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SMPS- Battery input to ac to dc converter

balasubramani r
Expert 1170 points

Hi Every one ,

As im planning to use DC source (120VDC)  to power up the smps. As far as my understanding there is not big difference in using DC apart from considering fuse and EMI filter .

Please advise  

  • 0
    Prodigy 5 points

    Hello  Balasubramani R,
    This is a very good question.

    The standard U.S. AC outlet voltage is 120VAC rms +-10%
    The peak voltage is Vrms x sqrt 2 = Vrms x 1.414.

    120Vrms = 170Vpeak
    240Vrms = 340Vpeak

    Many devices that use AC wall power have a simple bridge rectifier or bridge doubler at the input.
    The bridge rectifier will peak detect the AC, creating a voltage that is 170DC with a lower valley due to ripple.
    DC voltage will pass through the bridge.
    Thus you need an ~170VDC source to equal the rectified 120VAC voltage.

    However if the SMPS has a bridge doubler on the input there will be problems.

    As the picture below shows, when the switch is in 117V (U.S.) it acts as a voltage doubler however it only doubles on AC input, not DC.

    TI Power Supply Design Seminar Resources
    http://www.ti.com/ww/en/power-training/login.shtml?DCMP=pwr-psds-archive&HQS=pwr-psds-archive-psds

    Line Input AC to DC Conversion and Input Filter Capacitor Selection
    http://www.ti.com/lit/slup060

    When in the doubler mode, switch closed, applying a positive DC voltage will only charge C1 of the 2 series input capacitors.
    Circuits like this are common in consumer electronics.
    Desktop PC’s is a good example, most have a 110/220 switch on the rear of the system.
    The SMPS therefore is expecting a Vbus of 340VDC.
    You can place the switch in 230V mode and apply a 340VDC and the SMPS might work.

    However if the SMPS employs a PFC Power Factor Correction circuit on the input, then applying a DC input will likely not work. The PFC circuit is expecting AC, not DC.

    Another issue can be that some SMPS may have other housekeeping circuits that rely on an AC input.

    Yes the EMI filter does not really care if it sees AC or DC.
    The rest of the SMPS might.

    Bottom line is, check with the SMPS vendor on whether it will work with a DC applied.
    And if so, what the VDC should be.

  • 0 in reply to Ed Walker
    Expert 1170 points

    Hi Ed,

    Thanks for the detailed information. 

    My question is if it is universal standard input range it suppose to work from 85or90VAC, then for 85VDC the peak is 120VDC. Considering this 110VDC input may not create that much problem. 

    If my circuit employed with PFC booster then this 110VDC may be good enough to work because it provide steady voltage not like AC. 

    correct me if im wrong .  

  • 0 in reply to balasubramani r
    Prodigy 5 points
    Your math is correct, Balasubramani R.

    85VAC rms = 120V peak
    If the SMPS you are powering only has an input bridge rectifier then chances are it will work.
    Although the input currents will be higher at the lower Vin.
    The closer to 170VDC you can get, the lower the stresses will be in the SMPS.

    However if the SMPS has a 110/220 selector switch applying DC will not work.
    The SMPS may also have an "automatic line sense" circuit, that configures the bridge for voltage doubler vs standard bridge.
    With 120VDC it should select voltage doubler mode, which will not work without AC input.

    If the SMPS has a PFC front end it might work, or you might let the smoke out of it.
    The PFC IC's have components that rely on AC being present.

    We strongly suggest to consult the SMPS manufacturer for DC input operating conditions.