+ Post New Thread
Page 1 of 2 1 2 LastLast
Results 1 to 20 of 22
  1. #1
    Advanced Member level 5
    Points: 33,473, Level: 44
    Achievements:
    7 years registered

    Join Date
    Sep 2008
    Location
    cambridge
    Posts
    6,952
    Helped
    509 / 509
    Points
    33,473
    Level
    44

    NPN having its VBE protected against reverse voltage....but unecessary?

    Hi,
    The attached circuits provide an 8.3V rail which supplies approx. 10mA.
    The Left-hand version shows a diode D1, which is supposed to protect the NPN.
    Our contractor tells us that this is essential. However, though I don’t like over-reverse-voltaging NPN Vbe junctions, I find this diode D1 to be unnecessary in this case, do you agree?

  2. #2
    Advanced Member level 5
    Points: 24,506, Level: 38
    barry's Avatar
    Join Date
    Mar 2005
    Location
    California, USA
    Posts
    4,705
    Helped
    1038 / 1038
    Points
    24,506
    Level
    38

    Re: NPN having its VBE protected against reverse voltage....but unecessary?

    I suppose if the output (emitter) is sitting at some positive voltage and the opamp output suddenly went to zero you’d have a reverse bias. But why don’t you ask your contractor to explain it? Aren’t you paying them for their expertise?



  3. #3
    Advanced Member level 5
    Points: 40,248, Level: 49

    Join Date
    Mar 2008
    Location
    USA
    Posts
    6,495
    Helped
    1904 / 1904
    Points
    40,248
    Level
    49

    Re: NPN having its VBE protected against reverse voltage....but unecessary?

    The case where load is energized and then power is discharged
    could apply damaging reverse bias to the NPN.

    Low current reverse breakdown may only drift Vbe and cost low
    current beta - hot carrier charging of emitter-adjacent oxides.

    However I've done a fair number of circuits that use "zener zap"
    as a trim method, and this consists of milliamps forced backward
    across N+/P+ junctions (works well enough on N+/Pb if the base
    contact is made close enough). This turns the diode into an
    ohmic "short" permanently, as soon as you put enough energy
    into the small volume you'll pull aluminum into the silicon (all
    you need is for the silicon to hit the Al-Si eutectic temp, at
    even the smallest weak spot). The op amp forcibly depowered
    would have low enough reverse resistance (through ESD diode
    or explicit output devices) to program the kind of zap-zeners
    I use.

    You can't guarantee a particular transistor won't "program"
    at BVebo plus a skosh, as this is untested in production and
    depends on random defects for the low-ruggedness outliers.
    You might be able to build "population confidence" by adding
    a reverse-emitter-breakdown current test at 20mA (this
    ought to be comfortable margin against 12V/100 ohms) and
    use delta-Vbe, delta-low-current-hFE criteria (say, less than
    5% drift after some number of cycles representing power
    cycle timing with a fat output decoupling bank like the user
    might add externally).

    But I expect that if you don't want to pay for a cheap little
    diode, you don't want to pay for science projects either.
    How many diodes does it take eliminating, to pay for a man
    week of messing-about? Even if that messing-about pays
    off?


    1 members found this post helpful.

  4. #4
    Advanced Member level 3
    Points: 6,274, Level: 18

    Join Date
    Feb 2014
    Posts
    903
    Helped
    298 / 298
    Points
    6,274
    Level
    18

    Re: NPN having its VBE protected against reverse voltage....but unecessary?

    Other question: 12V to 8.3V at 10mA and you need this circuit?

    You could:
    Remove the NPN altogether and supply 10mA from the opamp directly
    Remove the opamp too and supply 10mA from the TL431 shunt circuit

    Or best: buy a single regulator IC that includes the reference



    •   AltAdvertisement

        
       

  5. #5
    Advanced Member level 5
    Points: 11,947, Level: 26

    Join Date
    Aug 2015
    Location
    Melbourne
    Posts
    1,955
    Helped
    713 / 713
    Points
    11,947
    Level
    26

    Re: NPN having its VBE protected against reverse voltage....but unecessary?

    the emitter base will zener at 6 - 7 volts the current limited by the 100E resistance and the output Z of the op amp assuming the emitter is held high by some external source

    if no external power ( associated with the load ) - then the ckt is fine without the prot diode as it is simply an emitter follower ... there may be a brief reverse pulse into the E-B zener due to the cap on the o/p but this is negligible ....



  6. #6
    Advanced Member level 5
    Points: 11,947, Level: 26

    Join Date
    Aug 2015
    Location
    Melbourne
    Posts
    1,955
    Helped
    713 / 713
    Points
    11,947
    Level
    26

    Re: NPN having its VBE protected against reverse voltage....but unecessary?

    Oh yes - the 431 could control the npn directly producing the 8v3 @ 10mA ....



    •   AltAdvertisement

        
       

  7. #7
    Super Moderator
    Points: 79,217, Level: 68
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    16,080
    Helped
    3642 / 3642
    Points
    79,217
    Level
    68

    Re: NPN having its VBE protected against reverse voltage....but unecessary?

