Building a simple SMPS

Discussion in 'The Projects Forum' started by BrainFog, Sep 23, 2011.

  1. BrainFog

    Thread Starter Member

    Jan 24, 2011
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    I wish to have a go at building a simple switch mode power supply but am beginning to wonder if there is such a thing.

    I understand the basics of how they work but this is of very little help when designing them and I am having difficulty finding clear simple schematics or design ideas online. It seems very common and fairly simple to build DC to DC switch mode power supplies just not mains AC to DC.

    My thought was to build one as a battery charger to see if my idea would work. I was thinking 40.5v output with adjustable current limiting of say 1 amp to 3 amps. That would be to float charge 3 12v lead acid AGM batteries in series each at 13.5v.

    How easy would it be to construct a SMPS such as the one stated above and approximately how would I go about doing it?

    Thank you
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    It is not very hard to build a simple DC/DC SMPS if you follow some basic PCB layout tips. Depending on your knowledge and experience, you can build one that works or you can build one that works with good performance.
     
  3. BrainFog

    Thread Starter Member

    Jan 24, 2011
    122
    4
    I was hoping to have a go at a making one to turns mains AC 220-240v to DC.

    From my understanding is you must convert AC to high voltage DC using a bridge rectifier which is then smoothed and fed to some kind of H bridge which is controlled from the isolated side. This then puts square wave AC through a transformer at a high frequency and so on.

    I think my biggest problems are the high frequency transformer and the circuitry that connects the and powers the isolated side during start up and how it controls the H bridge.

    Would it be possible to simply make the first half of the SMPS, on its own in order to get unregulated isolated lower voltage DC, by skipping the SMPS parts and setting the H bridge to a fixed frequency or does this have drawbacks such as saturation in the transformer?

    Thanks
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    You first need to decide on a topology. There are quite a few different topologies with numerous trade-offs; cost, complexity, efficiency and power ranges.

    http://www.smps.us/topologies.html

    You might consider a flyback type supply; it will be among the easier type to make at the cost of lower efficiency. Flyback designs should really only be used for relatively low power supplies.

    One IC that can be used for a flyback converter is the UC2842A:
    http://www.ti.com/product/uc2842a
    Datasheet:
    http://www.ti.com/lit/ds/symlink/uc2842a.pdf
    There's a typical application for the IC on page 8.

    Or, you might salvage parts from an ATX form factor computer power supply, and use many of them for your project.

    There are lots of application notes at any manufacturer's site who makes switching regulator IC's. The UCx842 family are current mode PWM regulators. These respond to transients in the output more quickly than voltage mode PWM controllers do, but have their own unique set of problems.

    There is a lot of information on the site I'd linked to above:
    http://www.smps.us/
     
  5. #12

    Expert

    Nov 30, 2010
    16,247
    6,743
    mag-inc.com

    There is a whole science to making the isolation transformer. The above website has calculators to help you. I got these ideas from an old issue of Nuts&Volts magazine.

    The basic idea that I'm trying to get across is that the most mysterious part for me is the magnetics. Core manufacturers have put up websites to help you design the isolation transformer and buy their parts. The one I listed is not the only one. If you want to do this, you absolutely must have a high frequency transformer, and the core manufacturers want to give you the information.
     
  6. oldtech33709

    New Member

    Sep 24, 2011
    26
    0
    If you are considering winding your own high frequency, high voltage (above 300V p-p on the primary), you will have to pay attention to the V/ml rating of the insulation used. One common problem with these designs is the production of coronal discharge. This phenomenon can, over time corrosively destroy the transformer and surrounding circuits.
     
  7. BrainFog

    Thread Starter Member

    Jan 24, 2011
    122
    4
    There is a lot of information and I am struggling to get my head round it all but my understanding is improving.

    The topologies link was very useful and after checking some other sites I find myself agreeing that flybacks are best in this case as they appear to be one of the easiest designs to get my head around. SgtWookie, the statement "relatively low power" is very subjective. Can you put this into numbers please?

    After searching "Current Mode PWM Controller" I have noticed that may have set input and output voltages. Does this simply refer to powering the controller and the output voltage of the signal it gives out or the actual voltage input and output of the SMPS?

    I managed to find this with a bit of googling http://www.nxp.com/acrobat_download2/expired_datasheets/UC3842.pdf it is a slight variant of the uc2842a and on page 7 it has a very useful schematic of a single output flyback. One problem I have is that I have no idea how to modify the circuit for 220-240v Ac input to 40.5v output or how to set the current limit. Could someone help me with this?

    Thank you
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    There is quite a learning curve to SMPS's, and there is a LOT of information out there to digest.

    Simplicity when you're starting out is "a good thing". It won't be the most efficient, but simply getting something working is far better than getting in way "over your head".

    Yes, it is. Since the efficiency won't be so great (maybe 70% to 80% at the very best) you should keep the target power max around 200W. If you need more than 200W, you should be looking at different topologies; at 70% efficiency your total power is ~285.7W, of which 85.7W is dissipated as heat from the supply itself. Since England's average temperature seems to hover in the 4.4°C/40°F range much of the time, that might not be all bad - but you'd be better off to rely on the HVAC system to supply your heat and cooling.


    The ICs themselves have a range of supply voltages that they need to operate properly. As far as input and output voltages/currents - that's in the design of your transformer, power switches (MOSFETs) and feedback paths.

    Did you download/install LTSpice yet, and join the Yahoo! LTSpice Users' Group?
    If not, now would be a good time to do so.

    There are actually LTSpice compatible models/symbols for the UC3842 in the files section on the group, and some example circuits with some hints on how to get them to work properly.
     
  9. BrainFog

    Thread Starter Member

    Jan 24, 2011
    122
    4
    Only 4.4°C where on earth did you get that statistic. :rolleyes:

    I can easily live with 70% efficiency and seeing as I only want 2 amps at 40.5v that will only be 81w output and about 115w input. Things are more likely to go wrong, knowing my luck, when the batteries saturate, the current draw drops to about 10mA and I see a puff of smoke.

    I have had LTSpice for a while and still sometimes play around with it and will join the Yahoo group tomorrow assuming I don't freeze to death in the night.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Last edited: Sep 26, 2011
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
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    In the south of England where the weather is a bit nicer! ;)
     
    SgtWookie likes this.
  12. BrainFog

    Thread Starter Member

    Jan 24, 2011
    122
    4
    After a bit of a tragedy I am back to my flyback project and am as confused as ever.

    I am planning on using a uc2842, however there seem to be at least 12 variations of this chip with the first 2 numbers vary from 18, 28 to 38 and the last number being either 2, 3 ,4 or 5. The datasheet I am using seems to be purposefully being ambiguous. What are the differences between them?

    I have also begun attempting to get my head around Power Factor, my research states that I will need some kind of power factor correction. Does the IC I intend to use have this?

    What would be the best line filter?

    One concept that I have realised that I must rewrite is my concept of what Alternating Current is. The flyback design I am using manages to make full use of a transformer without alternating current (current that reverses direction) instead it simply turns the current on and off. How does this compare to say true square wave AC and mains AC when using a transformer?

    Thank you
     
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