| Electronics > Beginners |
| Power supply ripple |
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| JS:
At what frequency the pulses do appear? There is a modulation of the pulses at about 100Hz but I'm asking about between pulses, I'd expect the output ripple of a SMPS have a triangular wave shape with spikes on the corners, plus the modulation seen in your captures. Also, check the noise between the AD-DC supply and the converter, you could need some filtering there as well. All of those problems should be dealt separately, is strange a 100Hz modulation after two switching stages, so for there I'd add some filtering in between, which could be a big cap, no problem there. At the output you want a small cap so it doesn't store too much energy to dump in the load before going into current limiting mode, low ESR preferably there, so MLCC could be a good choice. For it to work properly you will want to add a choke before. As you have screw terminals you could place the caps on those to test the setup, before committing to a PCB with fixed caps if that looks better for you. The choke should be added to the brown wire going to the front panel, the cap you could start testing it at the binding posts, and use the banana output to run the measurements. Then for the 100Hz I'd try to place a big-ish electro between black and blue in the front panel module terminals. To better know the proper values for the application the freq is needed for the math, and better at a few different loads, try open, 10%, 50% and 90% and document any differences between them. JS |
| ArthurDent:
I don't know if you've seen this video Dave posted but it is interesting. |
| JuanGg:
--- Quote --- At what frequency the pulses do appear? There is a modulation of the pulses at about 100Hz but I'm asking about between pulses, I'd expect the output ripple of a SMPS have a triangular wave shape with spikes on the corners, plus the modulation seen in your captures. Also, check the noise between the AD-DC supply and the converter, you could need some filtering there as well. --- End quote --- --- Quote ---To better know the proper values for the application the freq is needed for the math, and better at a few different loads, try open, 10%, 50% and 90% and document any differences between them. --- End quote --- I measured both the output of the module and the switch mode power supply with different loads as sugested (100% is 5A). Please find the measurements below. I tried measuring with two probes and substracting (floating supply), but even measuring the exact same point with both probes, noise was seen and the math fuction displayed diferences between both channels. (see first capture). The remaining measurements were made grounding the negative of the supply via the low-inductance spring (instead of the earth lead). --- Quote ---All of those problems should be dealt separately, is strange a 100Hz modulation after two switching stages, so for there I'd add some filtering in between, which could be a big cap, no problem there. At the output you want a small cap so it doesn't store too much energy to dump in the load before going into current limiting mode, low ESR preferably there, so MLCC could be a good choice. For it to work properly you will want to add a choke before. As you have screw terminals you could place the caps on those to test the setup, before committing to a PCB with fixed caps if that looks better for you. The choke should be added to the brown wire going to the front panel, the cap you could start testing it at the binding posts, and use the banana output to run the measurements. Then for the 100Hz I'd try to place a big-ish electro between black and blue in the front panel module terminals. --- End quote --- I tried with a 330 uF and 1000 uF electrolitics (what I had at hand that could withstand the voltage) on the output of the SMPS and it made no difference. Also tried several ceramic caps on the binding posts with the same result. Will looping the wire a couple times around a ferrite bead do instead of a choke? --- Quote ---I don't know if you've seen this video Dave posted but it is interesting. --- End quote --- Yes, I have seen it. I tried to measure as he recommends (20 MHz bandwidth limit, low inductance paths...within the equipment I have) Thanks again to everyone for spending time helping others, and sorry for measurements not being taken correctly or anything else for that matter...still learning how to use the scope. |
| JS:
For me looks like the module is switching at about 70kHz and making the SMPS sag a bit at that frequency, can you see how the dips grow in the SMPS as you increase the load? Also, something similar happens to the module, where with no load there are no spikes, then at 10% load the spikes are there but the ripple ignoring the spikes is very low, and once the load increases more there is some ripple to be clearly seen in the 50mV/div scale which was barely showing off the noise in the 10mV/div scale. It might be interesting to see how the SMPS behaves on it self, loaded with a easier load than the SMPS. To filter the spikes you need a capacitor that has low enough ESR and ESL to deal with them, it needs to provide 5A, now you have a ripple of 200mVpp which suggests 40 to 20 mΩ ESR+ESL on the output cap, the sharp spikes suggest main problem is ESL. You should try using MLCC SMD caps, as they are shorter and thus have lower ESL. Zooming in even further, to check for the spike itself could throw some numbers in the capacitance and inductance shown in there. Then you have the ringing between each spike, that's over the 70kHz, it can almost fit two cycles so about 140kHz, but as it's much lower than the spikes I'd leave them as they are till I get the spikes down. As the spikes are so fast, I wouldn't expect much inductance to be needed to deal with them, a few turns into a core in one of the wires coming from the module to the output should help, then dealing with the ringing it could need something different.. For the low inductance caps, I think the best way would be a double sided PCB, one side untouched as ground plane, the other with a slot in the middle for the cooper, so the caps go mounted side to side. You get into the PCB with the wires from the module and came the other way with the wires to the output or just mount the output directly to the PCB, holes to screw the banana jacks directly (you might want to have ±outputs side by side and the earth to the negative side as is sometimes done). In the PCB I'd install quite a few 0805 caps, or something similar, rated for 50V but with not much capacity, under 1µ for sure. 10 caps of 220nF would make 2.2µF which is a nice number for the output cap of a lab PSU while getting them distributed in a 50x50mm PCB as described will make seriously low ESL. You could even mix the caps, interleaving some 1nF and 10nF to go crazy with the 2.2nF ones. You don't need as many as the small ones as you are not trying to increase the overall capacitance with them but reduce the HF impedance of the solution. In any case, even as fun as this is and informative, you shouldn't expect this SMPS to be the low noise PSU from your lab, but the flexible one for the odd load. Having that output range (V and I) will make it's use inside the lab even being this noisy. I still believe some improvement could be made. JS |
| JuanGg:
--- Quote --- To filter the spikes you need a capacitor that has low enough ESR and ESL to deal with them, it needs to provide 5A, now you have a ripple of 200mVpp which suggests 40 to 20 mΩ ESR+ESL on the output cap, the sharp spikes suggest main problem is ESL... --- End quote --- I zoomed into the spikes and they appear to have a frequency of about 190 MHz (see attached capture). I added 5x 100 nF and 4x 10 nF on a veroboard on the binding posts and 6 turns around a ferrite core on the positive wire. (see photo). I know it is nor even near what you proposed, just used the materials at hand. I will add some more capacitors I am waiting for, or even make a proper PCB. The spikes haven't disappeared, but are a third of what they were before: Spike's Amplitude Load Before After 0% 17 mVpp 16 mVpp 10% 145 mVpp 50 mVpp 50% 330 mVpp 100 mVpp 90% 280 mVpp 95 mVpp --- Quote --- In any case, even as fun as this is and informative, you shouldn't expect this SMPS to be the low noise PSU from your lab, but the flexible one for the odd load. Having that output range (V and I) will make it's use inside the lab even being this noisy. I still believe some improvement could be made. --- End quote --- No doubt I am learning a lot, and that is what it's all about. Thanks again for the contribution. At the moment it is the only PSU in my "lab" (appart from an Atx PSU with binding posts...) I will eventually make a linear one with a couple LM317s if I need to. |
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