    Hi,

    Or with even less components: use an adjustable voltage regulator.
    Space saving, low cost...

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



  8. #8
    Advanced Member level 5
    Points: 33,473, Level: 44
    Achievements:
    7 years registered

    Join Date
    Sep 2008
    Location
    cambridge
    Posts
    6,952
    Helped
    509 / 509
    Points
    33,473
    Level
    44

    Re: NPN having its VBE protected against reverse voltage....but unecessary?

    Thanks, this is on the PCB now, which has been made, and we cant go back unfortunately.
    Anyway, i think we will increase the base resistor to 3k3 (R172) to limit the base-emitter zener current (if it ever occurs)
    I saw a video once where they show some electronics meters which actually use a base-emitter of an NPN as a zener diode for circuit protection...so it can't be that bad i am thinking, as Easy Peasy partially notes in his kind description above.



    •   AltAdvertisement

        
       

  9. #9
    Advanced Member level 4
    Points: 7,616, Level: 20
    Achievements:
    7 years registered

    Join Date
    Jul 2010
    Location
    Sweden
    Posts
    1,021
    Helped
    388 / 388
    Points
    7,616
    Level
    20

    Re: NPN having its VBE protected against reverse voltage....but unecessary?

    Quote Originally Posted by treez View Post
    Thanks, this is on the PCB now, which has been made, and we cant go back unfortunately.
    Anyway, i think we will increase the base resistor to 3k3 (R172) to limit the base-emitter zener current (if it ever occurs)
    I saw a video once where they show some electronics meters which actually use a base-emitter of an NPN as a zener diode for circuit protection...so it can't be that bad i am thinking, as Easy Peasy partially notes in his kind description above.
    I would never allow a transistor to go into "reverse VBE zener mode" unless that is the intended operation. It can permanently reduce the gain of the transistor, as dick_freebird mentions in post #3.
    I would keep diode D1.


    1 members found this post helpful.

  10. #10
    Advanced Member level 5
    Points: 11,947, Level: 26

    Join Date
    Aug 2015
    Location
    Melbourne
    Posts
    1,955
    Helped
    713 / 713
    Points
    11,947
    Level
    26

    Re: NPN having its VBE protected against reverse voltage....but unecessary?

    Dear std_match, assuming the junction remains below 115 deg C how EXACTLY does zener operation reduce the gain of the device? natural drift or osmosis of dopants is generally temp dependent not reverse bias dependent ... ?



  11. #11
    Advanced Member level 5
    Points: 40,248, Level: 49

    Join Date
    Mar 2008
    Location
    USA
    Posts
    6,495
    Helped
    1904 / 1904
    Points
    40,248
    Level
    49

    Re: NPN having its VBE protected against reverse voltage....but unecessary?

    Quote Originally Posted by Easy peasy View Post
    Dear std_match, assuming the junction remains below 115 deg C how EXACTLY does zener operation reduce the gain of the device? natural drift or osmosis of dopants is generally temp dependent not reverse bias dependent ... ?
    Pushing carriers acros the reverse breakdown potential makes
    them "hot" electrically and they can leave the silicon and embed
    holes or electrons in adjacent oxides.

    This charging changes surface recombination behavior and "steals"
    a portion of the base current, without collector gain, reducing the
    terminal hFE. This is mostly at low Ic, Ib (the recombination term
    is roughly fixed, so matters more at low currents). The beta-vs-Ic
    curve becomes "peaky" when what you want is a fairly consistent
    "flat top".

    The "zener" itself is not causing the degradation, it's the act of
    forcing the carriers backward across it and the hot carrier effects
    of so doing.

    Zeners designed for stability have a "buried" structure where the
    active zener junction is entirely subsurface, and can't charge
    an abutting oxide. Surface zeners, which are often opportunistic
    arrangements of the emitter implants in IC technologies, are much
    less stable and not used for reference products.


    1 members found this post helpful.

  12. #12
    Advanced Member level 5
    Points: 33,473, Level: 44
    Achievements:
    7 years registered

    Join Date
    Sep 2008
    Location
    cambridge
    Posts
    6,952
    Helped
    509 / 509
    Points
    33,473
    Level
    44

    Re: NPN having its VBE protected against reverse voltage....but unecessary?

    This video discusses using an NPN as a clamping zener diode, via its reverse biased BE junction
    https://www.youtube.com/watch?v=BGcK...ature=youtu.be



  13. #13
    Advanced Member level 5
    Points: 11,947, Level: 26

    Join Date
    Aug 2015
    Location
    Melbourne
    Posts
    1,955
    Helped
    713 / 713
    Points
    11,947
    Level
    26

    Re: NPN having its VBE protected against reverse voltage....but unecessary?

    Are there many oxides in a BJT ...? I would have thought not - in CMOS maybe ...


    1 members found this post helpful.

  14. #14
    Advanced Member level 5
    Points: 40,248, Level: 49

    Join Date
    Mar 2008
    Location
    USA
    Posts
    6,495
    Helped
    1904 / 1904
    Points
    40,248
    Level
    49

    Re: NPN having its VBE protected against reverse voltage....but unecessary?

