Power supply noise issue , how to remove it? - Page 1
Forum section: electronics / general

author: electronic_artist_
author_rank: Newbie
timestamp: July 24, 2019, 11:19:01 am
content: Attached is the circuit diagram of my power supply. it converts input 12V DC into 5V DC and then 3.3V DC with linear regulator.Red circled is power supply 5v and 3.3v powerSo current situation is, whatever noise is there on 12V input voltage shows up on 5V and 3.3V. How do i filter this ? how do i improve my design to get clean 5V and 3.3V even with a noisy 12V input. If i use clean power supply 12v, i am getting best results.But i cant give 12V clean power supply to everyone. Customers can pick up any noisy power supply and it should work with my design.below i am attaching two waveform, Yellow is 12V , Blue is 5V and Pink is 3.3V.If you look at the waveforms, the noise appearing on 12V is carried on to the 3.3V. i was assuming LDO will at least attenuate it more but, it is very high.how do i get rid of this, what ill have to change or add in my design to improve it?

author: David Hess
author_rank: Super Contributor
timestamp: July 24, 2019, 02:29:54 pm
content: Start by using a single point ground to remove ground loops.

author: electronic_artist_
author_rank: Newbie
timestamp: July 25, 2019, 01:27:51 am
content: hi,thanks for pointing out. But on my PCB i have full solid ground with proper vias ground stitching.still it will have effect?

author: David Hess
author_rank: Super Contributor
timestamp: July 25, 2019, 11:31:08 pm
content: Quote from: electronic_artist_ on July 25, 2019, 01:27:51 amA solid ground plane is not a perfect solution but that means the cause is elsewhere.To check for common mode noise, connect one oscilloscope probe with the probe tip and ground at the same spot. If this is a problem, then a common mode choke between the adapter and switching input may be needed.Beyond that, LCR filters may be needed at inputs and outputs.

author: ogden
author_rank: Super Contributor
timestamp: July 26, 2019, 12:03:09 am
content: It looks like probing problem. Are you even using scope probe ground clips? Seems like you don't. Note that frequency of noise bursts is exactly 100Hz, could be external noise leaking into your measurements. Start with single channel probing, make sure you are probing properly. You can find SMPS probing techniques on youtube and web. Just one of search hits:https://www.cui.com/blog/how-to-measure-ripple-and-transient-in-power-supplieshttps://youtu.be/VLzcV9Sxt5Q?t=1

author: ocset
author_rank: Super Contributor
timestamp: July 28, 2019, 01:46:36 pm
content: i think much of your problem is simply noise induced into the scope probe lead...you can eliminate a lot of it perhaps by using a home made coaxial probehttps://www.eevblog.com/forum/blog/eevblog-594-how-to-measure-power-supply-ripple-noise/..never forget at a huge telco, the gaffer left three ee grads and told them to get rid of the scope noise....they tried for three days by changing the filter......after three days the gaffer came and told them they were fools because it was just noise induced into the probe....and what they did with the filter would have no effect

author: electronic_artist_
author_rank: Newbie
timestamp: July 29, 2019, 02:31:50 am
content: hi,thanks for so much of feedback. yes wrong probing can really induce lot of noise. i have experienced it lot of time while doing measurement also whenever i light some of my light in the room they induce noise into my scope specifically LED lights. That's y i have started taking lot of care while doing such measurement.i grounding my probe properly, also ensured that there is no random ac noise coming in on oscilloscope, used 3 pin Plug to ground any internal leakage of the scope.the above screenshots attached shows lot of noise reason for that is i am purposely using a bad cheap Chinese 12V wall adapter noisy power supply because i want to make my circuit compatible with the most noisy power supply.the common mode noise as well as ripple are part of the 12V wall adapter power supply. If i connect a clean good quality power supply i am getting pure Dc without noise.Anything am i missing here?

author: ogden
author_rank: Super Contributor
timestamp: July 29, 2019, 08:52:05 am
content: Quote from: electronic_artist_ on July 29, 2019, 02:31:50 amCould be so that noise you measure is caused by bad insulation of transformer of wall adapter. Please redo measurement with only one channel, one probe lead - disconnect other leads from scope. Make sure scope lead is not tangled around supply or adapter cable - lay cables on the table carefully. Also make sure your scope and wall plug adapter is plugged into same mains line, do not use different phases. When measurement is made, swap line&neutral for wall adapter by rotating it 180 degrees, measure again. If nothing changes then you shall start posting photographs of everything - supply and your measurement setup.

author: xavier60
author_rank: Super Contributor
timestamp: July 29, 2019, 09:38:17 pm
content: The main cause of common mode noise from isolated SMPS is the switching noise current capacitively coupled between the transformer's primary and secondary. A Y capacitor is commonly fitted between the live and isolated sides to bypass this noise current, reducing the noise voltage.A common mode choke on the mains input side adds a high series impedance to minimise the noise current that would be caused by the noise voltage.The common mode choke usually isn't present in cheap AC adapters.The 100Hz modulation is caused by the action of the mains rectifier diodes gating the noise as they conduct during the mains voltage peaks.A common mode choke could be added on the DC side to reduce the noise.

author: electronic_artist_
author_rank: Newbie
timestamp: August 03, 2019, 02:23:58 am
content: thanks for suggestions.ill do the measurements again as you have suggested and put more details to clear rest of the confusion and making the main issue clear so we can point out that issue and apply fix for it.

author: electronic_artist_
author_rank: Newbie
timestamp: August 03, 2019, 02:29:54 am
content: your explanation gave me idea, what must be happening inside this cheap wall adapter.Yes, ill try the common mode choke and LC filter lets see what works.i have tried already common mode choke just for the curiosity sake 10mH!!!, but ya it does work but... it cleans output side and introduces more noise at the input of the filter... it is because i have wrong value of common mode choke , impedence difference is pushing lot of noise in the input.ill be trying proper values soon. will update results herethanks