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Inrush current protection with Switch mode Power supplys.. How to do this?

Discussion in 'Power Supplies' started by mrpackethead, Sep 23, 2010.

  1. mrpackethead

    mrpackethead Full Time Elf

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    Heres my problem,

    The front end of most SMPS is a bridge rectifier and a set of caps. When you first turn them on, the "inrush" current to charge the caps can be 10-20x times the normal run time current.. Typically just for the first half wave of AC. So, a 350W supply might be as much as 30A.. If you turn on 20 of these together, you've got 600A, and it will trip your circuit breakers for sure.

    What i need is a way to "soft-start" the power supply; so that i can reduce the load;

    Options I can see.

    (1) Stagger the turn on.. so, that they are just turning on one at a time; thuse reducing the total inrush current to a point where its probably ok.

    (2) Use power supplies with "soft start" on them.. Typically seem to be much more expensive.

    (3) Use A NTC device in series with the power supply. The theory behind this is that the NTC ( negetive temperture co-efficent ) resistor has a high resistance when cold, and a low reistance when hot. When you first turn on, its cold, so it effectively limits the current to a low value, as it warms up ( because its dumping power ), its reistance drops ( to a very low value ).. The effect of using this, is that the current draw on turn on is drastically reduced. .( it does mena that the power supply will also take longer to "turn on", but thats not really a bit problem ( we are taking 1-2 seconds rather than .05 sec ).

    There is a problem however, with these.. If you turn the power off, the NTC is still hot for up to 60 seconds.. If the power comes on during this period, they offer no in-rush protection.. ( becuase they are in their low reistance state ).

    So, i need a "power lock out". if the power goes off, its got to stay off untill i manaully reset it..


    Does anyone have any practical experience around this problem?
     
  2. David_AVD

    David_AVD Bite my shiny metal ass!

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    Power sequencing is the usual solution to this problem.
     
  3. OP
    OP
    mrpackethead

    mrpackethead Full Time Elf

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    Got another idea;

    NTC in series with the Power supply...
    Triac in parralle with the NTC

    When power supply turns on, ( which is a few cycles after the inrush ) the power supply can switch the triac on ( via an opto ). It shorts out NTC, so no current is flowing and it cools. If power goes off, and comes back on, the triac will be off, so the NTC will again be the only current path.

    Its simple and its easy i think.
     
  4. David_AVD

    David_AVD Bite my shiny metal ass!

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    It all depends on how far you want to go.

    Some amplifiers use an NTC (or power resistor) in series with the transformer primary. When the secondary comes up, they wait a second then short the NTC out allowing full current to be drawn.

    I have seen a TRIAC used instead of the relay contact too.

    The NTC parts are often called "Surge Guard" or similar. (not to confused with a MOV)
     
  5. AAH

    AAH I love blinky lights :) Community Project Designer

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    An NTC inline with the power with a relay contact driven off the DC supply. The DC output typically doesn't turn on for a second or so after the power comes on so the NTC has performed its function before the relay shorts out the NTC and the NTC then cools again. The alternative would be to use an on-delay relay off the mains to short out the NTC. If you're lucky you can sometimes pick up on delay timers for under $20. A couple dollar relay off the output of the power supply is about the cheapest alternative.
     
  6. JonB256

    JonB256 Full Time Elf

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    Not sure how effective or cost effective, but an inductive coil (choke) will act exactly as you describe, resisting initial current flow as it builds up its magnetic field, then offering only its own Ohm resistance (with a slight phase change) once current stabilizes. Put it between the Rectifier bridge and the first capacitor.

    Pluses - it is virtually failure proof; good for decades

    Minuses - not sure how big (how expensive) it would be to get the desired result. It also will (when power is turned off) continue to provide an output voltage until the magnetic field collapses (5 time constants).

    You can use the primary or secondary of an old, abandoned power transformer
     

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