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    Protecting SMPS against mains transients.

    As can be seen, this LSP05240 Mains Surge module lets through up to 1100V of ‘residual’ voltage.
    Obviously, the surge module has to go upstream of the AC mains filter in the SMPS. As such, would you agree that when the 1100v of “residual voltage” hits the AC input LC filter of the SMPS, then the voltage at the output of the diode bridge will actually be 2200V?
    We are trying to protect against this and believe that a TVS after the bridge rectifier is the way, would you agree?

    LSP05240 surge module
    https://www.littelfuse.com/~/media/e...asheet.pdf.pdf

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    Re: Protecting SMPS against mains transients.

    1200 V according to data sheet - for a very short time ( the applied test pulse is very short ) - the volts at the bridge depend on the amount of C you have after the bridge and its ESR/ESL

    A string of TVS ( essentially zeners, say 4 x 150V ) should clamp any residual to 600V at the bridge ...


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    Re: Protecting SMPS against mains transients.

    e.g. SMBJ120A x 4 on the HVDC


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    Re: Protecting SMPS against mains transients.

    Hi,

    Basically every overvoltage protection includes:
    * a serially connected device to limit the current (even wiring)
    * a parallel connected device to limit the voltage
    For sensitive devices you may need several stages.

    In your case - for short pulses - a serially connected inductor may be suitable to limit the current.
    But an inductor does not dissipate much energy. The benefit is, that during normal operation there is not much voltage los, not much heating.
    On the other side the pulse energy is not dissipatad, thus the time (current waveform) may be stretched and/or ringing may occur.
    RC combinations may suppress ringing.

    I have good experience with:
    * big TVS, or gas filled overvoltage protection pill, or any other rugged ovvervoltage protection device (par)
    * pulse rated carbon resistor (ser), take care that the overvoltage creates no arc
    * small voltage limiting device (tvs, zener .....), (par) for finetuning of overvoltage

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.


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    Re: Protecting SMPS against mains transients.

    1200 V according to data sheet - for a very short time ( the applied test pulse is very short ) - the volts at the bridge depend on the amount of C you have after the bridge and its ESR/ESL

    A string of TVS ( essentially zeners, say 4 x 150V ) should clamp any residual to 600V at the bridge ...
    Thankyou...we have [1.8uF film cap plus 200nF] behind a diode (its for the 10W flyback and the diode is so the Power factor is not detrmiented) And there is 130nF oif ceramic caps after the bridge aswell.
    So, this 1200v of let through........the TVS alone will deal with it?.....we dont need a TVS_and_current_clamp_combination?....(there is eg a tnyswitch which has 725v peak drain voltage)

    Basically, the " let through" is "weak" (?) and so just a few TVS's can sort it out and keep the Voltage below 725V?



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    Re: Protecting SMPS against mains transients.

    correct ...



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    Re: Protecting SMPS against mains transients.

    Thanks, presumably thats because the combination of the AC filter section and the TVS's ensures the downstream SMPS circuitry sees no more than ~600V (ie not the 1200v of let-through)?



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    Re: Protecting SMPS against mains transients.

    do you have any CM chokes with measured leakage L on the input ...? and of a certain resistance ...?



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    Re: Protecting SMPS against mains transients.

    Thanks, yes , our common mode choke is this one
    by wurth
    7448640415
    https://katalog.we-online.com/pbs/da...7448640415.pdf



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    Re: Protecting SMPS against mains transients.

    May i please ask, Which one of the input stages to a 60W offline Flyback SMPS is preferable?
    (please see attached schem and LTspice sim)
    They differ only in the TVS’s (ringed)

    The top one can shunt 5.1A at 584V so appears better than the bottom one, which can only shunt 4.6A at 648V. However, the top one uses TVS’s in series, and I suspect that due to part-to-part tolerances, this renders one of the TVSs as being susceptible to damage during transients. Would you agree?


    SMCJ TVS family datasheet:
    https://www.littelfuse.com/~/media/e...asheet.pdf.pdf



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