50uH LISN for basic testing

[Faringdon]

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.html

But 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?)

[jonpaul]

FTTS nonsense.
Real solution, a commercial LISN

https://www.solar-emc.com/LISN.html

Very good design and safety.

cheers

Jon

[Smokey]

https://www.tekbox.com/lisn/

https://www.emcfastpass.com/test-equipment/product-category/lisn-line-impedance-stabilization-networks/

More sources.

[NiHaoMike]

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.

[mtwieg]

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.

[jonpaul]

define "basic testing"?

Use schematic and inductor design from LISM mfg like Solar, not an app note or simulator.

j

[Faringdon]

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.

[T3sl4co1l]

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.

CISPR 16-1 gives an example (clause F.7):

The solenoidal winding of the inductor shown in figure 27 consists of 35 turns of a single layer of 6 mm diameter copper wire with an 8 mm pitch wound on a coil former of an insulating material. its inductance is greater than 50 µH outside the metal case and 50 µH inside the metal case.  The diameter of the inductor is 130 mm. in order to improve the electrodynamic stability of the winding, a 3 mm deep spiral groove is made in the coil former, and the wire is laid in this groove.

The higher frequency characteristics of the inductor are improved by sectionalizing the winding. Alternate sections, each of 4 turns, are each shunted by a 430Ω resistor. These act to suppress internal resonances in the inductor, which otherwise would cause the input impedance to deviate from the specified value at certain frequencies.

Tim

[jonpaul]

..."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 one 

 or Lease it one month 

j

[NiHaoMike]

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.

Simulating 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.

[Gyro]

https://www.elektor.com/products/elektor-dual-dc-lisn-150-khz-200-mhz ?

[Faringdon]

.."just want to check the LISN doesn't instigate input filter oscillations..."

Your premise  is nonsense as usual  have never seen this.

Thanks....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.

https://www.elektor.com/products/elektor-dual-dc-lisn-150-khz-200-mhz ?

Thanks, but we must use a 50uH LISN....as its higher impedance, and more likely to give the input filter instability.

[mtwieg]

Thanks, but we must use a 50uH LISN....

Really? What standard are you referring to?

[Faringdon]

Rail standard

[floobydust]

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...

[T3sl4co1l]

It's EN50121-3-2, which references CISPR16, right?

Tim

[Faringdon]

Thanks, its deffo the one that has the 50uH LISN.
Which is why we need to assure operation with 50uH LISN

[coppercone2]

beware that the inductance measured will change if you put the chassis on

[floobydust]

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.