DC-DC Converter simulation problem

Discussion in 'The Projects Forum' started by defster, Jan 25, 2010.

  1. defster

    Thread Starter New Member

    Jan 25, 2010

    I have a problem with a project I have in school. My project involves simulating a "hybrid full-bridge three-level llc resonant converter" and simulations are my weak spot. The converter operates in two modes, either a three leg mode or a two leg mode, in each mode different switches being turned on/off. The switching is accomplished using mosfet transistors. I attached a picture of the circuit and i will present de operation principle next. Switches Q1-Q4 (including their body diodes D1-D4 and their intrinsec capacitors C1-C4), the input-divided capacitors Cd1 and Cd2, the freewheeling diodes D7 and D8, and the flying capacitor Css form the two-level leg. The resonant inductor Lr, the magnetizing inductor Lm, and the resonant capacitor Cr form the LLC resonant tank. Dr1 - Dr4 are the output rectifiers diodes. Cf is the output filter capacitor. RLd is the load. Q2, Q3, Q5 and Q6 are phase-shift controlled; Q2 and Q3 are the leading switches; and Q5 and Q6 are the lagging switches. Q1 and Q4, which correspond to Q2 and Q3 are PWM controlled, so they are called chopping switches. When the input voltage is low, Q1 and Q4 are PWM controlled. A very small fixed phase shift is introduced between the leading and the lagging switches. vAB is a TL waveform. The output voltage is regulated by controlling the duty cycle of the chopping switches. In this situation, the proposed converter operates in 3L mode.
    When the input voltage increases or during start-up, overload, or short-circuit situations, the duty cycle of the chopping switches reduces to zero, and the phase shift will be controlled to regulate the output voltage. In this situation, vAB is near a two-level waveform; thus, the proposed converter operates in 2L mode.
    The specifications of the prototype are as follows: input voltage, Vin = 150-350 Vdc; output voltage: Vo=300Vdc; output current: Io=4A, switching frequency: fs=100kHz. Parameters used in the experiment Switching frequency fs=100kHz, resonant capacitor Cr=92nF, resonant inductor Lr=22uH, magnetizing inductor Lm=120uH, Switches Q1-Q4: APT30M75BFLL, Q5-Q6:SPW47N60C3, rectifier diodes Dr1-Dr4: CSD10060, freewheeling-diodes D7-D8: DSEP30-03A, Transformer turns ratio: 10:16.
    My questions are where to apply the functions generators at the mosfets in order to simulate the circuit? Do I need another resistor in the mosfet's G? I have to obtain the next waveforms: vAB, iLr, iLm, vCr, iDr1, iDr2 etc. where do i have to place the oscilloscope for each of those? I would be forever grateful if someone can offer me some assistance. If you have any questions please do not hesitate to ask.

    Thank you.
  2. 3ldon

    Active Member

    Jan 9, 2010

    you are going to need to add a lot of .0001 ohm resistors and bypass all the diodes with real world capacitor values, then slow down the time step and bring the "accuracy down to 10^-5th instead of 1x10^-12th, then you might get somewhere.
    Other than that start with normal full bridge and it should work.
  3. defster

    Thread Starter New Member

    Jan 25, 2010
    Thank you for your reply. Where exactly do I need to add the resistors and which diodes do I need to bypass? And yes, I am using Multisim.

    Thank you again.