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Modelling a Mains Input Filter in LTspice?...input Z of LED current regulator?

Discussion in 'Circuit Simulation & PCB Design' started by Flyback, Apr 17, 2017.

  1. Flyback

    Flyback Well-Known Member

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    Hello,
    Please advise on how to model our Mains Input Filter in LTspice for our LED power supply.
    In our Non_switch_mode, offline LED current regulator, we have the Mains Input Filter as in the attached diagram “_Mains Input Filter”
    We have done the AC sweep of it in LTspice by modelling the LED current regulator as a 10k impedance. (as in the attached LTspice schematic called “_Mains Input Filter _LTspice” We chose a high impedance because Current sources are high impedance.
    Our LED current regulator works by kind of switching in different banks of LEDs as the mains rises and falls in the mains cycle.

    So do you think it is reasonable to model the LED current regulator’s input impedance as a 10k resistor, as we have done in the schematic of the filter attached?
     

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  2. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Likely, it is not appropriate to sweep the filter in the frequency domain. Since you are "switching", you need to characterize your filter in the time domain...
     
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  3. crutschow

    crutschow Well-Known Member Most Helpful Member

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    As Mike noted, you need to simulate in the time domain.
    You need to simulate the complete circuit with AC input including the LED current regulator and switching.
     
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  4. dave

    Dave New Member

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  5. Flyback

    Flyback Well-Known Member

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    Thanks, i have simulated it in the time domain, completely, and its fine. But we have an EMC scan which shows that we are 35dB too high at 150khz...so therefore i need to design a mains input filter which assures 35dB of attenuation at 150khz. As such i need the frequency domain scan. I have done an excel calculation to get the Magnitude vs frequency plot, but wished to check my calculations with ltspice. My plot from excel conforms to the one in ltspice, but i am not sure what impedance to use to model the input impedance of the LED current regulator?
     
  6. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    So why are you modelling a 50Ω source impedance? If it is a real 50Ω generator, then move the "In" node name to the right end of R1. If R1 is part of the actual circuit, then make Rs of the source 10KΩ or higher to simulate a current source.

    Either use a voltage source with a big resistor in series with it, or use a current source directly.
     
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  7. Flyback

    Flyback Well-Known Member

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    its a 50 ohm source impedance because when the regulatory people do mains conducted emissions testing they use a LISN upstream of the unit under test, and it has an impedance of 50 ohms, or at least it does from about 100khz upwards.
     
  8. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    So if your simulated source is a voltage source with a 50Ω output impedance, then make V1's Rser=50Ω. Still haven't told us if R1 is part of your actual circuit that you are testing, or it was supposed to account for the source impedance?

    So what/where does a current source enter into the picture?
     
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  9. Flyback

    Flyback Well-Known Member

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    Thanks, the current source it what the Mains Input Filter is feeding into.....the output of the current source is the LEDs.
    The R1 is not part of the circuit under test, i put it there to represent the source impedance of the supply that the test house uses when they use their LISN.
     
  10. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Flyback,

    1)I think the noise is from your "light bulb" so I moved the signal source to the right hand side.
    2)The power line is very strange and unpredictable, For test purposes it is molded as 50 ohms. For you tests they force the power line to look like 50 ohms at the frequencies of interest.
    3)To get a model of your filter you must make real world capacitor and inductors. You must model the resonant frequency! You must have the internal resistance added.
    4) Right or wrong this is what I do.
    upload_2017-4-17_15-41-17.png
    We can talk about common mode vs differential mode noise, later.
     
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  11. Flyback

    Flyback Well-Known Member

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    Thanks, this paper....
    http://www.ti.com/lit/an/snva538/snva538.pdf
    ..shows the source being on the left. They also speak of the importance of knowing the input impedance of the power supply, because its impedance must be more than that of the output impednace of the filter otherwise input filter oscillations can occur as per the middlebrook criteria.
     
  12. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Thanks for the paper.
    True, it does not matter which end the noise is injected into. (just hurts my head to think backwards)
    Next time I will watch the input impedance of the power supply. In spice it is easy to set the noise impedance. (not as easy using a real signal generator)
    Thank you
    RonS
     
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