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Why don't they make power supplies like this...?

Discussion in 'Electronic Design' started by Mike Harrison, Dec 11, 2006.

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  1. With modern SMPSUs increasingly using active PFC front-ends, typically comprising a flyback step-up
    converter, it would seem almost trivial to also use this circuitry to implement :
    a) Soft-start to produce effectively zero inrush current. By effectively zero I mean no more than
    the normal full-load current draw of the PSU.
    b) Remote on/off, to allow sequencing of multiple PSUs, again to avoid inrush problems where
    multiple PSUs are running off the same mains feed.
    As far as I can see current PSUs with remote-on tend to do it after the reservoir cap.

    I've been looking at PSUs for an upcoming project that will need quite a lot of supplies
    distributed around in installation, and hence am concerned over inrush, and haven't yet found anyone
    that does the above...

    Have I missed something or does no-one else see this as a potential product benefit at negligible
    extra cost..?
  2. Joerg

    Joerg Guest

    Now you know why I end up rolling my own switcher almost every time :)
  3. What inrush current? For the line-side, this is usually handled by a
    cheap thermistor.

    It's unusual to have inrush problems on the DC side as the capacitors
    tend to be small.

    I had to add inrush limiting on an old huge Heathkit stereo-- big old
    80,000 uF capacitors made the house lights dim. Heathkit often
    bobbled a small detail or two.

  4. Look at the inrush spec on a typical 200W SMPSU - typically 10-20A at 230V
    Now run ten of these PSUs on the same mains power supply & watch the lights flicker when you hit the
    breaker to turn them all on.
    100-200A peak on a nominally 8A supply can't be a good thing.
  5. Mike Harrison wrote:

    I'd look at the size of the capacitors after the first bridge
    rectifier. Typically about the size of a chapstick up to maybe a C
    cell. If the PS doesnt have anything to limit the current before this,
    and you switch on the power at the peak of the cycle, yes, you're going
    to get a current spike. Most power supplies use a 40 cent thermistor
    to limit this current.

    If the PS has some power-factor correction circuitry before this point
    in the circuit, then you're right, you'd think that circuit could do a
    smart startup at a zero-crossing.
  6. There is an other problem too, with many of those on one phase (cable),
    that is that the current these draw is not a sinewave.

    More like this:
  7. Correction, no DC, no airgap, no.
  8. Genome

    Genome Guest

    Unfortunately the majority of PFC front ends use a boost converter as the
    front end. That means that the input supply is connected directly through to
    the bulk DC bus capacitor via the boost, or a bypass, diode at start up and
    there is therefore no inherent surge limiting.

    A flyback stage in this location might achieve what you are looking for but
    its output voltage is negative with respect to the input which makes
    interfacing some things tricky. Input and output current are both
    discontinuous as well which makes noise problems worse.

    Micro Linear (defunct) did have a method for doing things slightly
    differently but the main DC bus output voltage rode on top of the input
    rectified line so you end up with interface problems again. The method would
    avoid inrush currents and maintains continuous input current but the voltage
    rating of the power switch needs to be greater.

    The only topology (well one that comes to mind) is the SEPIC converter. It
    provides a positive voltage referenced to the same ground as its control
    electronics. However it is slightly more complex/harder to design and places
    more stress on the components used. In particular the main switch sees a
    peak of (VIN+VOUT) and has to carry a peak of (IIN+IOUT).

    It does overcome the major inrush current that would arise from charging the
    main DC bus capacitance but the coupling capacitor still has to be charged
    so there is still some inrush but it is of much shorter duration. If you
    have a look about the TI website you should be able to find a couple of
    papers by Lloyd Dixon that deal with the design and compenstaion of a SEPIC
    converter for use as a PFC front end.

