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85-265VAC and 24VDC combined power supply

Discussion in 'Electronic Design' started by [email protected], Aug 10, 2007.

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  1. Guest

    I think you will need primary coil divided into two sections. One for
    85V-265VAC and one for 24VDC. Maybe you could select the extra long coil
    via some zener + transistor. Ie if the voltage is over a certain level
    the zener will conduct to the transistor wich will activate etc..
    Just a quick brainstorm..
  2. Hello,

    I am currently working on a device where I am evaluating options for the
    design of the power supply. On the secondary side we need two power
    supply output with 5V/4A and 13,8V/2A (uncommon voltage). The input
    voltages are the standard 85V-265VAC range and 24VDC.

    We want every unit to be capable of being used in both environments.
    Therefore the power supply should allow both inputs. Because of 24VDC
    input I will need two DC/DC converters at the output side. One from
    24VDC/5VDC and one from 24VDC/13,8VDC. These converters are easy to
    design and can have a high efficiency (>90-95%).

    What I am unsure about is how to handle the transformation from the
    primary 85V-265V to the intermediate internal voltage of 24V. A
    regulated switched mode power supply is certainly a overkill because I
    don't need the 24V to be regulated. So I thought I could use a standard
    50/60Hz input stage with a simple full bridge rectifier and a good
    transformer. Size is not the problem but power efficiency and costs are.
    Or are there any better options? I searched for quite some time in
    goggle for possible AC input stages but did not come up with a good

    Kind regards,
    Christian Walter
  3. Phil Allison

    Phil Allison Guest

    "Christian Walter"

    ** Yes you bloody well do !!!

    You need the 24 volts DC to be REGULATED against a 85V to 265V AC input

    Only one way to get that at low cost.

    A SMPS !!!!!!!!!!!

    ......... Phil
  4. Eeyore

    Eeyore Guest

    Bizarre ! A 'DC transformer' ?

    Brain fart more like.

  5. Eeyore

    Eeyore Guest

    That'll give you an output that's ~ 24V +/- (265-85/(265+85)) * 100%. That's
    roughly +/- 50% or 12-36V

  6. Hello,

    Maybe I was not clear. I mean I don't need a very exact regulation and I
    do not care if it is 20V or 28V because the input range of the second
    DC/DC converters will be 16V - 30V. I think it also does not matter if
    there is a certain ripple on that internal powerrail because this will
    be taken care of by the second regulation and the frequency of the
    ripple is much lower than the switching frequency of the DC/DC converters.
    Using a line frequency power supply means I would buy a decent good
    transformer, add a full wave bridge rectifier and only have to use a low
    cap filter capacitor. So maybe that the output is around 18V and
    accepting 4Vpp ripple.

  7. I forgot too mention. There can be a switch for selecting the 110V/230V
    variant and I will connect the primary windings in parallel or in
    series. Customers either use 110V and 24V/DC or 230V and 24V/DC.
  8. Eeyore

    Eeyore Guest

    Not a wide enough margin.

  9. Eeyore

    Eeyore Guest

    That puts an entirely different complexion on the matter. In that case it'll be
    fine with a line frequency transformer, rectifier and filter caps.

  10. Phil Allison

    Phil Allison Guest

    "Christian Walter the Wanker "

    ** You were clear - but unbelievably STUPID !!

    ** What about the fucking 85V to 265 VAC requirement ???????????

    IMBECILE !!!!!!!!!!!!

    ......... Phil
  11. Phil Allison

    Phil Allison Guest

    "Christian Walter the WANKER "

    ** Will still cost MORE than a SMPS with 85 V to 265 V AC range and 24 VDC

    IDIOT !!!

    ....... Phil

  12. Have you considered an off-the-shelf universal input line to 24VDC
    module? You can get a cULus etc. approved Meanwell 65W open frame
    supply for USD 18. 1-9, and as you note the DC-DC converters can be
    cookbook designs.

