Electronics > Power/Renewable Energy/EV's

50uH LISN for basic testing

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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.


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.

(...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?)

FTTS nonsense.
Real solution, a commercial LISN


Very good design and safety.





More sources.

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.

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.


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