Interesting LC Power Converter

Discussion in 'General Electronics Chat' started by dannybeckett, Dec 5, 2012.

  1. dannybeckett

    Thread Starter Active Member

    Dec 9, 2009
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    Hi guys,

    I have come across a strange cascaded DC-DC power converter design. The goal of the converter is to drive a transformer, at, I believe, it's resonant frequency. I have drawn a simplified schematic:
    [​IMG]
    The circuit is a simple cascaded converter (a buck converter followed by a push/pull circuit to alternate the current between the different windings of the transformer). Q2 and Q3 are at a fixed frequency, 50% duty cycle, 180 degrees out of phase with each other (so at any point in time, one of them is guarantee to be conducting).

    What are L1 and C1 doing? How do L2, T1 and C2 interract? If T1 is fixed (hence L3, L4 and L5), how would you calculate the component values of L1, C1, L2 and C2?

    Thank you!
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    L1 and C1 are for filtering and smoothing of the main supply, C2 will be tuned with the coils L3 and L4 to make T1 resonant at the operating frequency of Q1, which pulses L2 via L3 or L4. When Q1 is non conducting the back emf from L2 goes through D3.
     
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    718
    It looks like a buck converter followed by a flyback converter.

    What is the voltage/current input, and the desired voltage/current output?

    What control is switching the transistors?

    See Simple Switcher Datasheet for a similar example of a regulated flyback converter.
     
  4. dannybeckett

    Thread Starter Active Member

    Dec 9, 2009
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    It's being controlled by an LM5041.

    http://www.ti.com/lit/ds/snvs248c/snvs248c.pdf

    Input voltage will be 12V (T1 is a 12V transformer) at 1.25A (15W). I need to calculate L1 and C1 to resonate at the same frequency at the transformers resonant frequency, and L2, T1 and C2 all need to be in resonance too. Do you know how I would go abotu calulating this? It's a strange circuit because of the push/pull config, and im not sure how to deal with T1 and C2 interractions!
     
  5. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    The example circuit on page 13... :eek:

    For the frequency calculation, Page 12 shows the formula, you'll need to know the inductance of the coil, once that is known the frequency can be calculated by choosing a capacitor form a high Q oscillator with the transformer inductance.

    Then the center frequency of the LC circuit is what is used to set the oscillator on the IC via RT.
     
  6. dannybeckett

    Thread Starter Active Member

    Dec 9, 2009
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    I know the inductance of the transformer - each primary winding is 16uH. That's the only set value. I just can't get my head around what's actually happening each cycle regarding L2, T1 and C2 - C2 will have it's polarity reversed each cycle, does it completely discharge? can you treat L3 and L4 as two separate inductors? (no energy transfer between them?) Once I have the frequency at which the transformer wants to oscillate, I should be able to calculate the rest of the circuit but I need a foothold in something first! I've attached a picture in an attempt to break it down further:

    EDIT: Pictures deleted :TIDE
     
    Last edited: Dec 6, 2012
  7. dannybeckett

    Thread Starter Active Member

    Dec 9, 2009
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    Those two previous pictures were an oversimplification, and not true to the previous circuit. I dont think this circuit can be simplified anymore than the illustration in my first post!
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Check This Section on resonance out in the e-book here.

    I'm unsure of your exact question, other than "what values do I use for all these parts"?

    I don't know that answer. With full information, it is a matter of filling in formulas and calculating.
     
  9. dannybeckett

    Thread Starter Active Member

    Dec 9, 2009
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    Thanks for that link. Really all I'm trying to understand is what exactly is happening during resonant operation of this circuit - How does the electricity flow between L2, T1 and C2, whilst this circuit is running at resonance? If I'm being too vague please probe with more questions
     
  10. dannybeckett

    Thread Starter Active Member

    Dec 9, 2009
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    Thanks for the resonance link, the refresher was useful
     
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