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diode-connected transistors

Discussion in 'Electronic Design' started by Terry Given, Nov 16, 2004.

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  1. Terry Given

    Terry Given Guest

    I often use diode-connected npn and pnp transistors. Sometimes I use the
    base-emitter junction reverse-breakdown as a (somewhat dodgy) zener,
    sometimes I use them as diodes. When used as a diode, I can either use
    the c-b junction (b shorted to e) or the b-e junction (b shorted to c).

    Here are my questions:
    1. Which configuration has the lowest capacitance ?
    2. which configuration has the lowest leakage current?

    In a more general sense, what are the pros and cons of either
    connection? Using say bc547 and bc557 transistors.

  2. Jim Thompson

    Jim Thompson Guest

    CJE is generally larger than CJC (see below), but not necessarily so,
    since capacitance is a function of both doping level _and_ area.

    A B-E junction has lower breakdown than B-C.

    ..MODEL QBC547A/PLP_XN NPN(IS=15.33f NF=1.002 ISE=0.7932f NE=1.436
    BF=178.7 IKF=121.6m VAF=69.7 NR=1.004
    + ISC=83.05f NC=1.207 BR=8.628 IKR=112.1m VAR=44.7 RB=1 IRB=1u RBM=1
    + RE=639.5m RC=650.8m XTB=0 EG=1.11 XTI=3 CJE=16.1p VJE=420.9m
    + TF=499.5p XTF=139 VTF=3.523 ITF=702.1m PTF=0 CJC=4.388p VJC=200m
    + XCJC=619.3m TR=1.e-032 CJS=0 VJS=750m MJS=333m FC=776.2m)

    ..MODEL QBC557A/PLP_XN PNP(IS=20.59f NF=1.003 ISE=2.971f NE=1.316
    BF=227.3 IKF=87.19m VAF=37.2 NR=1.007
    + ISC=13.39f NC=1.15 BR=7.69 IKR=76.46m VAR=11.42 RB=1 IRB=1u RBM=1
    + RE=688m RC=643.7m XTB=0 EG=1.11 XTI=3 CJE=14p VJE=591.2m MJE=357.2m
    + TF=704.6p XTF=4.217 VTF=5.367 ITF=194.7m PTF=0 CJC=11.13p VJC=100m
    + XCJC=628.8m TR=1.e-032 CJS=0 VJS=750m MJS=333m FC=794.7m)

    ...Jim Thompson
  3. Dingus

    Dingus Guest

    Diodes are cheaper, more predictable, accurate and reliable than cobbling up
    some home made diode-like device.
  4. John Larkin

    John Larkin Guest

    Some NPNs make superb reference zeners if operated as a zener, C-E
    with the base open. The tc of the forward-biased cb junction cancels
    the tc of the zenering be junction; this works right at about 6.2
    volts overall.

  5. Pooh Bear

    Pooh Bear Guest

    I believe at's actually avalanche breakdown.
    Can't think of a good reason to do it to be honest, although I've used the
    b-e junction to measure temperature ( because TO-220 devices are easy to
    mount in a thermally meaningful way ).

  6. John Larkin

    John Larkin Guest

    There is a TO-220 version of the LM35. Handy on heat sinks.

  7. Terry Given

    Terry Given Guest

    that all depends now, doesnt it. If you are designing with smt,
    minimising part types is extremely important. There are also often
    economies of scale. one of my customers uses a LOT (millions) of
    BC847bpn dual npn/pnp transistors in a little 6-legged package. If I
    need a few diodes it is a LOT cheaper to use one of the existing
    transistors, than add a whole new reel. Some of my other customers use a
    few of these parts - and converting a bunch of diodes into
    diode-connected transistors saves a slot on the smt machine (and for a
    contract manufacturer that's one less slot to screw up) and can
    significantly increase the volume of the one part purchased, thereby
    leading to further cost reductions.

    Economic considerations aside, there is nothing at all "wrong" with
    using a diode-connected transistor. pick an analogue IC, any analogue IC
    and look inside. Hell, any decent discrete audio amp likely has them in
    the current mirrors. Go read Grey & Meyer.....

