SMPS Multiple Voltage Output - A few questions

Discussion in 'The Projects Forum' started by LatAsh12, Sep 2, 2016.

  1. LatAsh12

    Thread Starter New Member

    Jul 28, 2016
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    I have to build a Switch Mode Power Supply with multiple outputs. I've seen many projects with just a single output , as well as projects with variable voltage output, such as using an lm317. However, I'm unsure as to whether the variable voltage output is the same as multiple voltage outputs - though logically it is very different. So, here are the basics of where I am with my project so far and how I would like to move forward:

    The SMPS has to output three voltages: 12V. 5V and 3.3V. In this respect I've researched flyback converters. I would like to know how well these work. I have checked out this site which comes closest to what I need (I think) -
    https://www.digikey.com/reference-designs/en/ac-dc-and-dc-dc-conversion/ac-dc-smps-multi-output/36

    I have not decided yet whether to step down from mains (220VAC) or from DC batteries. If DC is chosen, will the battery power have to be well above (12+5+3.3)V?

    These are all the questions I have for now. Thanks!
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,094
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    There are instructions around for changing the output voltage of a scavenged computer PSU. Those are all SMPS these days, and the hack does not involve a linear solution like the LM317. So at least the adjustable voltage part of your challenge should be doable.

    Do you need the three output voltages all at the same time?
     
  3. LatAsh12

    Thread Starter New Member

    Jul 28, 2016
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    Hi, yes I need three output voltages all at the same time. One to power a 5V MCU, a 12V speaker and a 3.3V SD card simultaneously
     
  4. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,034
    1,632
    Why do you need to build it? There are countless mass produced units available for both line power or battery based power for way cheaper that you will ever build your own for that provide those voltages together.
     
  5. Marcus2012

    Member

    Feb 22, 2015
    349
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    As Wayneh said using an ATX power supply for a PC would probably be the best option here as it always has 3.3, 5 and 12V outputs with decent power capacity. Just remember to deactivate undervoltage protection and ground the overvoltage sense pins.
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    The last ATX PSU I hand traced, summed the 3.3V and 5V rails into a TL431 circuit. The 12V rail relied on tight coupling to be "near enough".

    On high end network equipment - you sometimes find multi-rail SMPSUs with individual chopper circuits for each rail.
     
  7. LatAsh12

    Thread Starter New Member

    Jul 28, 2016
    6
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    It is a design project for my Engineering course
     
  8. LatAsh12

    Thread Starter New Member

    Jul 28, 2016
    6
    0
    Yes, I do need all three at the same time. the 12V has also been changed to 16V. There are more details in my reply.
     
  9. LatAsh12

    Thread Starter New Member

    Jul 28, 2016
    6
    0
    Because it is a design project, I have to use components from scratch to make this power supply.

    The specs are as follows:

    Topology: Buck Converter
    Input: 220Vac mains
    Outputs: 16Vdc, 2A (Powering an amp/speaker)
    : 5Vdc, 200mA (Powering an ATMega32 chip)
    : 3.3Vdc, 80mA (Powering an SD Card)

    I have an understanding of the components that are needed. The AC mains need to be stepped down with a transformer and then rectified to dc voltage, and then regulated. From there the buck converter will convert this regulated voltage to the voltage I need, which I'm choosing to be 16Vdc so I can step it down for the other 2 outputs. I also need a feedback loop.

    The first question I have is of the transformer. Can I or can I not use a secondary winding of 12Vac? Once rectified using a full wave IC (I have KBL06 in mind) it should be a higher dc voltage but at the expense of current. I don't really understand this part of the design at all - which transformer to use, what calculations to do to get to the dc I want out of the rectifier (which I want to be approx 24Vdc). This will be stepped down with the buck converter to my desired 16V output. The other concern here is the high current required of 2A.

    The second major issue I have is regarding the multiple outputs. I want to use regulators to step down the 16V to 5V and the 5V to 3.3V - is this approach acceptable? And what expense will this have on the current?

    I have seen many designs online but none have an output of 16Vdc from mains, and there's also the problem of the multiple output. I understand that it is designing from first principles to get cap, resistor and inductor values, and I have looked online for these calculations with no success.

    The more compact the design, the better. IC chips are the best bet, and I'm trying to find the correct and most efficient ones.

    Help will be greatly appreciated.
     
  10. Marcus2012

    Member

    Feb 22, 2015
    349
    26
    Hi :)

    My advice would be to buy a new or used Laptop PSU which is regulated for a 16VDC output at about 4.5A like this one. You could use a buck/boost IC like the MC34063A to gain your 5V for your uC as using the linear LM1117-5 would probably get a little hot. I'm assuming you're using a SPI interface for your SD card as it's the only free licence and 3.3V configuration but you may consider getting a prefabbed module like this as you will need to have logic level shifters as well as the regulator (use the linear 3.3V as it comes ready made, no need for SMPS here). Just make sure you get one that shifts the MISO too otherwise it holds the MISO hostage and you can't use any other SPI device (it was a early design flaw on some modules). You might also want to use filter capacitors between each stage so the amplifier doesn't affect the uC timing.

    FYI: THE_RB has published a completed projected using the 34063 that you might find a useful read and there are probably more completed ones in the forum somewhere too :)

    12v to 5v DC high efficiency SMPS buck converter using 34063 IC.
     
    Last edited: Sep 25, 2016
  11. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,438
    368
    You have a lot of don't knows.

    Usually, in a mains to DC switchmode power-supply, the mains is rectified first and then chopped at a high frequency which means a much smaller transformer/ inductor can be used. If you drop the mains voltage first using a conventional 50/60Hz transformer before rectification you lose that size and weight advantage.

    However, I don't recommend you do it this way and am surprised that the input is specified as 220VAC, is the college trying to reduce the student count this year?

    https://en.wikipedia.org/wiki/Switched-mode_power_supply
     
    Last edited: Sep 26, 2016
  12. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    And it's worth noting that we can't talk about the project here unless you use the isolating transformer as the first step, per the Terms of Service.
     
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