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Switch mode powersupply designs

Discussion in 'Electronic Design' started by Mich, Jun 10, 2004.

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  1. Mich

    Mich Guest


    I know it has been covered before, but i need help on the design of an
    switch-mode powersupply.

    Input : 380 up to 460 Vac (Euro and USA)
    Frequency : 50~60 Hz
    Output : 12Vdc 250mA + 5V 50mA (might use 78xx for the 5v)
    Temp : -30 to +75 degrees centigrade

    Design has to be small, but very reliable.

    Thankx for some help.

  2. Mariano

    Mariano Guest

    For the voltage and power requirements, I suggest a flyback SMPS. It is
    simple, reliable and has minimum components. You will need:

    -EMC filter for the mains input
    -Rectifier and capacitor
    -High voltage mosfet
    -Control IC
    -SMPS transformer
    -Diodes and caps for the output
    -Optocoupler for feedback

    I have designed many circuits using On Semi NCP1200 and found to be an
    excellent choice for this kind of SMPS. You can also use ICs that have a
    mosfet integrated in them (I believe NCP series of On Semi has some).

    To have two output voltages, design your own transformer and set the turns
    ratio to have 12v and 5v as outputs. Set one as master, regulate it and the
    other will track within 5%. I think you can do this as small as 2" x 2" if
    using SMT.

    Some links:

    NCP1200 100kHz Through Hole:,4450,NCP1200A,00.html

    A 70W Low Standby Power Supply w/NCP120x Series:

    Implementing the NCP1200 in a 10 W AC/DC Wall Adapter:

    Transformers for ON Semiconductor NCP1200:
  3. Mich

    Mich Guest


    Thank you very much for your extensive reply.
    This will me a lot.

    Thankx again,

  4. R.Lewis

    R.Lewis Guest

    Odd supply voltages. Where do they come from?
  5. Mich

    Mich Guest

    Euro 3 phase is 380 volts, i was told by my customer that USA 3 phase is
    about 440~460 volts, please correct me if i'm wrong.
  6. Mariano

    Mariano Guest

    As far as I know, Europe and most countries use 220VAC single phase, which
    gives 380VAC three phase. Remember to multiply by 1.414 to get the peak
    rectified voltage.

    Note that for 380VAC, you are getting 538VDC. You will need a rugged mosfet
    and a good snubber design. Watch out for the flyback voltage. Say you use a
    Coilcraft transfomer with 6.25:1 turns ratio, then for 12V, your flyback
    will be 538+12x6.25 = 613V. Add some leakage spike and you'll be working
    with 700~800V. You can solve this by changing the turns ratio or using an
    interleaved flyback, but I recommend you to get a rugged mosfet and keep it
    simple. Try a smaller turns ratio and a good snubber design.

    So far, in the US I found 208VAC three phase and 440VAC three phase. I guess
    one comes from 120VAC single phase. Ask around which one is more common for
    three phase in USA. 440VAC is a very high voltage to be rectified and

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