Any good articles to read up on for reducing switching noise? - Page 1
Forum section: electronics / projects

author: Pasky
author_rank: Regular Contributor
timestamp: March 25, 2014, 01:02:38 pm
content: I've got a PT78ST105V (http://www.mouser.com/ds/2/405/slts059a-206024.pdf) I'd like to install in a device that is pretty sensitive to analog noise. I'd really like to use a switching regulator as I don't have much room to put a linear one with a proper heat sink in there so I was wondering if maybe I could possibly use this and see if I could reduce the noise down to a level where it would not affect anything (mostly analog video out). I've tried searching but I can't find much info on reducing switching noise, only what switching noise is.Can anyone point me in the right direction? Thanks.

author: jmaja
author_rank: Frequent Contributor
timestamp: March 25, 2014, 01:43:44 pm
content: One solution is to use a LDO after the switching regulator. E.g, 12->5.5 with a switching one and then 5.5->5 V with a LDO. This will reduce the ripple very much and the LDO will not produce much heat. But It won't filter switching spikes, you can use e.g. ferrite beads for that.This is a very good application note about the issue:http://cds.linear.com/docs/en/application-note/an101f.pdf

author: akis
author_rank: Frequent Contributor
timestamp: March 25, 2014, 02:24:13 pm
content: On this very subject, I constructed recently a transistorised switched capacitor voltage doubler at about 13KHz. When I tried to use it I noticed that it was contaminating the input voltage power supply line with spikes. I then added a filter consisting of a ferrite at 22uH and a capacitor of 10uF at the input which eliminated all spikes from the supply line. My question would be, without doing a PhD on the subject, what are the right values of L and C to use in these cases even as a bulk figure, as a starting point? Typical power ferrite inductors (taking amps) range from 10uH to 10mH, the range is huge - what should I have used? I suppose I am asking "how do I specify an LC filter" ?

author: georges80
author_rank: Frequent Contributor
timestamp: March 25, 2014, 02:41:29 pm
content: Google LC filter and get various hits.Something like ->http://circuitcalculator.com/lcfilter.htmlets you enter in some values. BETTER yet, download ltspice and draw your circuit and simulate it and get a bode plot. You will learn more and 'see' how the filter operates and changes as you adjust its component values. You could enter in your entire voltage double circuit and add the filter to it.Your filter needs to remove switching harmonics, so knowing your switching frequency gives an idea of where you want to filter to rolloff.You don't need a PhD, but you do want to learn some of the basics. Ltspice is and amazing tool for the price (free) and I can't imagine why anyone wouldn't download it AND use it to learn.cheers,george.

author: jmaja
author_rank: Frequent Contributor
timestamp: March 25, 2014, 06:03:20 pm
content: Quote from: AcHmed99 on March 25, 2014, 01:50:57 pmYou can easily choose one that has 25-40 dB PSSR at 650 kHz.

author: cellularmitosis
author_rank: Unknown
timestamp: March 25, 2014, 11:34:32 pm
content: I recently fooled around with using an ICL7660 to generate a negative rail, and discovered similar noise spikes.I tried using an "RLC" filter (http://sim.okawa-denshi.jp/en/RLClowkeisan.htm) to filter them out. The LTSpice simulations looked great. I tried out values like R=1ohm, L=100uH, C=100uF. But the measurements didn't turn out anywhere near as good as the simulations (the spikes were still there, up around 90MHz).Eventually I tried putting a WIMA MK2 cap (4.7uF) in parallel with the electrolytic cap I was using, and the spikes basically disappeared. It turned out the relatively high ESR of the electrolytic severely reduced the effectiveness of the RLC filter. The super low ESR of the WIMA allowed the filter to work much better at high frequencies.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: March 25, 2014, 11:51:47 pm
content: Probably more the ESL of the Electrolytic than the ESR at 90Mhz.

author: mazurov
author_rank: Frequent Contributor
timestamp: March 26, 2014, 12:48:07 am
content: Switching spikes' bw in a typical converter are on the order of 100 MHz, the "noise" will be capacitively coupled to the output through any filter consisting of conductors placed close together (like, for example, an inductor). Linear Technology Appnote 70 (written by Jim Williams) explains the issue quite well and offers a solution. LT1533, 1534 are low-noise switchers, I used them and they work well. Linear also has newer parts utilizing the same idea.Another inherently low-noise high-frequency converter topology is resonant Royer, also covered by Jim Williams in Appnote 118.If you can't change the controller, a string of 3-4 ferrite beads will work too. The length of the structure determines the efficiency of the filter.

author: tree
author_rank: Regular Contributor
timestamp: March 26, 2014, 03:09:08 am
content: Try using some ferrite beads -> elliptic filter -> linear regulator -> LC filter -> ferrite bead

author: Pasky
author_rank: Regular Contributor
timestamp: March 26, 2014, 09:58:15 pm
content: I ended up using a pi filter. Did the trick.

author: tech_builder
author_rank: Contributor
timestamp: March 26, 2014, 10:59:56 pm
content: I too was looking for almost exactly this information. I found the replies very helpful. I also found another piece of information that might add to that already shared:http://www.digikey.com/en-US/articles/techzone/2012/may/hybrid-power-supplies-deliver-noise-free-voltages-for-sensitive-circuitryThey talk about using high PPSR LDOs (such as TI's TPS7A49) in series with the SMPS to significantly reduce noise for ADC circuitry etc. I'm going to give it a try.