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Mosfet gate voltage

Discussion in 'General Electronics Discussion' started by mathphobe, Mar 24, 2018.

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  1. mathphobe

    mathphobe

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    Sep 3, 2017
    Among several questions I'll likley have while working on a very simple circuit (can you say can of worms :/ )

    I want to fully saturate a mosfet from a 4.2v or 8.4v bus source, can I use a voltage trippler capacitor diode arrangement to supply the gate?
    This would mean a higher gate than source voltage and I'm not sure that works.

    Thanks in advance!
     
  2. BobK

    BobK

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    Jan 5, 2010
    I am sorry, my vision is really bad, I am having trouble seeing your very simple circuit.

    Bob
     
  3. mathphobe

    mathphobe

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    Sep 3, 2017
    Haha, good point.

    Drawing a circuit on my phone is fairly difficult, doing so only to find that the ideal basis that the mosfet is fully saturated isn't possible makes for a pointless hours work..

    I'll attempt it by hand and come back.
     
  4. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Voltage doubler/tripler etc require an alternating waveform to work but otherwise, yes, you can do it. But why would you want to use such a circuit if all you're required to reach is a 'lowly' couple of volts?
     
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  5. mathphobe

    mathphobe

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    Sep 3, 2017
    OK BobK, so drawing it isn't going well. I've fell down at making a voltage divider :( pretty sure I'll get there.

    A brief verbal description;

    4.2v (or 8.4v) lipo source. Cap&diode doubler/trippler to lm7805 supplying this part of the circuit. Ds1669 acting as a voltage divider from 0-1v. An LTC6992-1 (set at 25khz) receiving the control voltage from the ds1669 to control it's pulse width. Pulse out to jfet, jfet source from the trippler, drain to mosfet gate.
    I realise that's not the best description but it's as far as my mind got with it o_O

    I really want to exploit the lowest rds I can from the mosfet, but they seem to want 10v+ at the gate to achieve this.
     

    Attached Files:

  6. mathphobe

    mathphobe

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    Sep 3, 2017
    Alternating wave form?.. Dam, nothings simple is it :(
     
  7. Minder

    Minder

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    Apr 24, 2015
    Why not use a logic level gate Mosfet??
    M.
     
  8. mathphobe

    mathphobe

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    Sep 3, 2017
    I have been on all of my recent projects, but I'm seeing a fairly large voltage drop through the mosfet. My 4.2 is usually around 3.8 at the drain. With a 1.4mohm rds I should be getting higher Vout of the mosfet fully saturated no?
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Just a technical point. What you want is exactly NOT to have the mosfet in saturation mode.

    When a MOSFETs changed is saturated, it is effectively operating in a constant current mode with a large voltage drop.

    You want the mosfet to be operating in its triode region where the channel behaves like a resistor.

    Technical use of the word "saturation" aside, it's clear what you want. I'm not saying that what you want doesn't seem clear :)

    To get the lowest resistance across a MOSFET, you need to provide a suitably large voltage between source and gate, and limit the current so that the MOSFET doesn't saturate. If you have access to the datasheet, you want to ensure that the Vds vs. Id curve remains in the steeply increasing portion (where Vgs increases only slightly with increases in Id). In this part of the curve, the slope of the line is an indication or Rds, and you will note that Rds decreases (the curve gets steeper) with increasing Vgs. At some point the curves level off, with Id remaining almost constant for increasing Vds. This is the saturation region.

    Some MOSFETs require a gate voltage (Vgs) up to 15V to get the Rds(on) specified in the datasheet.

    If you need a high Vgs and you can't provide it, there are several solutions.

    1. You can increase your supply voltage. This may be inconvenient it impossible in some cases.
    2. You can boost the voltage you have. This may be complex.
    3. You can use a gate driver which creates its own boosted gate voltage. This can be expensive, and the parts may be hard to get or come in hard-to-use packages.
    4. You can choose a MOSFET with the same Rds(on) at lower Vgs. So-called "logic-level MOSFETs" are an example of a marketing label which is indicative of this feature. Beware that having a reduced Rds for a given Vgs may be accompanied with other changes in behaviour (such as reduced Vds(max) and/or Vgs(max), and increased device capacitance.
    5. Placing multiple MOSFETs in parallel. This works like resistors in parallel. Not that the gate capacitances will accumulate.
    You state that you want to get the Rds to the lowest possible value. It is best to work from the other end and determine (from allowable voltage drop and/or dissipation limits) the maximum Rds you can tolerate, and then determine how to achieve it.
     
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  10. mathphobe

    mathphobe

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    Sep 3, 2017
    And just when I thought I understood a mosfet I'm reduced to noob again!

    Obviously I was under the misguided understanding 'saturated' mean fully 'on', apparently not the case. I figured supplying the gate higher voltage would give me better source to drain resistance :(

    Beginning of paragraph five there, *steve*, is telling me that there's no way to gain better losses (if that's even a sentence o_O ) from the component I have. I can't use a higher voltage at source nor can I change current drain to be 'optimal'.

    I've paralleled six mosfet, seeing similar voltage loss as I mentioned before. I really need to improve the Vout from the mosfet, given this info I will have to use a higher voltage battery. I'm quite disappointed I have to say.
    Thank you for your time, steve.
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
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    Jan 21, 2010
    In general a higher gate voltage DOES give you a lower source to drain resistance. However this resistance is constant only at currents up to a certain point.

    Tell me what mosfet you're using and we can take a look at some of the stuff in the datasheets and explore what the mosfet can do.

    Also, give me some more information on the maximum voltage being switched and the maximum current being drawn. That will allow me to point to the appropriate points on the graphs in the datasheet.

    Don't worry. Some of the terminology around MOSFETS seems back-asswards compared to Bipolar transistors.
     
  12. Audioguru

    Audioguru

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    Sep 24, 2016
    Do not talk about a Mosfet being saturated or not. Simply say if it is fully turned on or if it is turned off.
    If its datasheet says that its gate-source voltage must he 10V then believe it or buy thousands of them and hope that you find one more sensitive than most of them. Many "logic-level" Mosfets are made that are fully turned on with a gate-source voltage of 4.5V.
     
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