Filtering 19kHz switching noise from UPS AC output - Page 1
Forum section: electronics / beginners

author: Novgorod
author_rank: Contributor
timestamp: June 29, 2019, 07:02:50 pm
content: Hi!We have some critical lab equipment, which we want to power through a UPS in case of mains power interruption. Our UPS is an Eaton 9PX series with 1.5kW max. but typically much less of protected load (100-200W). Everything works fine except for one device, which really doesn't like the AC power provided by the UPS but works fine directly on mains power. This device is basically a high-voltage supply (<10kV), which takes only 70W of mains power during normal operation. When it's powered by the UPS, the high-voltage supply still works but it can't stabilize the HV output, i.e. it fluctuates heavily. When switched back directly to mains (e.g. UPS set to bypass mode), it stabilizes immediately (behavior is the same with or without power cycling).I assume the HV is directly derived from the AC mains input without an intermediate DC conversion step and is thus very sensitive to the mains waveform. Before getting the company involved and losing sanity to tech support, I want to try to find a simple solution first. The UPS is more or less decent and is supposed to output a clean AC sine waveform, so I measured it with a scope (under light load, i.e. with the HV device connected) and compared it to the mains from the wall (we have 230V/60Hz here):Red: mains AC from the wallBlue: AC output from the UPSThe sine output from the UPS is actually cleaner with less harmonic distortions than the mains from the wall, but it clearly has some small but significant switching ripple at 19.2kHz, see the zoomed bottom image, with an amplitude of a few volts (roughly 1-2% or so). I don't have much experience with UPS but I suppose it's nothing out of the ordinary, as UPS are primarily meant for IT stuff with their switching power supplies, which don't care much about the input waveform anyway. I can only assume that the small high-frequency ripple is the reason why our high-voltage supply has problems with it.My question is: can this 19.2kHz ripple be sufficiently filtered out in a simple way? I guess a low-pass filter with a high enough power rating should do but I really wouldn't want to cobble together some custom solution because I have very little experience in playing around with mains power. Maybe that's a common industry problem that has been solved many times already but I don't know where to start looking. Could it even be as simple as putting some ferrites around the cable and what specs would I need?Thanks for the help!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 29, 2019, 08:25:13 pm
content: Probably the output has a big capacitor on it, so the output impedance at those frequencies will be low. So a series "trap" would be called for. This is a parallel resonant tank (L || C), connected in series with the source, and acts to increase the output impedance at its resonant frequency. Probably design it for 5 or 10 ohms at 19kHz, with some give-or-take for final tuning (adjust the capacitor or inductor by selecting values or making parallel/series combinations, or adjust inductor gap or turns/taps). So, that'll be L = (10Ω) / (2*pi*(19kHz)) and C = 1 / (2*pi*(19kHz)*(10Ω)).If that's not good enough, an R+C in parallel with the load may be helpful (R ~ 1-5Ω, C a few times (>= 2.5) bigger than the trap C), or a trap (series L+C, calculate the values similarly).Tim

author: ogden
author_rank: Super Contributor
timestamp: June 29, 2019, 08:26:38 pm
content: If you have 1:1 mains transormer around- try it first. Then there are power conditioning filters usually combined with surge protection. APC manufacture such for example.

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 29, 2019, 10:17:15 pm
content: Tim's suggestion of a parallel resonant trap is not a bad one except for assuming the output impedance of a HV supply will still be capacitive at 19kHz - there tends to be a lot more stray inductance with the large clearances and transformer winding arrangements required above 3-5kV or so.Consequently, I would recommend the boring old LC low pass filter on the output of the UPS (series L, shunt capacitor) with a corner frequency around 1/10th the ripple frequency (e.g. - 1.9kHz fc for 19kHz fripple) and a characteristic impedance 1/10th or less than the effective load impedance. In this case, the load is 70W with 230VAC mains so it has an effective impedance of 756R; a filter with less than 75R impedance should do the job. For examples using standard E12 values: 4.7mH + 1.5uF (Zout = 56R); 3.3mH + 2.2uF (Zo = 39R); etc.

author: Novgorod
author_rank: Contributor
timestamp: June 30, 2019, 09:46:58 am
content: Thanks for the advice. A custom-built filter box would be my last resort if I can't find an "off-the-shelf" solution. The proper design would be probably more complicated, since the power draw of the HV supply device ist not constantly 70W but can vary by a lot during normal operation (up to ~300W but typically below 150W and most of the time around 70W). However, it's a kind of composite device with additional functions (other than the HV supply) and it has a rated total power draw of up to 1kW (those high-power functions are used rarely and only for a few minutes at a time). I could probably integrate a custom mains filter just for the HV supply portion of the device, but that would require messing around inside the device itself.Quote from: ogden on June 29, 2019, 08:26:38 pmThis appears to be the component I'm looking for. I don't have anything like that "at hand" but I found plenty of power line EMC/EMI filter modules from various vendors, which might be suitable. These EMC filters are typically designed for high rejection at high frequencies (>1MHz), as I couldn't find any specifically for the few-kHz range. However, some of the "high-performance" type of filters seem to be good enough down to the 10kHz range, for example this one:https://hken.rs-online.com/web/p/power-line-filters/7024031/According to the data sheet, the 10A version has the best performance at low frequencies with ~20dB insertion loss for both differential and common mode noise around 20kHz, although I don't know whether this is load-dependent. This should reduce my UPS ripples by a factor of ~100, i.e. to some ten millivolts amplitude. Is this worth a try or am I completely mistaken?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 30, 2019, 07:54:16 pm
content: Yeah, my first thought was probably against using a filter module, because they're aimed at higher frequencies. But it doesn't hurt to try, and if it works then it works.There are wider bandwidth ones, usually aimed at military purposes...with price and availability to match.20dB is power, so 1/10th voltage.It's something, not all that much, but worth a try. It's also measured into 50 ohms (or it usually is, anyway), but with no idea what the load impedance is, there's no telling if it will be more or less with the real load.Transformer is interesting because it adds series inductance and some parallel capacitance, but somewhat randomly so; this can work like a resonant trap or lowpass filter, but the break frequency may be anywhere. Depends on type -- toroids have wide bandwidth, shell type modest, bank-wound (split bobbin) the least.Tim