    Although b-e breakdown voltage is not a parameter to rely on, especially
    between manufacturers. But nevertheless can still be useful.

  8. Terry Given

    Terry Given Guest

    Hi John,

    Thanks, I'll try that. Care to elaborate on "some" ?

  9. Terry Given

    Terry Given Guest

    odds on. Hell, any "zener" over what, 5V or so (too lazy to look it up)
    is actually an avalanche diode, but we still call them zeners. Think how
    tedious typing "zener (avalanche) diode" everywhere you use "zener"
    would become :)
    see above reply to dingus for the reasons.

    I've forward-biased the input ESD diode in a variety of semiconductors
    (micro, CPLD etc) to measure Tj.

  10. Terry Given

    Terry Given Guest

    Hi Jim,
    So usually the lowest capacitance diode is the B-C junction, ie B-E
    shorted, but as usual *it depends* on the actual device used :)
    I presume the B-C junction will break down at Vcbo (rather than Vceo)?
    So for these parts the npn B-C diode is about 4x less capacitance
    (4.4pF)than the B-E diode (16.1pF), whereas for the pnp they are both
    similar - 11pF vs 14pF (all at zero bias)

    Thanks very much
  11. Jim Thompson

    Jim Thompson Guest


    Yes, Provided you tie the emitter to the base. Let it float and see
    what funny business you get ;-)

    ...Jim Thompson
  12. Terry Given

    Terry Given Guest

    Hi Jim,

    Thanks for that.

  13. Pooh Bear

    Pooh Bear Guest

    A bit pricier than the device I was using though ! They produced pretty
    reproducible results ( good for about +/- 5C ).

  14. Jim Thompson

    Jim Thompson Guest

    You are quite welcome! I hope I was helpful.

    ...Jim Thompson
  15. John Larkin

    John Larkin Guest

    You can also make quick time-slice measurements of the substrate diode
    of a power mosfet to measure realtime junction temp and deduce
    transient thermal behavior.

  16. Terry Given

    Terry Given Guest

    Yep. And it also works with 600A IGBTs. Very important when you beat the
    hell out of your silicon :)

    I did an analysis recently of a DCM flyback converter using an infineon
    FET. My simulation closely matched the real waveforms, but I found a
    start-up problem (measured it too) which gave the FET a hiding. Infineon
    kindly supplied me with a 6th order thermal model, which was duly
    entered into the simulation, along with a function block to measure
    power loss. The work of a few minutes, but it then allowed me to look at
    the effects of various soft-start circuits on dTj (conclusion: the
    soft-start quartered peak dTj)

    Do you have any data on thermal cycling related fatigue of little
    devices (T0220, TO3P etc)? Big IGBT manufacturers have these nice
    lifetime curves - no. of operations versus delta-Tj (eg powerex have
    them in most datasheets). The failure mechanism is thermal runaway -
    large dTj plus differing CTE causes voids under the die, increasing
    Rtheta, leading to more hot spots...voila. Little dutch time bomb, tick
    tock boom. see
    figure 3.16, p.15:

    6,000 cycles at dTj = 100K;
    200,000 cycles at dTj = 50K;
    1,000,000 cycles at dTj = 30K

    so the no. of cycles halves for a 10K increase in dTj.

    I once designed a constant-loss modulator - it made switching frequency
    inversely proportional (ish - curve fit) to load current, so at light
    load there was a lot of switching loss and less conduction loss;
    vice-versa at high load. That way the IGBTs didnt cool down so much
    between load steps, thereby reducing dTj and dramatically increasing

  17. Joerg

    Joerg Guest

    Hi Dingus,

    The reasons Terry gave are the most common for using a transistor as a diode. Heck, I have even used them for logic if I didn't want to spring for another CMOS chips. Whatever saves a penny is usually the better solution.

    Also, if you had to demodulate or detect very tiny signals a reversed transistor exhibits a much better performance while a 'real' diode often is not very useful at the millivolt level.

