 2022, 04:37:55 pm
content: Hi! I'm trying to build a half-bridge SMPS with 12V and 30A for a university project. The problem I have are those HUGE overshoots at NPN turn off cycle. The inductor and transformer were taken from a Chinese smps. I've had times (in previous past measurements) when those spikes were negligible. The method of measurement was: placing 1 turn of wire (loose coupling) around the core of transformer and 1/2 turn on the inductor to be able to see the waveforms on the scope. p.s. Checking straight the primary side with the scope being directly connected (10x probes) will end up with the same waveforms. I hope below images will help you to understand....360W Load60W LoadIndcutor + Half-Bridgewaveform sensingOld waveforms when everything seemd ok

author: andyB2022
author_rank: Regular Contributor
timestamp: March 22, 2022, 05:30:17 pm
content: FIXED...The issue was the sens wire.. (green okay waveform, red not okay). If anyone can explain why this happens I'm welcome to hear it.That's how it should look like:

author: Faringdon
author_rank: Super Contributor
timestamp: March 22, 2022, 05:44:13 pm
content: Hi,The better way to measure current in the inductor and Txfmr is to have a torroidal current sense transformer...and bring a bit of circuit inductor through it, and measure the current like that.They way you try and measure is very poor....and those spikes are likely just from common mode noise getting induced into your flying wires and scope wires...also, you shoudl really also have primary side current sensing in the circuit, so you could use that to view the waveforms.Whilst your there, you could make yourself a cheap coaxial probe and get some nice switching node voltage scopes done.....you know how to make one, just buy a piece of coax, 75r or 50r, and cut it down, make sure one end has a bnc for your scope...then just strip out the other end so you can solder it to the node you want to see, and ground....put the scope on x1.If you want to get away from noise, avoid using scope probe with dangling ground clip......yes, you shoudl always twist any flying test wires.....it stops them bieng like an antenna into which noise can couple easily.So you do Half Bridge.....are you in current mode?....if so, how will you stop split cap imbalance?...i reckon youre better off with a 2 transistor forward at 360W.

author: andyB2022
author_rank: Regular Contributor
timestamp: March 22, 2022, 05:53:07 pm
content: It's not for current measurement. It's to be able to see the waveform in there without direct connectionThanks for the tip with coax.Not sure what you mean by "are you in current mode?" (I'm a beginner in SMPS). The powersuply is designed to offer constant voltage at the output.Quote from: Faringdon on March 22, 2022, 05:44:13 pmI guess you reffer to primary smoothing capacitors which are in series, still I don't know the answer to your question.Quote from: Faringdon on March 22, 2022, 05:44:13 pmYou mean 2 switch forward topology?

author: Faringdon
author_rank: Super Contributor
timestamp: March 22, 2022, 05:58:05 pm
content: Here is an SMPS course which tells about half bridge, and split cap imbalance, etc etc..and current mode.SMPS course_Big foldershttps://drive.google.com/file/d/17ij_pQLkAzzYeL_TRNci86sXHMirKHmD/view?usp=sharingSMPS course_little foldershttps://drive.google.com/file/d/1WvElhDNfHI0EBei4rouSNEdWWTO1oCtJ/view?usp=sharingAlso, i can see some electro caps by the mains connector....i dont think they should be there......the green inductor is the boost PFC choke i believe?Measure the voltage on each split cap when you are on full power...it shoudl be the same...or else you suffer imbalance....current mode can cause that imbalance.

author: andyB2022
author_rank: Regular Contributor
timestamp: March 22, 2022, 06:01:42 pm
content: The yellow and blue cap are EMI X2 class caps to reduce common mode interference, they are in parallel to the mains input. The green inductor is the main output inductor right after main transformer secondary. The smps has no PFC unfortunately (no space on a 2 layer board and way to complex to design for me).