    BJTs still have the "field oxide" insulating the metallization
    from silicon. CMOS adds the engineered thin oxide (active
    area) as the gate insulator, a field oxide etch followed by
    controlled growth (thickness, rate, chemistry).


    1 members found this post helpful.

  15. #15
    Advanced Member level 5
    Points: 11,947, Level: 26

    Join Date
    Aug 2015
    Location
    Melbourne
    Posts
    1,955
    Helped
    713 / 713
    Points
    11,947
    Level
    26

    Re: NPN having its VBE protected against reverse voltage....but unecessary?

    surely in a BJT the metallisation goes straight onto the Si, then if you want to protect the Al metal, you form an oxide on that, any non metallised Si can be oxidised - on the surface - but is there is not much near the junction in a planar device - in fact most of the junction is buried ... ?


    1 members found this post helpful.

  16. #16
    Advanced Member level 5
    Points: 40,248, Level: 49

    Join Date
    Mar 2008
    Location
    USA
    Posts
    6,495
    Helped
    1904 / 1904
    Points
    40,248
    Level
    49

    Re: NPN having its VBE protected against reverse voltage....but unecessary?

    Quote Originally Posted by Easy peasy View Post
    surely in a BJT the metallisation goes straight onto the Si, then if you want to protect the Al metal, you form an oxide on that, any non metallised Si can be oxidised - on the surface - but is there is not much near the junction in a planar device - in fact most of the junction is buried ... ?
    The entire surface must be insulated. Consider the usual
    vertical NPN. To get to the emitter, the centermost active
    region, you must traverse isolation, collector and base.
    If the surface were not generally ozidized you would short
    all four. Because it is oxidized, you make contact cuts.

    Even simple discrete devices will have field oxide and
    contacts. Aside from the electrical insulation there is also
    the need to exclude external contamination post-fabrication.


    1 members found this post helpful.

  17. #17
    Advanced Member level 5
    Points: 11,947, Level: 26

    Join Date
    Aug 2015
    Location
    Melbourne
    Posts
    1,955
    Helped
    713 / 713
    Points
    11,947
    Level
    26

    Re: NPN having its VBE protected against reverse voltage....but unecessary?

    you seem to be allowing my point - which is - by far most of the B-E junction is buried - away from any oxides ... ?


    1 members found this post helpful.

  18. #18
    Advanced Member level 5
    Points: 40,248, Level: 49

    Join Date
    Mar 2008
    Location
    USA
    Posts
    6,495
    Helped
    1904 / 1904
    Points
    40,248
    Level
    49

    Re: NPN having its VBE protected against reverse voltage....but unecessary?

    The point of relative areas is semi-irrelevant. Inducing non-active
    (usually) regions to "steal" base current from the region that
    actually has current gain ruins low current beta, and "programming"
    just needs one weak spot between emitter and base contacts with
    lower breakdown than the junction bottom "plate".

    More is worse, but only "none" is good (i.e. same as un-abused).



  19. #19
    Advanced Member level 5
    Points: 33,473, Level: 44
    Achievements:
    7 years registered

    Join Date
    Sep 2008
    Location
    cambridge
    Posts
    6,952
    Helped
    509 / 509
    Points
    33,473
    Level
    44

    Re: NPN having its VBE protected against reverse voltage....but unecessary?

    Thanks , this is very interesting. I’ve now heard it from a few different sources that driving current through the VEBO (zener) is bad in that it degrades the hfe.
    The thing is, it’s a question of how much it degrades it…..
    …..If I make the R172 of the right-hand schem of the top post equal to 3k3, then that’s less than 1mA going through the zener, just until the 200nF on the output rail discharges down. Surely I am thinking sub 1mA is nothing, but I suppose the damage caused is cumulative?



    •   AltAdvertisement

        
       

  20. #20
    Advanced Member level 5
    Points: 11,947, Level: 26

    Join Date
    Aug 2015
    Location
    Melbourne
    Posts
    1,955
    Helped
    713 / 713
    Points
    11,947
    Level
    26

    Re: NPN having its VBE protected against reverse voltage....but unecessary?

    I have never seen a BJT fall to un-useable levels of gain through being OCCASIONALLY used as a zener, when it is used as a zener - the gain is often irrelevant.

    Fluke use 2 x npn in an AC zener clamp arrangement in a lot of their meters - the gain is used as one of the devices has to be on for the clamp to work - an elegant ckt actually

    - - - Updated - - -

    sorry to harp on - but those carriers in avalanche won't be attracted, per se, to oxide regions, they will go from emitter to base, N to P type in an npn xtor - certainly avalanche & zener operation is energetic for the carriers but the suggestion they will punch through the receiving semiconductor and then on to the oxides ( at the fringes where the oxides exist ) seems ( to me at least ) to be overstated. However if the empirical evidence is there that the gain reduces after some use as a zener, I won't dispute it.



--[[ ]]--