help in boost converter

Discussion in 'General Electronics Chat' started by grad-student, Nov 22, 2014.

  1. grad-student

    Thread Starter New Member

    Nov 19, 2014
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    I've designed this circuit and expecting an output of 200V. I am getting a very odd response at steady state. It may be a problem with the model. Please let me know if you have any suggestions.
     
  2. MikeML

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    What are the 24V power supplies for?
     
  3. grad-student

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    Nov 19, 2014
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    I am using temperature dependent MOSFETs. The temperature connections are working as voltage pins which are the 24V.
     
  4. MikeML

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    Ok, I see that now. I took some liberties with you circuit to make it run. Your drawing style leaves something to be desired as I couldn't follow it until I redrew it. Biggest problem was that your drive signals overlapped, so you had huge shoot-through currents.

    I discarded your .options statement and used the alternate solver...

    Solver.gif

    Here is the edited schematic file:
     
  5. grad-student

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    Nov 19, 2014
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    Also could you please tell me why you have added a delay of 5u in Vgs2? Without this delay the voltage wouldn't get as high as 200V.
     
  6. MikeML

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    The way you did the gate drive, the top and bottom NFETs were on at the same time; a real no-no. This causes huge currents to flow through the FETs, called shoot-through current. You have to set up the gate drives so that the upper one turns completely off before the lower one turns on and vice-versa with some dead time in-between where both are off .

    It is totally unrealistic to drive the gates from a pure voltage source. Any real gate driver will have some finite source impedance. It is also unrealistic to have 1ns rise and fall times at the gates....

    Have you checked the power dissipation in the FETs. I fear that is unrealistic, too..

    This thing is just a pwm followed by a low-pass filter. Is that what you intended?
     
  7. grad-student

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    Nov 19, 2014
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    My object was to keep one transistor on while the other off as you said. I was trying to drive the MOSFEt from an ideal point of view. This is actually a bidirectional dc-dc converter (step down) in this case. The duty cycle regulates the step down factor. One of my concerns in your schematic is that, shouldn't Vgs be pulsed. If we reduce the value of the gate resistors say 1ohm, we can get pulsed waveforms. I was wondering why you chose resiators value to be 100ohm?
     
  8. MikeML

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    It will be tough (in a real circuit) to get gate drivers that can create a dv/dt at the gates much faster than an ideal voltage source in series with a few tens of Ohms.
     
  9. MikeML

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    I took some more liberties with your circuit to make some points: First, I dont understand why the lower NFET is even there, and why it is ever turned on. This type of converter only needs to have diode where you have the lower NFET. If the gate of the lower NFET is kept at zeroV (never turns on), then the Drain to Source Parasitic diode provides the required diode for the converter to work.

    Look at this much simplified sim. I suggest you use it to clarify the principles of what you are trying to do before worrying about the Infineon parts. Note that the circuit only has the upper NFET, simulated as a simple voltage-controlled switch. I added some On resistance and some gate capacitance. The lower NFET has been replaced by an Ideal diode. Note the resulting voltages and currents.

    Bi.gif

    Now look at the turn-on transient, as the output capacitor charges to the voltage determined by the duty cycle...

    Bia.gif

    Finally, look at the detail of the various node voltages, and branch currents. Note the current in the diode and in the inductor. You will have real problems finding real parts that can support these current levels...

    Note the current in the filter capacitor. You will have real problems finding a real part that can support these current levels...

    Bid.gif
     
  10. grad-student

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    Nov 19, 2014
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    Thanks Mike but my main object is using a bidirectional dc dc converter and study its behavor using Si MOSFETs. Bidirectional means it works as a buck on one direction and boost the other way. I have found many papers that use something similar to my design. I am using temp dependentMOSFETS because I want to see how they work under different temperature. Does that make things clear?
     
  11. MikeML

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    Ok, but you started out turning on both FETs at the same time. That will never happen during normal operation of either a buck or boost converter. When running as a buck, the lower one never should turn on. When running as a boost, I dont know, I would have to think about it. My point is that you need to work out what needs to happen first before you worry about the details of the specific FETs.

    And you avoided the question about where you obtain parts that come close to operating at these power levels (except the FETs)
     
  12. MikeML

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    They were overlapped due to charge storage. When turning one off, you have to wait before turning on the other. Yours were concurrent.
     
  13. grad-student

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    Nov 19, 2014
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    Vgs1 and Vgs2 keep on overlapping. I am trying different ways to prevent that. Any advice?
     
  14. grad-student

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    Nov 19, 2014
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    I am trying to work on a project that use boost converter and my object is to get an output of 6ooV. The maximum output voltage I am getting is 400V. I have attached my design. Can anyone help me in this please?
     
  15. GopherT

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    I won't open zips, sorry.

    Your load may be drawing more current than your supply can produce at 600V.
     
  16. grad-student

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    Nov 19, 2014
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    I have tried to change several things in this circuit but could not figure out the causes.
     
  17. spinnaker

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    What is in that zip file? Most people are reluctant to open a zip file from a newbie. Unless it is something like png they will need the software you have anyway.

    Just attach it as a png. Bonus if you embed it inline.
     
  18. Papabravo

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    What is the required output power in watts?
    Assuming 80% efficiency, What would the input power required be?
    Is that amount of power available?
     
  19. spinnaker

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  20. grad-student

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    Nov 19, 2014
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    well, I am not focusing on power. I only used the power to get output load (RL) but it does not matter if we can change it since the project did not ask about the power calculations.
     
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