  9. Guest

  10. On a sunny day (Tue, 12 Dec 2006 13:18:49 GMT) it happened Jan Panteltje

    Oops, an other correction:

    --- ~ + -- L ------
    AC bridge ===
    rectifier ---
    --- ~ - ----------

    And yes an airgap in the inductor.
    Hope I got it right this time.
  11. Ross Herbert

    Ross Herbert Guest

    Fairchild have the ML4826 and the ML4841 which combine PFC with soft

    Looking at the MOQ of 1000pcs @ $5.52 (ML4841) it is not hard to see
    why there are so few PC psu's with such functions. The PC
    manufacturers would prefer to use a much cheaper option to keep costs
    down. If that means providing only the PFC function as specified by
    the new requirements then that's all they need do to conform and the
    addition of the "nice to have but not necessary" soft start gets
    dispensed with.
  12. nospam

    nospam Guest

    A typical PFC front end has a bridge, a small inductor, a diode then the
    main smoothing capacitor. There is nothing you can do with the PFC MOSFET
    to prevent inrush when the smoothing capacitor is initially discharged.

    I don't know much about mains switchers, I presume the smoothing capacitor
    can be smaller with PFC because it gets charge over more of the mains cycle
    so inrush could be less anyway?

  13. Ross, I see no soft start in those ML46XX ICs. that you pointed out. They
    are the standard boost topology of PFC ICs but a soft start can be attained
    by inserting an input current limiting resistor, like 25R in series with the
    low side of the bulk cap to ground. A power MOSFET is added in parallel with
    the resistor and turned on after the bulk cap has charged.
  14. legg

    legg Guest

    The smartest thing is not to duplicate the AC front end n times, Have
    one power supply connected to the mains and run the others as dc-dc
    off of it's outputs.

    You say these are 'distributed' around an 'installation'. Perhaps
    reviewing methods of distribution are warranted. Perhaps your intended
    method doesn't really create a inrush problem.

  15. legg

    legg Guest

    I think you're looking at this the wrong way.

    You are designing the power interface for this equipment. It might be
    worth while doing a bit of research before just bringing in a bunch of
    120VAC power supplies.

    Everything does not run off of 120V, unless you make them do it.

  16. JoeBloe

    JoeBloe Guest

    Most well designed supplies do soft start.
    With soft start, shouldn't be needed. If you are running a circuit
    at close to it max rated spec, you should be placing some of your
    load(s) on a different mains circuit branch.

    A PSU can be designed and configured to respond any way you want it
    to, within the reason of the technological limitations.
    You want a set of custom behaviors in an off-the-shelf mentality
    search mindset. You are going to have to compromise and get something
    that almost gets you there, or actually get a custom design
    requirements evaluation and quote on a customized off the shelf item,
    or a full custom all-for-you design.
    Engineering time is not cheap, and YOU would STILL have to bear the
    initial tooling/NRE costs, if any in a custom design. It wouldn't
    matter what subsequent benefits the vendor may or may not reap later

  17. JoeBloe

    JoeBloe Guest

    Only as large as needed by the power requisite and the noise figure
    Hey! You leave my favorite childhood company alone! :-]
  18. Ross Herbert

    Ross Herbert Guest

    I haven't actually used either of these parts so I am going on the
    info the manf provides.

    The data sheet contains this info on p.10;

    Soft Start

    Start-up of the PWM is controlled by the selection of the
    external capacitor at SS. A current source of 50uA supplies
    the charging current for the capacitor, and start-up of the
    PWM begins at 1.5V. Start-up delay can be programmed by
    the following equation:

    CSS = tDELAY x (50uA/1.5V )

    where CSS is the required soft start capacitance, and tDELAY
    is the desired start-up delay.
    It is important that the time constant of the PWM soft-start
    allow the PFC time to generate sufficient output power for
    the PWM section. The PWM start-up delay should be at least
    Solving for the minimum value of CSS:

    Css = 5mS x (50uA/1.5V) = 157nF

    Caution should be exercised when using this minimum soft
    start capacitance value because premature charging of the SS
    capacitor and activation of the PWM section can result if
    VFB is in the hysteresis band of the VIN OK comparator at
    start-up. The magnitude of VFB at start-up is related both to
    line voltage and nominal PFC output voltage. Typically, a
    1.0ìF soft start capacitor will allow time for VFB and PFC
    out to reach their nominal values prior to activation of the
    PWM section at line voltages between 90Vrms and
  19. JoeBloe

    JoeBloe Guest

    No. Thy retain soft start. NTBF goes down the tubes otherwise.
  20. This refers to starting the PWM IC but the bulk cap has already been
    charged by the input supply, look at the schematic!
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