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Need Help in mosfet switching strobed LEDs

Discussion in 'LEDs and Optoelectronics' started by MiZZi, Sep 13, 2015.

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  1. MiZZi

    MiZZi

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    Sep 14, 2014
    I have a bunch of High Power LEDs that are being strobed. I need to switch them using Mosfets. Well, I really have a little idea in using Mosfet that is why I carelessly only bought N-Channel Mosfets for switching them. This is how the circuit looks like:

    [​IMG]

    But it turns out that it does not work. What should I do for this circuit to work? Would it be better to replace the N-Channel near the VCC with P-Channel mosfets like this?

    [​IMG]

    I'd appreciate your thoughts guys. Thanks!
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    check out the resource on LEDs which has a circuit which can be used to both switch and current limit a LED.

    N channel mosfets are ideal in most cases.
     
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  3. Old Steve

    Old Steve

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    Jul 23, 2015
    Yep, use P channel for the upper MOSFETs, then pull the gate of the P channel low to activate. (Exactly opposite to the lower N channel's operation.)

    Best to use a tie-up resistor on the gate of the P-channel MOSFET, then an NPN transistor in common-emitter mode to activate it with a positive level on it's base.

    The N-channel won't work on the upper side because the gate needs to be taken higher than the supply voltage to activate it. 4-5V higher for a logic-level MOSFET, otherwise about 10V for a normal one.

    A 'high-side driver' could be used, but that's added expense and complexity.
    Choose P-channel MOSFETs with as low an on-resistance as possible.

    Using a P-channel MOSFET rated for a much higher current than it will actually need to switch is a way to get one that will have a lower Rds-on. Check the datasheets carefully before buying, otherwise you might have to add an unnecessary heatsink to the upper MOSFET if you're switching much current.

    Edit: I was thinking more about the half-bridge switching action, and not enough about 'what' was being switched. *steve* is right.
    But I'd still use P-chan/N-chan for a half-bridge.
    (Too preoccupied thinking about another problem.)
     
    Last edited: Sep 13, 2015
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  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    We need more information. Why do you want to do both high and low side switching? And as an aside, how are you limiting the current through the LED?
     
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  5. Old Steve

    Old Steve

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    MiZZi likes this.
  6. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Hello
    I don't quite understand why you need two switches as Steve mentioned. Also you will need to flip your P-Channel over because the body diode will be conducting. You need to connect the source to +V and not the drain. If you do want hide side switching then choose a logic level FET and connect this to the output pin of the controller. You also don't need the 10K resistor. depending on the capacitance of the gate and the output drive capability of the driver you maybe able to reduce this to say 100R or remove it altogether. And don't forget the current limit for the LED.
    Thanks
    Adam
     
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  7. Old Steve

    Old Steve

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    Well spotted. I didn't notice that. :oops:
     
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  8. MiZZi

    MiZZi

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    Sep 14, 2014
    Hi guys sorry for the late reply. I am actually doing this for a school project. I am using two drivers (P-channel and N-Channel) in order to multiplex LED's. What I am switching are just 12V LED Strips. I believe they already have current limiting resistors so I don't think I need to add it in my circuit. By the way guys, thank you for all you reply. I have already bought a Pchannel MOSFET which is this one:

    https://www.sparkfun.com/datasheets/Components/General/FQP27P06.pdf

    Apparently, this is the only one available in our place so I won't be able to choose any other mosfets. Now, I have another problem I can't turn off the P-Channel Mosfet. I tried supplying it with the voltage on the source but still it turns on. What should be the problem guys?
     
  9. MiZZi

    MiZZi

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    Sep 14, 2014
    Oh, I forgot to tell you that I have already connected the source to 12V and the Drain to the led strip. Supplying 5V to the gate won't turn it off even the 12V.
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    you should have a resistor between the gate and the source and use an NPN transistor to pull the gate low to turn it on. The base of the NPN transistor will need a resistor.
     
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  11. Old Steve

    Old Steve

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    Jul 23, 2015
    What he ^ said. (I already outlined this in my post #3 as well.)
     
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  12. MiZZi

    MiZZi

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    Sep 14, 2014
    [​IMG]

    This is what I did guys. The MOSFET is FQP27P06. For some reason, connecting the gate directly to the source (12V) won't even turn the MOSFET off. What could be the problem?
     
  13. Old Steve

    Old Steve

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    Dead MOSFET!

    With the circuit unpowered, measure between drain and source with a multimeter. I'll bet it measures a dead short - dead MOSFET.

    Attaching the gate to the source, +12V in your case, should switch it off if it's a healthy MOSFET.

    Got a spare? (Never buy just one of a part when you're prototyping.)

    A bi-polar transistor likes to fail open-circuit, but MOSFETs usually fail short-circuit.

    And note that for high-speed switching, R1 would probably need to be a lower value, to discharge the MOSFET's gate capacitance fast. Perhaps 1K. 10K is fine while testing this problem though.
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    note that mosfets are static sensitive. You should be taking the basic precautions applicable to static sensitive components.
     
  15. MiZZi

    MiZZi

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    Sep 14, 2014
    I did the checking using this video as reference:



    Well, it seems fine. Four mosfets actually have the same reading. I am still gonna try the other three to the circuit. Thanks for your reply Steve by the way.
     
  16. Old Steve

    Old Steve

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    Jul 23, 2015
    This shouldn't be this hard.

    You followed his tests exactly and it tested similar to his measurements? That's odd - your's is a P-channel and he's testing an N-channel.

    Getting back to the MOSFET not switching off in-circuit, if the drain-source isn't short-circuit, then something is fishy. If you definitely used a good P-channel, connected in the circuit you showed in post #13, and the LED wouldn't switch off when you connected the gate to +12V, then you had the MOSFET in backwards, with the drain connected to +12V instead of the source, which would cause the internal source-drain diode to conduct and keep the LED on.

    Double-check that you're connecting it correctly and that it is definitely a P-channel.
     
  17. Old Steve

    Old Steve

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    Jul 23, 2015
    Note the internal diode in this drawing:-

    P-Channel MOSFET Internal Diode.JPG

    Picture if the MOSFET was in backwards - the diode would be forward-biased and conduct.
     
  18. MiZZi

    MiZZi

    24
    2
    Sep 14, 2014
    I think I know what really is the problem now. For some reason, doing the mosfet testing using N-Channel procedures actually worked out well for my P-Channel Mosfet. This made me think that my P-Channel Mosfet actually has an N-Channel Mosfet characteristics. To verify it, I constructed this circuit instead:

    [​IMG]

    wherein the Drain is connected to VCC and the source voltage is connected to the LED. Well, it works. It is switching like an N-Channel Mosfet. I guess this is what I should expect from buying clone MOSFETS (these are the only mosfets available in our area). Anyways, thanks for all of your replies guys.

    I have one last question how can I be able to switch N-Channel Mosfets using this form?

    [​IMG]
     
  19. Old Steve

    Old Steve

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    Jul 23, 2015
    That first diagram of your's just above doesn't make sense - the internal diode would be conducting and the MOSFET would not switch off.

    I'm afraid you've lost me.

    And in the second of your above diagrams, you're right back where you started - the top MOSFET couldn't be switched on until the gate was taken above the supply voltage.
     
  20. MiZZi

    MiZZi

    24
    2
    Sep 14, 2014
    Indeed, sir! This must definitely be an N-Channel. Thanks so much for helping me point this out sir!
     
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