Cheap 5V SMPS with high ripple: design flaw? - Page 1
Forum section: electronics / projects

author: sorenkir
author_rank: Regular Contributor
timestamp: January 27, 2024, 10:59:19 am
content: Hello,I have bought two of these cheap 5V 1A TMF0321/TL431 based power supply modules:Actual implentation is almost identical to the one in the datasheet (https://datasheet.lcsc.com/lcsc/2209291030_TM-Shenzhen-Titan-Micro-Elec-TMF0321_C5174607.pdf), with different values for components.Both have the same issue: when loaded, there is almost 1Vpp of ripple at the output, at about 300Hz (lower frequency as the load current decreases).Example at 0.5A load:On the primary side of the transformer, there are 1ms bursts of high frequency (62KHz) high level 346V signal.In sync with what I see on the optocoupler led input:I am assuming that it is not the expeceted behaviour, but I don't know what could cause this.What should I look first ?Thanks,Michel.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 27, 2024, 01:01:56 pm
content: You have two of them. Do they both behave in the same way?You also have the datasheet of the IC, so you should be able to figure out how the thing is supposed to work.Also, if the stabilization around the TL431 is defective, you are likely to see low frequency instability.

author: uer166
author_rank: Super Contributor
timestamp: January 27, 2024, 03:20:05 pm
content: The chip looks like some kind of PFM control type. They show hysteresis between 1.2 and 1.4V in datasheet for FB node. That 0.2V hysteresis at 1.2V translates to about 1V hysteresis at 5V output if you have feedback gain of 1.Very likely that's normal and how it's designed, though it's pretty crappy for applications that care about ripple.One way to improve is to increase loop gain (i.eget a better optocoupler with higher transmission ratio).

author: magic
author_rank: Super Contributor
timestamp: January 27, 2024, 08:00:47 pm
content: Replacing optocouplers is a rather cumbersome way of manipulating loop gain.I wonder if 100nF isn't too much for C8. No experience with PSU tuning, but I would probably start there.

author: DavidAlfa
author_rank: Super Contributor
timestamp: January 27, 2024, 11:02:14 pm
content: Yeah try removing C8!

author: magic
author_rank: Super Contributor
timestamp: January 28, 2024, 09:04:36 am
content: I would start with reducing to 47nF to see if frequency increases to 600Hz and ripple decreases to 0.5V.If this works then keep tweaking.

author: ArdWar
author_rank: Frequent Contributor
timestamp: January 28, 2024, 09:08:54 am
content: Replacing optocouplers to tune loop gain isn't exactly reliable either. They degrade rather quickly!

author: sorenkir
author_rank: Regular Contributor
timestamp: January 28, 2024, 10:21:01 am
content: Hi,Thanks for the suggestions.In the actual implementation, C8 is 100nF and R9 510 ohms.I have tried without cap and then with 10nF. In blue, the optocoupler input.Indeed, there is less ripple, but still not ideal.Thanks,Michel.

author: magic
author_rank: Super Contributor
timestamp: January 28, 2024, 10:30:41 am
content: The schematic shows a second cap on the output of the optocoupler, this could be tweaked too.I don't know if it would be of any help to increase output capacitor C7?

author: Zero999
author_rank: Super Contributor
timestamp: January 28, 2024, 12:02:07 pm
content: Considering the price, they appear to be well designed, both from a safety and EMC perspective. This is assuming there isn't anything bad in the transformer i.e. relying purely on the thin enamel for insulation between the primary and secondary and the Y-capacitor isn't fake.If it's not possible to reduce the ripple to a satisfactory level, you could tweak the voltage up a little and use a LDO regulator.

author: DavidAlfa
author_rank: Super Contributor
timestamp: January 28, 2024, 12:10:48 pm
content: Also try:

author: mariush
author_rank: Super Contributor
timestamp: January 28, 2024, 12:14:11 pm
content: I think these are meant to be used with led lightning ( like for example to light up led spotlights to shine light on merchandise) ... I don't think they intended / designed it to be for precise 5v output.

author: sorenkir
author_rank: Regular Contributor
timestamp: January 28, 2024, 03:47:06 pm
content: Hi,Bad news: I have managed to fry both modules one after the other while tweaking C4I don't know exactly what happened. Maybe I have made some short with C1 still charged.Anyways, many thanks for your help and your potential solutions (which will remain theoretical)!Michel.

author: DavidAlfa
author_rank: Super Contributor
timestamp: January 28, 2024, 04:52:09 pm
content: Great, now buy decent modules instead the same crap again

author: sorenkir
author_rank: Regular Contributor
timestamp: January 28, 2024, 05:12:53 pm
content: I have bought a cheap and decent 9V one in the past (not the exact same "brand": it was Ruihong).This time, bad luck, it was crap.Michel.

author: sorenkir
author_rank: Regular Contributor
timestamp: January 28, 2024, 05:52:32 pm
content: Quote from: mariush on January 28, 2024, 12:14:11 pmYou are right!I have read the datasheet for comparable modules from Meanwell designed for LED lightning (https://www.meanwell.fr/content/files/pdfs/productPdfs/MW/APV-8/APV-8-spec.pdf) and the ripple is specified at about 250mVp-p, better than what I had, but not very good.Michel.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 28, 2024, 06:48:16 pm
content: Quote from: mariush on January 28, 2024, 12:14:11 pmYes, the original SCH has 12V as the output, which seems to be LED focused.Hysteretic controllers will always have some ripple. That's the price of simplicity.

author: uer166
author_rank: Super Contributor
timestamp: January 28, 2024, 07:45:10 pm
content: Also remember that this kind of controller is inherently stable and predictable, so you can't really screw up the compensation network too much. I actually like these if the ripple doesn't matter. A decent LDO would easily reject this low frequency ripple.