Single MOSFET latchup

Discussion in 'General Electronics Chat' started by eng_stu, Jan 10, 2013.

  1. eng_stu

    Thread Starter New Member

    Jan 29, 2012
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    Hi All,

    I have searched the forums and google but to avail so far.

    Is it possible to have latchup in a single MOSFET package? (In this case PMOS). All the explanation images have 2 mosfets next to each other and the latchup comes from their interaction through the substrate. But I figure it might be possible in single MOSFET packages depending on their construction. My voltages are well within the absolute maximums of the PMOS, although there is a large inductance on the drain side of the PMOS (but the body diode should take care of that).

    My problem is I have a boost converter with a high side PMOS switch to turn it on and off by supplying or killing power to it. It is supposed to turn on for a minute every ten minutes but sometimes (every 2-4 weeks but fairly randomly) it will turn on and stay on until power is removed and reapplied. This made me think of latchup but I'm just wondering if it is possible or likely as I have no experience with it.
     
  2. JMac3108

    Active Member

    Aug 16, 2010
    349
    66
    Post your schematic and let us have a look at it. I would like to see the gate circuit on your PFET.
     
  3. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    Do you have a schematic for members to look at?

    It is best to add another diodes to take care of inductive voltages and not count on the body diodes.
     
  4. eng_stu

    Thread Starter New Member

    Jan 29, 2012
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  5. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Don't you need a resistor to hold the gate low?
     
  6. eng_stu

    Thread Starter New Member

    Jan 29, 2012
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    But its PMOS. You need a resistor to hold it high (off state) and then short to ground to turn on
     
  7. tubeguy

    Well-Known Member

    Nov 3, 2012
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    If you are controlling the PMOS from the PIC, is it possible the PIC is getting 'stuck' randomly? Is it running with a watchdog timer? Proper PS bypassing?
     
  8. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Either way, I think a MOSFET gate needs a pull up or down resistor to keep it off until it's intentionall turned on. In other words, don't let it float.
     
  9. ScottWang

    Moderator

    Aug 23, 2012
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    The Gate(pin 6) of LT1619 that the inside have a driver, probably its structure like as NE555 can be pull high and draw to GND.

    The internal structure on page 5, and the connection samples of gate on page 1 and page 14 and they didn't add any resistor.

    LT1619 datasheet:
    http://www.datasheetcatalog.org/datasheet/lineartechnology/1619fa.pdf
     
  10. eng_stu

    Thread Starter New Member

    Jan 29, 2012
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    Scotts right, I just followed the datasheet example. Plus it has to be the PMOS not the NMOS. The PMOS has R3 pullup.

    Tubeguy, you could be right. A code/micro issue is the other explanation. I just wanted to know if I can eliminate a hardware problem as a candidate. I have run through the code and there is nothing that is obvious (ie getting stuck in loops etc) I will try to work in a watchdog timer but its a little bit tricking because it is sleeping for long periods.
     
  11. JMac3108

    Active Member

    Aug 16, 2010
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    Your mosfet circuit looks fine.

    When you get into this "stuck" condition, have you probed the circuit to see what's happening? You say it gets stuck and stays stuck until power cycled, so you should be able to probe it and see whats happening.

    When its stuck, measure the gate to see if the PIC is driving the mosfet on or off (low or high). This will tell you if the problem is hardware or software.
     
  12. JMac3108

    Active Member

    Aug 16, 2010
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    R3 is his pull-up to keep the P-FET off.
     
  13. JMac3108

    Active Member

    Aug 16, 2010
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    Scott,
    I believe he was talking about the other PFET, the one used to turn off the input voltage to the circuit. This one does require a pull-up and he has it. You are correct that the boost switching FET does not need one. The gate driver in the IC will always hold it to the proper state.
     
  14. ScottWang

    Moderator

    Aug 23, 2012
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    What kind of MOSFET would need a resister to draw to GND, in this case that it must be NFET, and the NFET was connecting to the LT1619, so I checked that IC and the situation just as I decribed, and that's why I guessed that he was pointed to the NFET.

    The gate of PFET already pull 100K to Vcc, and the value of resistor is too high, maybe 4.7K is ok, and the low will be came from pic, so i don't guess this one.
     
  15. JMac3108

    Active Member

    Aug 16, 2010
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    100K is fine for a pull-up or down to hold a mosfet gate off. I use this value in my designs all the time. All it has to do is prevent the gate from floating.
     
  16. ScottWang

    Moderator

    Aug 23, 2012
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    Yes, you are right.
    The PFET is using as switch for the power source, and the output of pic could be a OD, it just need a resistor to hold it not get into the floating state, so the resistor no need too low, in the begining that I saw the signal came from pic, so i consider that it maybe the R3 value need more lower.

    Long time ago, if the value as R3 that I may used as 470K or 1M, but now I will check many circuit to figure out a better value.
     
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