Modifying 500W ATX psu to output +/- 45v  - Page 1
Forum section: electronics / projects

author: iWei
author_rank: Contributor
timestamp: November 22, 2021, 05:17:05 pm
content: I have modified a regular 500w ATX psu to have 2 45v outputs that can be either connected in series for 90v or parallelled for higher currents.Seeing as ADI have a couple of 48v forward converter designs and danyk.cz has a too simple 350w +/-35v design, I thought that this would be feasible.Of course in dealing with mains i have taken the basic safety precautions eg isolating the smps and grounding the scope, keeping thick isolation/gaps between primary and secondary turns, and keeping the outputs grounded to PE.I did expend much effort in rewinding the main transformer and replacing/modifying just about every component in the secondary power section.At the moment, the power supply works (i.e outputs 90V into a 30R load for 5 mins), except for1. terrible multi-Mhz spikes in the scale of volts!! on the output (not much kHz scale ripple though),2. the PWM (oscillating?) above certain duty cycles.So Im looking for some advice/directions to head in solving these1. Im certain that is partially because i dropped the transformer and now it cracked and has an airgap of maybe 0.1mm and i merely taped the cores together instead of gluing(I only have viscous 2 part epoxy that creates a much bigger air gap). The transformer emits buzzing sound in pulse skip mode and while oscillating. But will a tiny airgap lead to so much leakage inductance to result in crazy spikes? I have put 4x 100nF NP0 caps across the 4 output capacitors and nothing changed.Also hand wound transformers and all but i pretty much copied diodegonewilds way of rewinding his transformer, could this result in more leakage inductances? Or rather i didnt scope the output correctly?(i scoped at the screw terminals)2. I dont even know where to start with the crazy maths hidden behind a few innocent looking RC compensation components so I left the feedback compensation components as is except for changing resistor divider ratios and slapping a 18v zener to prevent overvolting the TL431.Given my capabilities im hoping to just fix the voltage spikes unless I can upgrade from copy pasting recommended compensation components in datasheets to wizarding my own.Photos/videos/writeups here due to size limitations:https://drive.google.com/drive/folders/1r4lavv5KmiYpO9YYpDlyQv_8QHw-mEbR?usp=sharingThank you for reading!

author: jonpaul
author_rank: Super Contributor
timestamp: November 23, 2021, 08:39:45 am
content: iWei, bravo to try this but you need to learn a lot to modify a production 500 W SMPS.>>>>1. terrible multi-Mhz spikes in the scale of volts!!Not trivial to measure noise this way,We use a 1 x coax to BNC cable and NOT a scope probe or the output wires to measure this, place the ends of the coax across the output filter caps. Beware of the 90V on the cable, AC copple the scope at 1 Mohm (NOT 50 Ohm) input.2. the PWM (oscillating?) above certain duty cycles.Loop compensation is depending on the Vout, you have changed so much the loop is now marginally sable or unstable. Change loop comp.>>>> dropped the transformer and now it cracked .....airgap lead to so much leakage inductanceDropped ferrite is useless, need to replace the ferrite core.>>>>copied diodegonewilds (WHO IS THIS?) way of rewinding his transformer, .... inductances? .... i didnt scope the output correctly?(i scoped at the screw terminals)Yes all three are not good!Bon chance,Jon

author: iWei
author_rank: Contributor
timestamp: November 23, 2021, 03:09:15 pm
content: Quote from: jonpaul on November 23, 2021, 08:39:45 amI have tried scoping right where the cables leave the board, using a DIY pigtail stype probe, and things are much better (~600mVpp of hf noise and 150mVpp of ripple)The oscillations have also disappeared, the only thing different is that Im powering it from a ~190Vac (loaded) output from the isolation transformer (the 2 primary coils from 2 MOTs I slapped together have a different turns ratio). Thing is though, there is active PFC in front meaning bus cap voltage should remain the same...Is that helpful in pointing me in the direction eg to increase/decrease RC values?Maybe i should directly drill holes in the side panel and cut out the long wires that are picking up noise inside the case.Btw here is Diodegonewilds series on reparing a very dodgy 600w forward converter:

author: iWei
author_rank: Contributor
timestamp: November 24, 2021, 02:56:41 pm
content: I have tested the power supply using AC straight from the wall vs with an isolation transformer in between wall and power supply, and it makes a significant impact on the stability,With SMPS connected to mains with earth, primary side earthed via a couple of Y caps, and secondary side grounded via the scope, noise measured is slightly higher and the PWM is unstable. It also takes a long time for the scope trace to stabilize(i.e whenever a large DC voltage is added to an ac coupled input, it takes time for the rms voltage to rise/drop to 0v. It takes multiple seconds)With an isolation transformer, the primary is floating, with the secondary scope grounded via the scope. Noise measured is lower, PWM is stable and the scope traces rises and settles very fast.Im not sure what causes this, does anyone know? I would like to avoid keeping the primary side ungrounded