    Topology is typically flyback for universal input off line switchers..
    you could certainly design one. 50W or 60W isn't too bad. Another
    option is to change the specifications to dual input voltage and use a
    manually switched voltage doubler input stage as in PC power supplies
    (the rail is always 400VDC whether the input voltage is 120V or 240V).

    If you use a 50/60Hz transformer it will have to be ~200VA for FW
    rectification (nominally 240VAC 50/60Hz to ~21VAC), so it will be
    fairly large and heavy (and expensive). The transformer can be fairly
    efficient but you'll lose some power in the rectifier.

    Best regards,
    Spehro Pefhany
  13. Phil Allison

    Phil Allison Guest

    "Spehro Pefhany"

    ** Shame about the bridge rectifier, filter caps, AC switch , fuses,
    connectors, AC voltage selector and metal ENCLOSURE - PLUS never
    forget EMC and god knows what else red tape kinda approvals you must have.

    A mass produced in China SMPS with world wide AC supply compatibility and
    all approvals on tap is the go.

    I doubt anyone can sell you a " linear " unregulated 24 volt DC supply for
    under DOUBLE the cost.

    ....... Phil
  14. It's practical to do this, at least for low power consumption devices,
    but the efficiency of the following DC-DC converters suffers because
    they have to accommodate the 3:1 input range.

    Best regards,
    Spehro Pefhany
  15. Hello,

    Thanks a lot for your detailed answer. I though about using an open
    frame power supply but I am unsure about the mechanical aspects. One
    requirement is that there are terminal connectors with L,N,PE,+24V (DC),
    GND(DC) at one side of the PCB. Connectors are from phoenix contact and
    the series is the MKDS/3 (the typical green ones used in industrial
    applications). Using an open frame part means that I have to add some
    internal wires and have to think about howto mount it. I have not found
    any PCB mountable version of AC/DC converters with 40W and 24V output.
    I use a full-wave bridge rectifier and assume an average output current
    of 2.8A at 16V on the DC rail (I want the DC/DC converters to work
    optimal and so I try to have the input and output range to be near to
    each other). I use a bridge rectifier with schottky diodes to keep the
    voltage drop low. I used the following calculations:

    Efficiency DC/DC converters (for 5V/4A and 13,8V/2A) = 90% (guessed)
    POut = 40W (by design because in total the output modules will not use
    more than this. I mention this because 5*4 + 13,8*2 is greater than 40W).

    Therefore I have to supply an average power of 40W/0.9 = 44.4W at the
    internal power rail. Choosing 16V as the internal voltage when the AC
    supply is used gives me an estimate for the transformer with

    PTrans = 1.2 * 2.8A * 17 (2 * 0,5V for schottky) = 57,12VA

    Because the peak power will only be used for short times I though about
    using a 50VA toroidal transformer. The one which came to my mind is form
    talema, the 70083 (approx 20$ for a single unit). Or is this is to low I
    would go with the next bigger size rated at 80VA with a diameter of 90mm.

    Kind regards,
  16. Guest

    Any sane person will consider that it's about a switched psu. And that the
    turns ratio for 85VAC rms vs 24VDC will be too huge to be resonably
    compensated by pulse width.
    Rather use coil part A if 24VDC, and part A+B if input is 85VAC => 120Vpeak.

    Maybe you should fart some energy into your manners.
  17. And the transformer size drops to ~100VA.

    Best regards,
    Spehro Pefhany
  18. Eeyore

    Eeyore Guest

    I totally agree with this suggestion.

    The OP hasn't menioned any 'switch-on surge' requirement btw. It might be wise
    if he considered if his application needs it.

  19. Eeyore

    Eeyore Guest

    A line frequency transformer is going to weigh more than an open frame SMPS and
    that's important for pcb mounting.

    You do realise that an 40W DC load requires something larger than a 40VA
    transformer ?

  20. I think you'll have trouble getting this high over the input range.
    You need to check how to size transformers when used with this kind of
    load. 100VA is about right.
    Best regards,
    Spehro Pefhany
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