Show me your PSU noise - Page 1
Forum section: electronics / general

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: May 08, 2016, 07:36:09 pm
content: I have restored a few PSU these recent years, but it is always difficult to estimate the normalbehavior of the unit.I will show here the output of some of them on the oscillo (No load, 10 V) and propose you to do the same for comparison.All the records are made in the same conditions with a simple 50R coax directly from the PSU to the oscillo. I preferred not to put any load to avoid additional noise.I will also appreciate if someone tells me how to get rid of the 100khz spikes in the SL1731/SL1730 PSU .A) SL 1731SB - linear PSU, very common, low price, cheap through hole construction, but does the job.https://www.eevblog.com/forum/repair/tracking-problem-in-a-dual-bench-power-supply/msg529140/#msg529140Two channels 2 x 30 V -3A + 5V . Same as Velleman PS23023, HY3003F-3, and maybe some other ones.B) SL 1730SB - The same with only one channel, but fine/gross adjustmentC) Delta Elektronika 030-10 SMPS 0-30V x 10A. Recently restored.https://www.eevblog.com/forum/repair/delta-elektronika-es-030-10-cc-problem/msg934836/#msg934836D) ISOTECH IPS-2303. Linear PSU 0-30V x 3A 2 channels . Much more advanced and better construction quality than SL 1731SB.E) TOPWARD TPS-4000 : linear PSU. Analogic display. 0-30V x 3A 2channels + 5VF) Fluke 415B 0-3KV linear PSU (30mA) . Only tested here on 10 V as the others.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 08, 2016, 07:41:39 pm
content: Out of the lab at the moment, but bumping for future reference...Tim

author: jitter
author_rank: Frequent Contributor
timestamp: May 08, 2016, 07:57:21 pm
content: Quote from: JacquesBBB on May 08, 2016, 07:36:09 pmI can see some settings (like e.g. coupling, timebase vertical scale and no bandwidth limit), but what are your acquisition settings? Any averaging?I would like to duplicate all settings exactly on my Rigol.

author: AF6LJ
author_rank: Unknown
timestamp: May 08, 2016, 08:02:05 pm
content: Ether later today or tomorrow I'll grab an old school capture of the noise on my 1975 vintage Lamba OEM power supply I use to power my HF radio...

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: May 08, 2016, 08:02:28 pm
content: Normally no averaging. Only one pass, but I may need to do it again to be sure that I kept the same setting all the time.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 09, 2016, 03:03:04 am
content: My from-scratch bench supply, linear, +/-20V >5A,2.64mV RMS into 50 ohms (10ms/div, 1000 pts, pk detect, 20MHz BW, via 20dB RF amp (1kHz-100MHz BW))2.5mV RMS (same settings, no amp)Noise level includes bursty switching noise (harmonics in the 10s of MHz, repeat rate in the 0.2MHz range), probable diode recovery noise (lumpy ~1MHz ringing), and other generic EMI, RFI, and ambient radio signals.Sorry, that's quite shoddy of me! With the drill unplugged*, it's now 1.4mV RMS. So, that was ambient RF mostly!(*Fun stuff. I picked up a Horrible Freight "Drill Master" for $20 bucks, a year or two ago. Naturally the charger doesn't contain any control at all, so it quickly cooked the NiCd pack. Being the intrepid engineer I am, I soldered wires onto the drill's battery terminals, thus turning it into a corded cordless. Banana plugs on the end allow me to use it on the bench supply as I please. Bonus benefit: unlimited torque!)The power supply design is basically an unusually low impedance audio amplifier, with DC coupling and a TL431 adjustable DC source. (It also has a BNC input, selectable by switch instead of the reference, for general amplifier use.) The TL431 itself should be the dominant noise source, or maybe the crappy discrete diff pair input stage, or recovery noise in the power supply diodes (which I had noticed before, and added RC dampers across.) And ambient noise, of course.Tim

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: May 09, 2016, 10:29:41 am
content: I have updated the scope plots to show also the rms.Thanks Tim, but I would as well like to see the full wave form of your PSU noise.The idea for these plots is not a challenge, but educational. I would like to understand how to remove all the unwanted components ofthis output signal.For this it is nice to have some reference values.I would like to understand for example why the two channels of my SL 1731SB behave so differently. And how to fix the bad one.

author: AndyC_772
author_rank: Super Contributor
timestamp: May 09, 2016, 11:34:04 am
content: This is also quite an interesting subject from the point of view of accurately measuring noise under these conditions.Power supply noise is normally, and properly, measured directly across a capacitor located at the load. For example, it may be shown as a differential voltage measured across a 1uF ceramic capacitor, with wires 50cm long between PSU and measurement point, and a measurement bandwidth of 20 MHz.Without consistency in how the measurement is taken, it's impossible to meaningfully compare & contrast results.It's also worth measuring the influence of common mode noise on your measurement, however you take it. By all means attach a scope across the output and GND terminals of the PSU and plot the result. Then, remove the scope probe tip from the positive output, and connect it to the GND terminal at exactly the same place as the probe's ground. You may be surprised at just how much noise you still see, and this isn't a voltage that would ever be developed across the load.

author: tautech
author_rank: Super Contributor
timestamp: May 09, 2016, 12:45:18 pm
content: This ^^^^For results to have any meaning there must be consistency in how one would test for ripple or noise.20 MHz BW is a must as it's unlikely any noise beyond 20 MHz is from the PSU.AveragingYou'd be surprised how many cheap PSU's won't meet advertised specs at their output terminals without averaged ripple tests .DSO averaging enables the fundamental frequencies of the ripple to be more plainly seen and measured.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 09, 2016, 04:38:40 pm
content: Quote from: tautech on May 09, 2016, 12:45:18 pmWell...Trap for young players and all.Small DC-DC and AC-DC modules (yes, even ones claiming CE marks and standards compliance!) typically throw off more than regulatory limits allow, when placed into a typical application. Very small (<= 3W) DC-DC modules especially -- no room for filtering, so why bother!This is a big problem for power modules on mixed signal PCBs. 100MHz+ crap can spray out from the converter, dutifully carried by low impedance ground planes, right into your ADCs, FPGAs and so on.Most power supply designs, even from big names like TI, ADI and LT, have absolutely no consideration of slew rate control or snubbing, and generate very fast pulses: the LTC3810 dev board makes <4ns pulses on its input supply rail.Tim

author: mmagin
author_rank: Frequent Contributor
timestamp: May 10, 2016, 10:52:57 pm
content: Quote from: AndyC_772 on May 09, 2016, 11:34:04 amThat is true, but it always seems like a way for crappy power supplies to advertise more favorable specs. And some cheap stuff doesn't even bother to accurately specify the measurement conditions.Personally, I would rather know what the product does, not what it does with some extra LC low-pass filtering.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: May 11, 2016, 04:18:21 am
content: Quote from: mmagin on May 10, 2016, 10:52:57 pmMe too. This is why I propose to show here the raw output of the PSU with the full signal and high frequency spikes.

author: AndyC_772
author_rank: Super Contributor
timestamp: May 11, 2016, 07:03:14 am
content: Measuring across a cap at the load isn't about making the PSU specs look more favourable, it's about making a measurement which is actually representative of how the PSU will behave in a real system.A voltage spike which is presented with high impedance is easily removed by the decoupling caps that are present on any real load. A low impedance spike of equal (unloaded) amplitude results in a greater amount of noise in a real system, but simply connecting a scope across the output of the PSU won't show any difference.