Assignment Help

Discussion in 'Homework Help' started by traitorousdevil, Mar 9, 2011.

  1. traitorousdevil

    Thread Starter New Member

    May 24, 2010
    Well, my assignment title is actually to design a switch mode step down DC-DC converter. My lecturer have given us the block diagram as below and ask us to follow. By the way, can anyone help me analyze it and tell me the function for each part of the block diagram.
    (What I'm guessing is that:
    Oscillator - convert DC to AC signal
    Charge Pump - Step Up voltage
    N-Mosfet - switch
    filter - ??)

    And I have designed the circuit by using Multisim. I have no idea how to simulate oscillator by using multisim, it comes to error even though I have the function-able circuit. Please give some idea and comment.

    Oh yea, I have circle out some of the problem in my circuit.
    1. Oscillator come out with only 1 output, but connect to the charge pump which require two terminal. Any ideas?
    2. Where is the output of MOSFET? Is it only 1 terminal output or 2?

    Aha, wait for you guys comments. =)
  2. traitorousdevil

    Thread Starter New Member

    May 24, 2010
    Hello? Cn anyone help me here? =(
  3. SgtWookie


    Jul 17, 2007
    Since this appears to be a homework assignment, it should have been posted in the "Homework Help" forum. One of our Moderators will probably move it there when they see it.

    You really should use a more descriptive title when creating a topic/thread, as "Assignment Help" gives no clue as to what you might be needing besides help with a homework assignment. Such titles often cause members to overlook your post entirely, thinking that they've read it before - as so many threads have titles/subjects that are less than descriptive of the contents.

    The oscillator's function is to generate a rectangular wave output, used to drive the gate of the MOSFET. The ratio of ON-time to OFF-time should be adjustable. A 741 opamp is a poor choice, as they are ancient, slow, and have generally meager performance characteristics, as well as not having rail to rail inputs/outputs. In your schematic, the Vcc and GND terminals are not connected, so you will not get any output.

    Charge pump -l MOSFETs must have its' gate voltage referenced by the source terminal. For N-ch standard level MOSFETs, when Vgs=10v, the MOSFET is considered fully turned on, when Vgs=0v, it's considered fully turned off. The charge pump is required to get the gate voltage higher than the supply voltage.

    The filter consists of an inductor connected to the source terminal of the MOSFET, with a cap to ground on the other terminal of the inductor. You'll also need a fast or Schottky diode from ground to the source terminal, otherwise when the MOSFET is suddenly turned off, the inductor will have large voltage spikes that will most likely kill the MOSFET.

    Ditch the 741 opamp. Use something better.
    The 2N7002 MOSFET has too low of an Id rating to meet the requirements.
    Output is supposed to be ~8v-13v, 1W to 5w. That's anywhere from ~77mA to ~625mA.
    The gate drive is all wrong.
    You don't have an inductor or filter capacitor.

    MFETs generally have three terminals; gate, drain and source. You'll be using the MOSFET on the "high side" of an inductor. In this case, the drain is connected to the positive supply, and the source terminal to the inductor. The gate controls whether the MOSFET is on or off.
  4. traitorousdevil

    Thread Starter New Member

    May 24, 2010
    I have done in detail the oscillator part and the charge pump part. Yet, I don't know how do I cascade both of them. It looks weird in my connection. Please correct me if I have connected wrongly. The output from charge pump and oscillator will be uploaded below.

    (Sorry for my step by step corrections.. >.<)