MOSFET with 1.5MV Gate Voltage in Buck-Boost Converter?!?!?!

Discussion in 'The Projects Forum' started by deux_ex_machina, Jul 20, 2012.

  1. deux_ex_machina

    Thread Starter New Member

    Jul 11, 2012
    3
    0
    Hi,

    I am designing a Buck-Boost converter and am new to Power Electronics (mostly have a background in Digital and some analog signals). I was using a BJT in my Buck-Boost and it was working fine in my SPICE model but discovered that they have a large on-resistance implying a large power disappation. FETs I hear don't have that issue.

    When I put a FET in and modeled the circuit, it seems that I need 1.5MV to fully turn the FET on. As you can see by the screen-shots I posted of my circuit/simulation. With 15V applied to the Gate, I get an output in the FEMTO volts--yes, femto.

    When I apply 1,500,00V, I get the desired result. I think I have the NMOS biased wrong but can't seem to figure out how to bias it correctly.

    Any suggestions?


    Thank You,
    George
     
  2. bretm

    Member

    Feb 6, 2012
    152
    24
    What's the voltage at the NMOS source? What's the gate-to-source voltage difference?
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    You are using the N-MOSFET as a source follower which is not normally used for this type of circuit. Use a P-MOSFET instead with the source as the input and the drain as the output.
     
  4. bretm

    Member

    Feb 6, 2012
    152
    24
    Awwww... I was trying to lead him there.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    Sorry I jumped the gun. :p
     
  6. deux_ex_machina

    Thread Starter New Member

    Jul 11, 2012
    3
    0
    I attached a screenshot of my new circuit with the pmos. I think my problem is how I have the gate biased. I think I need to put a resistor in and bias it towards the drain or source but I have tried several combinations with no success.

    Thank you both for your help. This is racking my brain. :)
     
  7. deux_ex_machina

    Thread Starter New Member

    Jul 11, 2012
    3
    0
    To be clear, I noticed if I put a large value series resistor between the pmos and ground (or just set L1 to have a large series resistance), I can get the circuit to behave properly. However, this would not work in practicality because it will have 10's of watts flowing through said resistor.
     
  8. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    Post your .asc file for the circuit.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You really need to select a model of a MOSFET. The default NMOS and PMOS leave much to be desired.

    Right-click on your NMOS or PMOS to get the "MOSFET Properties" dialog box open, then click the "Pick New MOSFET" button. You will then get a scrollable list of the available MOSFETs. Try to select a MOSFET that has a Vdss rating that's 20% to 50% more than your supply voltage, and then make a trade-off selection between Rds(on) and nC (total gate charge).

    Then right-click on your diode and pick a model for it.

    Tip: always reference your gate voltage to the source terminal of the MOSFET; as that is what matters to the MOSFET. It does not "care" about ground. If the difference in the voltage between the gate and source terminals exceeds ±20v (±15v for some) even for an instant, it will be destroyed. Your simulation model won't blow up because of such abuse, which is just one item that will conflict with what you would experience in a lab with real parts.
     
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