    Regards, Joerg
  18. Terry Given

    Terry Given Guest

    Yep - I've got an inverter made from a BC847bpn and a 10k 1206 quadpack
    buried in the synchronous rectifier gate-drive circuitry for the 55W
    smps inside each module. I also use them as gate clamping zeners (in
    addition to the fast gate turn-off) in my charge-pump-driven soft-start
    bypass FET. I want to try out John Larkins suggestion of E-C breakdown
    (reverse-breakdown of B-E, forward bias of B-C) to see if I can throw
    out a zener diode (thats biased to 5mA for low tempco) now :)
    Hmm, notice the terms VJE and VJC in the models Jim posted:

    bc547: VJE = 420.9mV, VJC = 200mV
    bc557: VJE = 591.2mV, VJC = 100mV

    According to "semiconductor modelling with SPICE" (Massobrio et al)
    these are the base-emitter and base-collector "built-in potentials".

    They crop up in the capacitance calcs as the factor (1-Vbe/VJE)^MJE and
    likewise (1-Vbc/VJC)^MJC

    So the improved behaviour of the inverse-connected base-collector
    junction is apparent.

    Thanks Jim for the idea (doh) of looking at a spice model. Its quite
    complex. Does it work for inverse connections? (methinks it should)

    the inverse pnp (100mV) appears better than the inverse npn (200mV),
    even though the converse holds for conventional use (591mV vs 421mV). Is
    this actually so?
  19. John Larkin

    John Larkin Guest

    6000 cycles! Yikes!
    No, no data on thermal fatigue. I thought that problem was solved ages
    ago. My NMR pulsed-gradient drivers have huge dissipation spikes,
    typically going from, say, 40C to 130C Tj in 20-40 milliseconds, once
    a second, in big 300 watt mosfets. I've worried about fatigue, but so
    far haven't had many fet failures, or signs of failure rate increasing
    with use. I run software that simulates Tj in real time and shuts down
    the amps at simulated 140C junction temp. Why don't big fets have
    on-chip temp sensors like CPUs?

    Maybe bigger slabs of silicon, like huge SCRs and IGBTs, have more
    severe thermal stress issues. IGBTs have real high power densities,
    don't they?

    I did discover that a lot of 300-watt rated power fets will explode in
    under 100 msec when they really dissipate 300 watts, bolted directly
    to a cold copper block.

  20. Terry Given

    Terry Given Guest

    Its pretty bad all right. Normally in a drive this only happens when
    there is an overload. But overload ratings are a great area for
    specmanship. nd you can see how its quite possible to get great overload
    ratings for a while...
    A lot of big IGBTs do. Semikron make a range of neat goodies. You might
    want to take a look at their SemiTOP range.
    These powerex die are about 1cm on a side. Conduction loss is 1200W or
    so at 600A. Switching loss is a lot higher, but obviously for short
    durations. And yeah, its certainly related to the size - dx/x is
    constant but larger x has a correspondingly larger dx.
    I've seen those sorts of silly numbers for TO220 parts with Rtheta_j_s
    of 1K/W. Go figure.....mind you I once worked with a guy who had built a
    25kW liquid-nitrogen cooled inverter using little FETs.

    Semikron once sold us a "rupture proof IGBT package" so we smacked it
    into a prototype 100kW inverter DC bus assembly and dumped a full bus
    charge into it - 1.4kJ or so. It went BANG and the whole assembly jumped
    a few feet in the air. So we replaced the dead-but-not-ruptured IGBT
    with another one, and then sat an anvil on top of the PCB. This time the
    package exploded - in front of the salesman. So much for rupture-proof :)

    Mind you I once tested an smt 1500W TVS with a 600J pulse. There was
    NOTHING left - not even the J-leads. I had covered the experiment with a
    pyrex jug, and there were several distinctive splats - copper,
    carbonised plastic and re-solidified silicon. The office lady was pissed
    I had wrecked the jug, so I bought her a new one :)

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