CSI3003SM Budget PSU Tweaks - Page 1
Forum section: electronics / projects

author: Bynun
author_rank: Newbie
timestamp: April 24, 2014, 04:23:29 pm
content: I recently got a CSI3003SM which is a super cheap switch mode psu, 0-30V 0-3A, however being a switch mode psu makes it very noisey with around 200mv p-p noise.What i was wondering is if it's possible to clean this up using better components and maybe small changes to the output circuit to get it below 50mv p-p and maybe even less then 20mv? I know it'll never get super smooth, if that was possible there wouldn't be linear psu's anymore, but something better then what it is i think should be possible.Assuming cost isn't an issue (within reason) what would be the best method of achiving this?Some thoughs i had were replacing the caps with some high quality, low ESR, branded ones (nichicon?) and maybe adjusting the values?Changing the inductor, although my knowledge is very limited i belive there is limited scope here as they are usually tuned to the output voltage so on a variable output supply you have to get one that best fits, but maybe changing to a torroid with higher inductance might help?I also considered some kind of dynamic regulator but they might prove too costly?Adding extra shielding around the connecting wires?Added some images below which include output side circuit layout, the cheapo capacitors and an attempt at an oscilloscope output at 5v 500ma load (far from acurate but gives you an idea)

author: Bored@Work
author_rank: Super Contributor
timestamp: April 24, 2014, 05:27:11 pm
content: Well, that thing is a turd. I think I wrote about it here before. Regarding your schematic, did you miss the additional bodged cap they have directly at the output binding posts?Anyhow, if you look a the schematic you see the designer already tried to throw a good bunch of filtering on the problem, with not so much success. The theory says low ESR caps should reduce the output ripple. You should try them, but I doubt it will be enough.Some speculation, and it is only speculation:I have the feeling the pulses are a result of the SMPS control circuit driving the transformer very hard, maybe in some burst mode, and the transformer not being up to the task. Because of the pulses I would try a common mode choke in the output.Another thing to try, but that would be rather advanced, is to get some slope into the primary switching. I.e. making the primary side rising the current into the primary winding slower. That will require some extensive study of the whole circuit.More speculation:There is also a chance these pulses come from some other source in the PSU. The PSU includes an auxiliary SMPS. That one generates the supply voltage for the logic, etc. It is known to be wrongly dimensioned, creating an audible noise under some load conditions. You would have to do some measurements to figure out more.Which leads me to: Be careful when working in that PSU, it contains dangerous voltages. That includes measurements inside it. You might badly need an isolated differential probe for some measurements.

author: Bynun
author_rank: Newbie
timestamp: April 24, 2014, 09:48:48 pm
content: It is indeed a turd, but finding a way to improve it could be usful to all those like me who are limited to the cheap things in life. As they say, you can't polish a turd but you can roll it in glitter.The bodge cap is included in the schematic.Some useful suggestions there, i'll try upgrading the caps to low esr and playing around with different values to see if i can reduce the output capacitance along with adding a common mode choke.As for adding a slope to the input, would something like a 10n 1kv cap across the terminals work the way i think it would, with the charge and discharge cycles adding a slight ramp to the square wave, or would i need something more complex?I'll also check the other power lines and see how bad the noise is on those, as they are just supplying the logic components i could probably get away with hacking a 78 series regulator in line to smooth it out.

author: jwm_
author_rank: Frequent Contributor
timestamp: June 22, 2015, 11:48:05 pm
content: I have one too and was thinking of some modifications, have you considered tracking a linear regulator at 2V below the switching output? The linear reg will have to drop at most 6 watts so can do so with a small heat sink.