50uH LISN for basic  testing - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: May 22, 2024, 08:05:29 pm
content: Hi,We wish to test our 24vin, 12vout 150W out Buck converter with the 50uH LISN in its input circuit so that we can check for input filter oscillations when the LISN is attached.We wont actually be doing any pre-compliance EMC measurements at this point.We have the well known LISN schem as below.https://www.analog.com/en/resources/technical-articles/how-to-get-the-best-results-using-ltspice-part-1.htmlBut there should be a damping resistor across the 50uH....do you know what it is?Or is it five 10uH inductors in series each with 1k across them?J_Diddy_B kindly did a thread on this but for a mains LISN.https://www.eevblog.com/forum/rf-microwave/50uh-and-250uh-inductor-design-for-lisn/100/(...they say LTspice can be used to get conducted Common mode emissions...surely thats wrong?, how could you possibly get an accurate model of the strays to get accurate common mode plots with LTspice?)

author: jonpaul
author_rank: Super Contributor
timestamp: May 23, 2024, 05:44:25 pm
content: FTTS nonsense.Real solution, a commercial LISNhttps://www.solar-emc.com/LISN.htmlVery good design and safety.cheersJon

author: Smokey
author_rank: Super Contributor
timestamp: May 23, 2024, 07:57:42 pm
content: https://www.tekbox.com/lisn/https://www.emcfastpass.com/test-equipment/product-category/lisn-line-impedance-stabilization-networks/More sources.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 24, 2024, 01:16:13 pm
content: Unlike mains, the conducted EMI limits for DC depends on the application. (For example, a vehicle electrical system would be far more tolerant of EMI than a power rail inside instrumentation equipment.) I would say just get a variety of RF chokes and film/ceramic capacitors, exactly what would be used to solve DC supply EMI issues in the real world. Maybe also some regular inductors in case some user doesn't know the difference. (RF chokes are intentionally lossy to prevent them from ringing, while regular inductors try to minimize losses.)For simulation, the datasheet for the chokes should give some values that can be used to build a simulation model.

author: mtwieg
author_rank: Frequent Contributor
timestamp: May 25, 2024, 01:06:17 pm
content: I've never seen an LISN with damping resistors in parallel with the inductors. Some damping is provided by the resistors in series with the capacitors.Ultimately the standards specify the output impedance vs frequency of the LISN, see below for an example. How you choose to construct it is up to you. Though I would only use air core inductors and film capacitors.

author: jonpaul
author_rank: Super Contributor
timestamp: May 25, 2024, 01:11:18 pm
content: define "basic testing"?Use schematic and inductor design from LISM mfg like Solar, not an app note or simulator.j

author: Faringdon
author_rank: Super Contributor
timestamp: May 26, 2024, 11:14:22 am
content: Thanks, by "basic testing", we just want to check the LISN doesnt instigate input filter oscillations in the DCDC when we run the DCDC .Also, when we apply vin as a sudden step input, with the LISN in the supply, we dont want to see high overvoltage peaks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 26, 2024, 04:08:04 pm
content: Quote from: mtwieg on May 25, 2024, 01:06:17 pmCISPR 16-1 gives an example (clause F.7):QuoteTim

author: jonpaul
author_rank: Super Contributor
timestamp: May 26, 2024, 04:35:28 pm
content: ..."just want to check the LISN doesn't instigate input filter oscillations..."Your premise is nonsense as usual have never seen this.Note DC and AC LISN have different spec and construction.Recommended procedure;Examine Solar Electronics and other mfg LISN, select appropriate model.just Purchase oneor Lease it one monthj

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 27, 2024, 03:13:04 am
content: Quote from: Faringdon on May 26, 2024, 11:14:22 amSimulating the impedance of a DC supply can easily be a lot more difficult than for AC. Mains wiring is bound by electrical codes, therefore compliant installations would use one of a few approved types of wire and routing. That means most of the time, the characteristic impedance of the line would be pretty predictable.DC has no such universal rules. In particular, it's common in vehicles to run one line for positive and use the chassis as negative, often giving a large loop area and a very unpredictable impedance. The steel chassis being ferromagnetic would further increase the inductance, albeit rather lossy.

author: Gyro
author_rank: Super Contributor
timestamp: May 27, 2024, 08:13:19 pm
content: https://www.elektor.com/products/elektor-dual-dc-lisn-150-khz-200-mhz?

author: Faringdon
author_rank: Super Contributor
timestamp: June 01, 2024, 12:30:20 pm
content: QuoteThanks....when one is using the absolute minimised input capacitance with the SMPS, then attaching a 50uH LISN can definetely tip it over the edge and give input filter oscillations...there is a direct correlation between input filter instability and the impedance of the SMPS's input filter ("seen" looking back to it from the SMPS's power stage "input terminals")....the LISN is effectively part of the SMPS input filter....the SMPS doesnt "know" its a LISN.QuoteThanks, but we must use a 50uH LISN....as its higher impedance, and more likely to give the input filter instability.

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 02, 2024, 01:29:07 pm
content: Quote from: Faringdon on June 01, 2024, 12:30:20 pmReally? What standard are you referring to?

author: Faringdon
author_rank: Super Contributor
timestamp: June 02, 2024, 05:09:28 pm
content: Rail standard

author: floobydust
author_rank: Super Contributor
timestamp: June 02, 2024, 07:32:15 pm
content: What standard is that? How much does it cost? Never heard of it except mentioned in an Ozzy song, something about going off the rails...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 03, 2024, 12:58:57 am
content: It's EN50121-3-2, which references CISPR16, right?Tim

author: Faringdon
author_rank: Super Contributor
timestamp: June 03, 2024, 05:11:00 am
content: Thanks, its deffo the one that has the 50uH LISN.Which is why we need to assure operation with 50uH LISN

author: coppercone2
author_rank: Super Contributor
timestamp: June 03, 2024, 05:25:39 am
content: beware that the inductance measured will change if you put the chassis on

author: floobydust
author_rank: Super Contributor
timestamp: June 03, 2024, 09:34:35 pm
content: Trains here are 125VDC power systems.I wonder how you test to a standard when you don't even have it? OP is notorious for undersizing SMPS input caps, the ripple makes a mess. Even china uses larger value parts and does not design to the milliohm.CISPR does give a max. impedance spec for the inductor. It's what you would expect.