Need help with MOSFET high-side switch

Discussion in 'Digital Circuit Design' started by trepetti, Sep 27, 2017.

  1. trepetti

    Thread Starter New Member

    May 19, 2017
    9
    0
    First off, I am REALLY new to circuit design, having a basic understanding of the components.

    I am building a project where I need to switch an LED on and off at a high flash rate, and have built and tested the attached circuit. To simplify the design, I went looking for a Logic-level MOSFETto eliminate the NPN transistor and current limiting resistor. So far, so good.

    I ended up buying some RFP30N06LE's even though they are N-Channel.

    So, can someone tell me if it is possible to use an N-channel MOSFETon the high side, and if so, lead me in the right direction?

    Thanks
     
  2. ScottWang

    Moderator

    Aug 23, 2012
    6,711
    1,033
    The Vds will be as 3V when the N MOSFET is turn on, but the power is 12V and you using a logic level Vgs, so it should ok, but the uC control logic will be inverse, high(1) should be change to low(0), and low(0) should be change to high(1).

    You should add a 4.7K resistor at the base of transistor to protects the base, another question is how is the spec of led(D2, lables as led1, led2 is better)?
    HighSide_NMOSFET-switch.png
     
  3. ScottWang

    Moderator

    Aug 23, 2012
    6,711
    1,033
    The below is the normal application circuit for an N MOSFET or you can say that it is a low side switch circuit.

    LowSide_NMOSFET-switch.png
     
  4. trepetti

    Thread Starter New Member

    May 19, 2017
    9
    0
    Thanks for the information Scott. If I need to invert the trigger signal, that will not be a problem.

    However, as the N-Channel MOSFET I selected is advertised as being able to work with logic level signals (datasheet attached), I was aiming for the revised design attached.

    I am guessing that the principals of your design are unaffected by WHERE the gate is connected. Is that correct?

    And again, thanks for the swift response and the help.
     
  5. bertus

    Administrator

    Apr 5, 2008
    19,682
    4,047
    Hello,

    You can not make a high side switch without a gate driver.
    The gate voltage must be at least 3 volts higher ( 5 Volts would be better, according to the posted datasheet ) ,as the source voltage to get the mosfet conducting.

    Bertus
     
  6. wayneh

    Expert

    Sep 9, 2010
    16,099
    6,212
    That won't work. Consider that the LED drops about 3V across itself. Your micro controller will never be able to get the MOSFET gate voltage high enough to turn it on.

    The source pin of the MOSFET should be at ground. In your drawing, just swap the position of the LED and the MOSFET, and connect the grounds of your LED circuit and the µC.
     
  7. ScottWang

    Moderator

    Aug 23, 2012
    6,711
    1,033
    If you want a positive control logic input as the uC output high(1) then led on, output low(o) then led off, please check the circuit below.

    R1 - If you used a general led as 2V/20mA or 3V/20mA then you will need a current limiting resistor, 500Ω for 2V/20mA led, 470Ω for 3V/20mA led.

    LowSide_NMOSFET-switch_Positive ctrl logic Input.png
     
  8. trepetti

    Thread Starter New Member

    May 19, 2017
    9
    0
    Thanks, but the circuit I am building needs to interface into the vehicle's wiring, and there is no practical way to get onto the low side of the LED.

    I will see if I can find a P-Channel version of the MOSFET I selected. If I can find one that will allow TTL on the gate to allow source to drain flow then I believe that my revised design might work.

    Again, any help / advice would be appreciated.
     
  9. ScottWang

    Moderator

    Aug 23, 2012
    6,711
    1,033
    According to your application and using logic level N MOSFET to drive the led, the circuit that I uploaded in the #2 will be work, why you need to cancel the bjt(npn)?
     
  10. trepetti

    Thread Starter New Member

    May 19, 2017
    9
    0
    Just looking to simplify and make the board as small as possible. Thanks for the help Scott
     
  11. ScottWang

    Moderator

    Aug 23, 2012
    6,711
    1,033
    What do you mean by that?

    How are the voltage/current of led?
    Any special reason that you can't use the circuit on #7? (pin out problem?)

    If the led power is 12V and I assuming that the resistor included inside of the led module, and then the circuit on #2 maybe doesn't work, and the R3 will no needs, you can add a capacitor 10~47uF across on Vgs to try.
    [​IMG]
     
  12. Sensacell

    Moderator

    Jun 19, 2012
    2,359
    760
    You cannot drive a ground referred load with an N-CH FET unless you have a voltage higher than the supply, to drive the gate above the source.

    A bootstrap circuit can be used if the circuit is constantly switching, to keep the bootstrap cap charged.
    But this will fail if the circuit is static.

    Use a PNP transistor or a P- CH FET.
     
  13. trepetti

    Thread Starter New Member

    May 19, 2017
    9
    0
    Thanks, but the datasheet mentions that the N MOSFET can be driven directly from a microcontroller. I read that to mean that I can provide TTL at the gate to control source to drain. Am I reading that right?
     
  14. trepetti

    Thread Starter New Member

    May 19, 2017
    9
    0
    For the first question, I meant that I could eliminate the NPN and drive the N MOSFET gate with the PIC.

    For the second part, the LED is a replacement for the incandescent bulb and is connected to the 12v supply directly. I measured the draw at about 200mv, so nothing will need to pass a large amount of current. I am hoping that means I can use #2 without R1 and R3. Does that make sense?
     
  15. ScottWang

    Moderator

    Aug 23, 2012
    6,711
    1,033
    Do you mean draw current is 200 mA?
    If the bulb draw 200 mA then it is a lot of current, using bulb to replace the led then R3 is no needed, you still need the R1 to protects the base of npn bjt, otherwise you don't know when the bjt will be damaged.

    When you using the N MOSFET as high side then the Vcc will be needs as Vcc = 12V+Vgs(3V) = 15V.
    If you using the uC to send the pulse then you can try to add a 10uF~47uF capacitor across on Vgs, but this is no guarantee, and if the uC just send the fixed high or low then the Vgs will not enough voltage to be triggered, so the Vds will not turn on, this circuit works or not related with the working voltage of bulb, if the voltage less than 9V can be light up then this circuit could be work.
     
  16. BobTPH

    Senior Member

    Jun 5, 2013
    1,808
    472
    Yes, when the source is at ground. The voltage on the gate must be about 3V above the source, not above ground. Do you see the problem with driving it from the uC now?

    Bob
     
Loading...