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Dual output switcher; 5V and 3.3V

Jeff Starkell
Intellectual 490 points
Other Parts Discussed in Thread: LM2594HV, LM34910C, LM46000

I have been asked to provide a power supply with the following specifications:

  1. Non-Isolated
  2. AC/DC input
    1. The AC input terminals already have a bridge rectifier input on the ac side is 12 to 30Vac
    2. Input range for the DC terminals is 12 to 50Vdc
    3. Input current is limited to 100mA via a polyfuse
  3. Output 1: 3.3Vdc @ 250mA
  4. Output 2: 5.0Vdc @ 50mA

So there are many possibilities.......

  1. Use a simple switcher for example a LM2594HV for the 5.0V output and then use a LDO linear regulator to step down to 3.3V
    1. +Simple
    2. - Not very efficient
  2. Use two simple switchers for example a LM2594HV for the 5.0V output and another for the 3.3V output
    1. +Simple enough
    2. +Efficient
    3. - Not sure if having two of these running in parallel will create EMC issues
  3. Use a dual output switcher possibly a LM34910C

I would to hear your thoughts and recommendations from the power supply guru's

  • TI__Genius 16025 points

    Hi Jeff,

    Happy to help! Approach 2 is the best. Here is our thought process:
    a) Your current needs are low and hence efficiency will be good using a switcher.
    b) Battery/DC power would benefit from a switcher thanks to higher efficiency hence longer life.
    c) EMC issues will be at a minimum since currents are really low.
    d) advanced feature set would allow customization.
    e) Why not use the LM46000? Slightly more expensive IC, but passives are cheaper and solution size tiny with 80%+ efficiency at light loads, plus it is EMI optimized.

    Hope this helps,
    Thanks,
    Anston

  • in reply to Anston Lobo
    Intellectual 490 points

    Anston,

    Thanks for the prompt response. I'm going with the LM46000.

    I have a question regarding the Input Transient Simulation of current (IIn), How do I change the scale, defaults to +/-50A, and 1A would be more appropriate.

    Thanks

  • in reply to Jeff Starkell
    Intellectual 490 points
    Anston,

    So I have 2x LM46000, one for 3.3 and one for 5.0. They're both being fed from 18Vac via a fullwave bridge.

    What EMI filter(s) would you recommend for this circuit can I get away with one feeding both simple switchers or should there be one for each?
  • in reply to Jeff Starkell
    TI__Genius 16025 points

    Hi Jeff,

    Any switcher powered off the main supply would need an input EMI filter. The chopping action of any DC-DC switcher will propagate to the input and output, however the output has a shielded inductor and capacitor which is a already a complex filter to filter out EMI. Hence only the input needs to be protected individually for each.

    Let me know if you have any further questions.

    Thanks,

    Anston

  • in reply to Jeff Starkell
    TI__Genius 16025 points
    Hi Jeff,

    Where you able to figure out the scaling on the input transient simulation?

    Let me know,
    Thanks,
    Anston