Problematic Switch Mode Power Supply Noise - Page 1
Forum section: electronics / beginners

author: Kane
author_rank: Contributor
timestamp: April 26, 2023, 03:11:05 pm
content: Hello,I have a Hanmatek HM310 switch mode power supply that I got off amazon less than a year ago and I'm trying to understand what is happening regarding the noise on the output. I watched EEVblog #594 to try and measure the ripple and noise correctly but my probes only have 6MHz bandwidth in 1x mode so bear that in mind. All the tests I have done are with a 1A load in constant voltage mode.I was expecting to see a thin band of common mode noise with some switching noise on top of it but instead I saw this noise that is at the rectified mains frequency in the UK of 100Hz shown as the cyan waveform.For comparison I hooked up an old Maplin N93CX I got from a school and its output waveform is exactly what I would have expected to see from a switch mode power supply shown as the yellow waveform.I then noticed when i had them both on at the same time to do a comparison that the noise from the Hanmatek HM310 (cyan) is leaking onto the mains and onto the output of the Maplin N93CX (yellow) as shown below.Can somebody help me understand what is happening? Is the power supply badly designed or is it broken?

author: jmelson
author_rank: Super Contributor
timestamp: April 26, 2023, 03:47:39 pm
content: There are some SERIOUSLY crappy power supplies out there! I had an application where it really mattered, and I hacked the supplies for remote sensing. But, the control loop was quite marginal, and with a bit of capacitance at the load and a bit of wire between supply and load, the control loop would oscillate at audible frequencies and produce audible whine. I later found a much better supply (by trial and error) and the loop was VASTLY more stable, and the output had much lower switching noise, too!Given the zoomed-in trace, I think this may be a truly horrible supply. It might be fine for digital circuits, but the fact that the switching noise greatly responds to the mains ripple seems to indicate something nasty going on inside. Of course, make sure you have applied the minimum load current. Many switchers have a minimum load, below which they perform badly.Jon

author: Kane
author_rank: Contributor
timestamp: April 26, 2023, 06:39:28 pm
content: Thanks for the reply.I was using a 1 ohm power resistor at 1V for the 1A load current so I'm definitely above the minimum load.Is there anything i can do to stabilise the oscillations or at least prevent it from leaking out onto the mains and being picked up from other equipment?

author: JustMeHere
author_rank: Frequent Contributor
timestamp: April 26, 2023, 07:13:01 pm
content: Try your test again at 50%, 75%, and 100% of the power supply max voltage. The more you step down the voltage, the more "switching" the power supply has to do.

author: janoc
author_rank: Super Contributor
timestamp: April 26, 2023, 07:18:35 pm
content: Isn't Hanmatek the brand that is known to blow up, taking your device under test with it?I recall someone on Reddit asking how to fix one - and then saw that multiple people left reviews under the Amazon posting that thing just goes BANG at the slightest provocation and sometimes even without.That supply likely lacks any sort of filtering, certainly using the cheapest possible caps too. I am not sure I would even bother trying to fix a supply where every possible corner has been cut.

author: Kane
author_rank: Contributor
timestamp: April 26, 2023, 09:28:13 pm
content: Quote from: JustMeHere on April 26, 2023, 07:13:01 pmI don't have an electronic load so I wasn't able to test the different voltage settings with a constant current. Instead I tested it with 40 x 1/2W 2.2k 1% resistors in parallel which was 55 ohms.Firstly, I want to note that the noise is on the output even when the output is off, see below:I tested in 1V increments from 1V to 32V which is the maximum the supply can output.What's interesting is that the peak to peak noise through 1V to 15V increased from 50mV to 130mv then through to 24V it decreased down to 50mV before again increasing through to 32V where it reached a disturbingly high 150mV peak to peak.I've taken some more screenshots of the noise at various timebases when the supply was outputting 32V and 0.58A shown belowThis seems way too much to be normal?The manual states it has a 10mVRMS ripple does that mean it should have maximum of 14.1mV peaks or 28.2mV peak to peak or does it not work like that?

author: JustMeHere
author_rank: Frequent Contributor
timestamp: April 26, 2023, 11:15:21 pm
content: By chance do you have any LED or fluorescent lighting near you? Turn it off and see if it helps.

author: Kane
author_rank: Contributor
timestamp: April 27, 2023, 12:21:06 am
content: Quote from: JustMeHere on April 26, 2023, 11:15:21 pmThat was the first thing I tried, turning off all the lights and unplugging everything else but it is definitely from the supply, it has a big clunking mains switch and the noise disappears if I turn it off and as soon as I turn it back on it appears on the output of the good smps, the yellow waveform in previous images.It's alright, I'm assuming it is just terribly designed. I think I might try to make my own linear power supply, I hear it's a great beginner project.