author: Novgorod
author_rank: Contributor
timestamp: July 01, 2019, 01:35:48 am
content: Oh, somehow I assumed the rejection figures referred to voltage, not power- thanks! 1/10 might be still enough, it's worth a shot anyway for the price.So a transformer is basically like a box of chocolates, eh? Another issue is that a transformer would have to be very beefy for the occasional high-load operation even though most of the time the load is low - or we would have to manually disconnect the device from the UPS/transformer line and plug it directly into the wall (including a power cycle of course) for the high-load operation and switch it back afterwards, which is a bit impractical. 1:1 transformers in the 1kVA range are heavy beasts and starting to get pricy (here's the cheapest RS model with 14.6kg of beefinessshttps://hken.rs-online.com/web/p/din-rail-panel-mount-transformers/0504234/), and it's not guaranteed that it will filter out the 19kHz ripples. I guess I'll try the passive filter first then...

author: David Hess
author_rank: Super Contributor
timestamp: July 01, 2019, 01:56:49 am
content: That is pretty typical for a sine wave output UPS. It is not always visible if a transformer is used to couple the oscilloscope to the line for safety.I have not faced that exact problem but have considered various ways to handle it. One problem I kept running into is that the impedances at line voltage are inconvenient for filter component values. So I thought about using a step-up and then step-down transformer in series with the filter between them operating at a higher voltage and lower current. Transformers intended for tube power supplies would be suitable.Even the best common line filters only have a couple dB of attenuation at 20kHz at most so I think the best option if to build your own low frequency 2 or 4 pole LC filter.

author: eV1Te
author_rank: Regular Contributor
timestamp: July 02, 2019, 07:08:55 pm
content: You could try some off the shelf mains filter first (normally designed for 100+ kHz), because perhaps it is not the 19 kHz that you see that is the issue, perhaps the 19 kHz is more of a square wave and has a lot of higher frequency content as well which causes the real problems and which you do not see in the traces you measured.Filtering 19 kHz from mains power is difficult as many have mentioned, you need large inductors and capacitors in several stages/poles.

author: Novgorod
author_rank: Contributor
timestamp: July 03, 2019, 07:26:41 am
content: Quote from: eV1Te on July 02, 2019, 07:08:55 pmThe 19kHz ripple has some harmonics, but they drop off pretty quickly:(same color coding as in the 1st post)The scope doesn't have the greatest dynamic range and I only measured at 500kS/s, but anything beyond 100kHz seems insignificant compared to the 19kHz ripple. I will try the filter first anyway. Note that the 60Hz fundamental looks very broad - that's because the entire trace was only a couple of cycles.

author: ogden
author_rank: Super Contributor
timestamp: July 03, 2019, 07:59:50 am
content: Quote from: Novgorod on July 03, 2019, 07:26:41 amThat UPS can easily output nasty high-frequency spikes that are outside of your "scope" bandwidth. Any chances to look at waveform using at least 20MHz scope? BTW I would test that "primadonna supply" using regular (pseudo-sine) UPS, also double-check that there is no grounding/earthing fault in the UPS/suply or just discontinuity.

author: firas
author_rank: Contributor
timestamp: July 04, 2019, 02:04:17 pm
content: A simple electric load filter from this company meets the needs. Previously I read on the site about this problemhttps://www.schaffner.com/

author: Novgorod
author_rank: Contributor
timestamp: July 04, 2019, 03:09:54 pm
content: Quote from: firas on July 04, 2019, 02:04:17 pmI looked at the EMC/EMI filters from that company before but according to the rejection curves, they perform worse at 20kHz than the one I've already posted.

author: capt bullshot
author_rank: Super Contributor
timestamp: July 04, 2019, 03:59:15 pm
content: Back to original post: I wouldn't bet on the 19kHz ripple to cause the problem, might also be something else. Totally different impedance characteristics of the UPS output vs. normal mains could cause trouble also with strange loads that I can't imagine of right at the moment, but there's always something new that one hasn't seen before.So an oscilloscope plot of voltage and current (if possible) of the load running at the UPS could be at least interesting, maybe this can give some hints (voltage + current would help more than voltage alone). Also insightful might be a peek into the guts of the load.Anyway, one thing to test would be a simple series inductor placed (rated for the required amperes at some ten milihenries - a low (1V ... 5V) voltage secondary of an ordinary transformer might do, maybe with the primary shorted if the secondary is above 5V rated) in series to the UPS output. Or just a simple series resistor in the single Ohm range, with appropriate power dissipation rating.