Synchronous Buck, correct floating?

Discussion in 'The Projects Forum' started by basline, Oct 6, 2012.

  1. basline

    Thread Starter New Member

    Aug 16, 2012
    20
    0
    I'm slowly leaving my noob status behind, but still don't know exactly what I'm doing. I think I'm finally (after several months now) close to having this circuit figured out. I'm planning on using a microcontroller to control a synchronous DC buck converter to power some peltier devices. My schematic is attached below.

    I have a couple of remaining questions I was hoping I might find some help with. A point of clarification, the software I used to create this schematic has limited representations of power supplies. Where I have a voltage source which connects back to itself, I'm trying to illustrate that that loop is floating, not that it is necessarily a battery.

    -Is my microcontroller correctly isolated?

    -Are things floating correctly?
    I'm pretty lost here. The power supply into the buck converter is two 15V supplies in series which I think I can wire without connecting either of them to earth.

    -Am I missing pull up or pull down resistors?

    -Do my capacitor sizes seem reasonable?

    Any help would be appreciated. I was hoping to have learned all of this and completed this project about 6 weeks ago. Thanks in advance
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,009
    3,233
    Normally if you have isolated grounds you show them by two different symbols (such as a standard ground ground symbol for one and a triangular ground symbol for the other). That makes it much easier to follow the circuit. Please do that for your circuit.
     
  3. basline

    Thread Starter New Member

    Aug 16, 2012
    20
    0
    Hi crutschow,

    I'll see if I can import the triangular ground symbol, and redraw the circuit, sadly thats going to take some doing. I'll upload a new schematic once I have it figured out.
     
  4. basline

    Thread Starter New Member

    Aug 16, 2012
    20
    0
    Updated schematic, with floating ground symbols.

    Should be clearer now. Any help with my questions in the original post is much appreciated.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,009
    3,233
    I don't see anything obviously wrong except the opto isolator connections appear incorrect.

    If you want isolation between the μC and the rest of the circuit then the only μC connections to the opto isolator should be the LED anode and cathode (note that you also need a resistor in series with the anode to limit the LED current).

    All other opto isolator connections should be only to the converter power and common, not the μC power and common as you show.

    Are the 12V and 30V neg terminals tied together? They must be for proper circuit operation.
     
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