DC-DC converter with a PV source

Discussion in 'The Projects Forum' started by Akhilamuthu, Apr 5, 2016.

  1. Akhilamuthu

    Thread Starter New Member

    Apr 5, 2016
    please help me in the capacitor design of the buffer circuit, where the capacitor charges when switch S0 is off and supplement the load when S0 is on. S1 is an uncontrollable switch. The image of my circuit is attached here with.
  2. Dyslexicbloke

    Active Member

    Sep 4, 2010
    You need to do some reading on DC-DC converter Circuits.
    I am no SMPS engineer but I know there is good reason that almost all switch mode supplies are based on inductors.

    Buck converter 101...
    firstly you are almost there wih your circuit but you need to charge the capacitor with a switch, a silicone one, and an inductor.
    Before I go any further I need to state I am just learning ths stuff so anything I say needs to be used only as a starting point tor further research.

    Buck and boost convertr circuits come in many variants an diferent operating modes but probably the simplest buck, step down, requires a single active switch, a single passive switch, a diode, an inducto, a capacitor, and some form of adjustable PWM controller.

    When it starts up the high side switich cloeses and curent flows to the output capacitor via the inductor. The inductor is effectivlly charging, bulding flux, which limits the current due to the back emf and the capacitor us charge in the process.
    the second pasive switch is reverse biased so no current flows.
    At this point the load is connected in seriese with th supply via the active switch and the inductor.
    the output capacitor is effectihly part if the load in paralell.
    At some point the desired output voltage is reached and the active switch opens...
    this immediately causes the flux in th inductor to collapse inducing a forward curent into the load and the output capacitor. Since the supply is isolated on the high side the passive switch now conducts carrying the full load current.
    Since the change in flux is now negative the inductor still limmits the current because the forward curent now oposes the collapse...
    Que low voltage threshold, or possibly a current based trigger, and tne cycle starts again.

    Please note this is a huge over simplification and only reprsents one of sevral topologies and operastin modes but it is accurate enough to give you enough to do some research on. If you visualize a capacitor attemtping to draw huge uncontrolled currents and the inductor ckntrolling it whilst they both store energy whilst rapudly cycling on and off you will not be far off.
    Personally I find the maths and some of the finer points a bit too much right now but I would recomend playing with some...

    Hope this help you get on the right track.

    I have an active post now asking where I am going wrong trying to get several buck circuits to operate in paralell..
    like I said no expert.

    Have fun,
  3. Akhilamuthu

    Thread Starter New Member

    Apr 5, 2016
    Thank you for your reply and support.
    I actually want to charge the capacitor when S0 is off. The main point is to be noted is that S1 is simply a manual controlled switch. there is no control over it. I need to charge the capacitor by PV source by providing a suitable control to S0 and evenif we put different loads the capacitor voltage must remain within the tolerance limit..i.e, control must be provided likewise.
  4. Dyslexicbloke

    Active Member

    Sep 4, 2010
    I hate to be blunt but I fear you are missing the point...
    In this case knowing what you want is a world away from being able to achieve it.

    Capacitors store energy, the amount is a function of the size of the capacitor and the voltage you have charged it to.
    This dictates a fundamental property of any capacitor circuit, if you charge or discharge it the voltage must change.

    It sound as though you need a battery...

    The only potential solution I can suggest if you must store energy in a capacitor, for some reason, is to charge it to a much higher voltage than you need and then discharge it through a voltage regulator but even then you will have to use buck circuit or it will be horribly inefficient.

    If I am missing the point then you need to explain your goals and constraints much more clearly.

  5. wayneh


    Sep 9, 2010
    This sounds like homework? I don't understand the question. The title of the thread implies you want a DC-DC converter, but your schematic doesn't show one. What are the constraints and what are you trying to do?