SMPS output ripple - Page 1
Forum section: electronics / repair

author: tony359
author_rank: Frequent Contributor
timestamp: August 05, 2020, 10:14:05 pm
content: Hello all,I have a Switching power supplies which works ok but I read excessive spikes or ripple on all the outputs, about 1V. I have a working identical model and I can confirm its outputs are perfectly clean.The PSU has 3 different outputs (+5, +15, -15) and I can observe the issue on all of them, same amplitude. That makes me think that the issue is with the primary and not the secondary. The spikes are at the same switching frequency and matches a "ringing" that I can observe on the switched signal going into the transformer.I went probing the PWM output of the controller chip - an ML4841CP and I believe there is something wrong with it. Below you can see the comparison with the good PSU. I tried replacing the ML4841CP but that didn't change anything. I also replaced the switching Mosfets (IRF 830) and the secondary rectifiers.When I probe the PWM output of the ML4841CP I can see a few cycles that look good but immediately after that they become as in the picture.I see those spikes everywhere on the pcb so it's a bit difficult for me to pin it down to something. And I am just learning electronics - yes, I am very aware that a switching PSU has lethal voltages in it and I am being very careful and not grounding anything!Does anybody have any ideas? Thank you!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 06, 2020, 01:59:12 am
content: Check the capacitors.Tim

author: tony359
author_rank: Frequent Contributor
timestamp: August 06, 2020, 09:42:16 am
content: Hi thereI replaced all the capacitors a while ago. To be honest, I do not know whether those spikes were there before or not: the SMPS had another issue (outputs intermittently dropping) and I re-capped it hoping to fix. That did not improve, it turned out it was a tantalum capacitor on the feedback line which I identified and replaced.Don't you find weird that the PWM signal is so messed up compared to a working unit?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 06, 2020, 10:21:20 am
content: Neither one looks particularly attractive. The "good" one not dropping to GND looks strange to say the least -- but that's on the assumption it's supposed to be a logic-level signal. You said "PWM signal" but without a circuit it's impossible to say from here what that means.The spikes being "everywhere" are because the board itself has that voltage on its local ground reference -- common mode noise. You need a high performance differential measurement to avoid it. Using a pair of probes differentially can help. (Ferrite beads on the probe cables tends not to help very much, but can slow down the trash enough to visually separate it from the visible signal.)Tim

author: tony359
author_rank: Frequent Contributor
timestamp: August 06, 2020, 01:59:10 pm
content: Hi Tim,I may be able to explain why the "good" PSU PWM does not reach ground: I do not have a differential probe and I am using a 100X probe referenced to mains ground. I suppose that gives me some inaccurate readings. It's just that I do not have the budget for a proper setup at this time.Unfortunately schematics for my PSU are not available but the ML4841CP operations are detailed as perthis datasheetThe signal I scoped was from pin 11.The PSU in question is a CE150-3002 from Integrated Power Designs

author: tautech
author_rank: Super Contributor
timestamp: August 06, 2020, 05:13:52 pm
content: It could be the result of gate Miller capacitance.Have a read for a few posts starting from here:https://www.eevblog.com/forum/repair/tektronix-2215-scope-repair/msg1824401/#msg1824401

author: trobbins
author_rank: Frequent Contributor
timestamp: August 07, 2020, 02:53:53 am
content: I'd suggest you clarify how you probed, and use a known technique for probing smps. It could well be that you are picking up spike content that is not related to the actual PSU output.

author: tony359
author_rank: Frequent Contributor
timestamp: August 07, 2020, 02:30:08 pm
content: Tautech - I'll have a better read soon. However I could probe those spikes before I replaced the mosfets. May I have caused the issue by swapping the capacitors though? I do not remember whether those spikes were present before I re-capped.Trobbins: Using a normal oscilloscope probe, I clip the ground to output ground and the tip to +5V, +15V and -15V. If I do the same on the known good PSU I do not see those spikes, the signal is VERY clean.For the PWM signal, I am still using the secondary ground - I know it is not ideal but again I have probed both PSUs the same way and the signal looks different on the scope.That said, I just went ahead and ordered a micsig differential probe so I may be able to get some better readings next week.Thanks all for your help!

author: tony359
author_rank: Frequent Contributor
timestamp: August 14, 2020, 10:20:48 am
content: Hi all - final update!I purchased a differential probe (MicSig) and indeed the PWM and PFC signals look identical and perfectly fine on both PSUs.After lots of probing and tinkering, someone suggested that the spikes I was observing could be picked up by air and maybe coming from the primary area even though the primary signals looked ok.While I was still thinking of that I did something I never tested before: I tested the outputs of the "good" PSU while the PCB was out of its metal cage and with my surprise I could scope the same spikes that I could observe on the "bad" PSU.I then realised that I had always been comparing a fully-assembled PSU (in a metal cage) with a fully-disassembled PSU (unfortunately you cannot access the back of the PCB when it's mounted).I've re-assembled the "bad" PSU and guess what: no spikes anymore. Maybe something very minor but we are talking about 20mV which I am not worried about. So basically when I disassembled the PSU to repair it in the first place (outputs were jumping when hot - it was a ceramic capacitor which I replaced a long time ago) then the PSU was perfectly working - I just didn't realise as I was testing it out of its cage!I have also noticed that while I have several "common" pins on the output, if I scope the last pin on the right (+15V) using the leftmost common pin on the left, I do see spikes on both PSUs even when properly assembled. If I move the scope's ground clip on the rightmost common pin, which is next to the 15V pin, then the spikes disappear completely. This is despite all those pins are all linked together on the PCB. Can I assume that the short PCB path between the rightmost and leftmost pins act as an aerial and pick up the spikes? If you think about it, when it's installed ALL those ground pins are connected to the instrument and they must be connected together at the other end.Anyways, it was a good learning experience. I'm attaching pics of PWM, PFC and one of the outputs - the worst oneI have also found a good isolation transformer - just the transformer, I'll build my own set. I was thinking of making something nice with several incandescent bulbs that can be individually added or removed from the circuit.Cheers all for your advice, particularly those on safety