Question on LTSpice & MOSFET

Discussion in 'General Electronics Chat' started by toffee_pie, Nov 28, 2018.

  1. toffee_pie

    Thread Starter Senior Member

    Oct 31, 2009
    212
    8
    All,

    Following from the last thread I posted I am trying to some LTSpice simulation to see the impact of overloading a circuit, I am just picking off the supply line blocks.

    A few questions.
    1. I have a N Channel MOSFET that is used for charging (its feeding an ADC input on a PIC so most likely used for voltage sensing to determine when the battery is charged and to disconnect an isolator switch) - when I simulate this with a 2N7002 FET I get about 1.2V on the drain with a 40V input on a voltage divider going to the Gate, the Drain is pulled up to 5V with a resistor so the output cannot be more than 5V - how is it that the output voltage is lower if the input is higher
    2. I cannot for some reason alter diode parameters on Spice, the right clicking etc and altering parameters does not work, I also put in the type as 'X' not D which I think causes problems.
    sorry for the dumb questions..!
     
  2. eetech00

    Senior Member

    Jun 8, 2013
    1,474
    290
    Hi

    Post the .asc file and the model file so we can take a look...

    eT
     
  3. toffee_pie

    Thread Starter Senior Member

    Oct 31, 2009
    212
    8
    apologies for the delay

    attached is a spice models, i am looking to see the impact of overloading the circuit as per previous thread - the charge block is not impacted as there is a blocking diode preventing voltage from the motor going back to the charge FET in the spice simulation here but want to carry out simulation anyway to learn more about this, is it normal for the Gate N7002 output to vary from 1.27mV to 560mV (20V to 40V DC Analysis) ? Just trying to gauge what voltage is going into the PIC, I know it should be throttled to < 5V anyway due to the pull up.

    also, I am trying to model 2 more blocks, can anyone share some spice models for the 2 regulators in the schematic? Is it feasible to estimate the increase in power consumtion on the regulators with the increased voltage? that is what I am trying to figure out, and likewise the the last schematic, the diode at the output is connected to some switches but also going to a PIC ADC input so again want to see the impact on the increased voltage at R6 and how it may effect performance.

    thanks for the help.

    REG.jpg

    DRIVE.jpg
     
  4. eetech00

    Senior Member

    Jun 8, 2013
    1,474
    290
    1. I think you meant 1.2mv. Since the mosfet is a voltage controlled device and has a low gate threshold voltage, it is turned on once
    the voltage on the gate exceeds 0.8v. When its off, there's very little drop across the D/S, so the drain is close to 5v. When its on, the drain to source resistance drops to RDSon (as low as 1.2 ohms), so the drain is very close to ground. The output is lower because the mosfet in on.

    2. Two things.
    A. LTspice has a built in 2N7002. So you don't have modify its attributes or add a .inc statement to the schematic.
    Just place a nmos symbol on the schematic and select the 2N7002 as the mosfet device.

    B. Place the diode on the schematic, then select and highlight the diode model you want to use, but instead of double clicking the model, type "control-C"to copy the diode model params. Click anywhere on the schematic, then type "t" to open a text box. Click in the text box then type "Control-V" to insert the copied model params into the text box. Select "Spice directive" radio button, then place the text on the schematic. The diode model params should now be on the schematic where they can be edited. Change the name of the diode model to a unique name and then change the name of the diode you placed on the schematic to the same name. That's it...

    eT
     
  5. toffee_pie

    Thread Starter Senior Member

    Oct 31, 2009
    212
    8
    thanks for that, on the middle schematic i dont need to worry about damaging the Regulators as D20 is a TVS so will clamp the voltage at ca 24V. I need to try the last one on SPICE and also there is another one I need to get done.

    i never knew about them shortcuts also, so will be useful for sure. Thanks
     
  6. Bordodynov

    Well-Known Member

    May 20, 2015
    2,105
    641
    For example:
    2018-12-04_09-03-19.png
     
  7. toffee_pie

    Thread Starter Senior Member

    Oct 31, 2009
    212
    8
    I think i better understand the Mosfet voltage question, basically when its on the output is like a voltage divider pulled up to 5V with a pull up resistor and with RdsON being so small and effectively going to ground it explains the low voltage but enough for a PIC to decipher.
     
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