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  1. #1
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    Inrush into offline SMPS EMC filter is too much for inrush resistor?

    Hello,

    We have the following EMC filter for our 60W offline Flyback SMPS LED driver prototype(attached).
    (LTspice Simulation of inrush event also included)

    Do you think the 2R7 ROX3SJ2R7 inrush resistor will be able to survive the inrush at power on, which happens once per day? (I mean, will it survive for many years of this?)

    The inrush event is only 30us long with peaking to 800W and 1000W

    ROX3SJ2R7
    https://www.mouser.co.uk/datasheet/2...1_E-777485.pdf

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  2. #2
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    Re: Inrush into offline SMPS EMC filter is too much for inrush resistor?

    The mfr doesn't see fit to rate anything for pulsed stress,
    other than terminal voltage. I wouldn't speak for them.

    I would probably start by looking at the simulation result
    (not shown) for the Joule energy deposited in the resistor,
    figure "thermal time" is milliseconds and use the resistor
    mass and some WAG at specific heat to figure a temp
    rise, and go back to the datasheet's derating curves
    which seem to top out at 200C for long term reliable.
    Maybe figure a 100C rise as allowable for the pulse
    event (70C to 170C) having margin at both ends.

    Then see what you see.

    Or go to a vendor who provides pulsed current
    stress ratings.



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    Re: Inrush into offline SMPS EMC filter is too much for inrush resistor?

    Really shouldn’t be hard to find an alternate with a pulse rating or maybe better, an inrush NTC.

    Also in general the circuit and filtering there seems a bit complicated..


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    Re: Inrush into offline SMPS EMC filter is too much for inrush resistor?

    Dale Resistor provided joule stress ratings
    now Dale is owned by Vishay:
    https://www.vishay.com/docs/49076/_w...l0396_1604.pdf

    and this - page 8 looks a lot like the chart I used 30 years ago
    https://prod-ng.sandia.gov/techlib-n...981/810569.pdf


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    Re: Inrush into offline SMPS EMC filter is too much for inrush resistor?

    I agree with post #2, a quick sim ( 300VDC applied for 2mS ) will tell you the joules in the R, and from this you can approximate the Trise per pulse, also the DC bus caps for your 60W converter seem small....?

    to a first approximation, C = 660nF, therefore energy in R = 34.8mJ ( 1/2 C V^2), in one cycle of mains this is 1.74W - so very likely survive - for a long time.
    Last edited by Easy peasy; 21st April 2019 at 22:18.


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  6. #6
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    Re: Inrush into offline SMPS EMC filter is too much for inrush resistor?

    Thanks, Yes the bus caps are small because we needed PFC since it is for driving LEDs.
    Last edited by treez; 21st April 2019 at 23:04.



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    Re: Inrush into offline SMPS EMC filter is too much for inrush resistor?

    Well, if the input capacitance to the led converter is too small - you will have issues with your control loop ...



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