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Transistor breakdown voltage

Discussion in 'Electronic Repair' started by Rudge, May 25, 2007.

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  1. Rudge

    Rudge Guest

    I need a T039 NPN transistor with an 80 volt BVceo rating such as a BC142.

    My nearest stockist holds the BC141 which has a BVceo rating of 60volt.

    If I buy a BC141 and test the breakdown voltage, what are the chances that
    it will exceed 80volt?



    I would also like to know how manufacturers make these almost identical
    transistors with different voltage ratings.



    Do they just make a batch of BC140s and those which exceed 80v get stamped
    'BC142'?



    Or do they make separate production runs for each type?



    Also, do the higher voltage versions loose out on some other parameter such
    as gain?



    Many thanks,

    Rudge
     
  2. Eeyore

    Eeyore Guest

    No idea, but specific devices are indeed selected by parameters from a general
    device group by testing, so you're in with a chance.

    If the yield at higher voltages was good for that batch, you may well find that
    the '141 may meet a '142 spec.

    What application is it used in btw ?

    Graham
     
  3. Arfa Daily

    Arfa Daily Guest

    I would suspect that breakdown voltage is a 'designed in' characteristic,
    and will depend on physical dimensions of the transistor structure. The
    "cherry picking" that the manufacturers do is usually related to gain and
    gives something like an A, B or C suffix like BC108A or BC109B. How would
    the manufacturer actually test a batch for breakdown voltage, as if you take
    the device to breakdown, then it is destroyed. Would the manufacturer be
    prepared to stake the breakdown voltage of a production batch, on the basis
    of a few tested examples ? I'm not sure that he would. It's a bit like
    having tested fuses really, isn't it ?

    Arfa
     
  4. Pilgrim

    Pilgrim Guest

    You are right to a certain extent. However, Mfg's do sort by
    breakdownvoltage. They do this with a constant current supply set at a
    very low current and measure the voltage.

    Chuck P.
     
  5. Eeyore

    Eeyore Guest

    You're very mistaken.

    The test is done with a current limited supply and the test is for a given
    leakage current.

    Due to necessity (the situation was forced on me) I've had both bipolar and
    mosfet devices selected very successfully for breakdown voltage.

    By these guys in fact.
    http://www.semelab.co.uk/

    Graham
     
  6. Eeyore

    Eeyore Guest

    Correct.

    Graham
     
  7. mike

    mike Guest

    There are a BUNCH of issues here.
    What's the actual voltage applied in the application?

    Given volume discounts and inventory reduction, it's not unusual
    for a vendor to use a part in an application that would do fine
    with a MUCH lesser part.

    Do you really care about Bvceo? That's relevant if e is really o.
    Excellent metric, but sometimes not the metric you want.

    If you're running it anywhere near 80V, you might wanna think
    about secondary breakdown. Devices with similar text specs
    can behave quite differently at high voltages.

    Worry about temperature. If you're selecting breakdown voltages,
    make sure you measure at temperature extremes.

    One simple way to test breakdown is to put your voltmeter in series
    with the power supply. IF the meter is 10megs and you have the reading
    and the power supply voltage, you have enough info to calculate
    the current at various voltages. If you need more current shunt
    the meter with a resistor. If you try to use a current meter and the
    device goes into avalanche, you've just toasted your transistor and
    maybe your meter.

    I don't know how they do it today, but back in the day, there were
    a few "formulae" for transistors. For a given die, you ran it
    across the test bed and put it in whatever bin the specs met.
    If it didn't meet any, it went into the 2n3904 bin.

    Whether you can up-spec a part by testing depends a lot on the
    yield of that part. Some parts have high yield and they
    have to ship better parts than the spec requires. But that's
    not always the case. Back when I didn't know better, I tried
    to select 1% resistors out of 20% for a test jig.
    Turned out there was a BIG hole in the distribution that was
    10% wide. 10% resistors had a big hole in the distribution
    5% wide and so on. Somebody got there before me ;-)
    But I fooled them, just recalculated the ratios to be
    10-20% off the standard value.

    mike
     
  8. Arfa Daily

    Arfa Daily Guest

    Ah, ok. I can see how that would work. Do manufacturers ( rather than
    distributors like SEME) really still do this ? Seems like a lot of trouble
    to go to. There are so many (hundreds of thousands ??) of transistor types
    available now, I would have thought that it was possible to select one for
    whatever breakdown voltages, gain, and whatever other parameters you might
    require, without difficulty ? Maybe not. Thanks for the info.

    Arfa
     
  9. Eeyore

    Eeyore Guest

    Absolutely they do ! Semelab manufactures transistors too btw. They and Zetex
    probably represent the entire UK semiconductor manufacturing sector.

    The Semi makers have to test the devices for function and things like gain, so
    they test for loads of stuff at the same time. It's all automated.

    Here's an example of what sometimes goes on with device selection.

    Decades ago we were using a Motorola part, an MJ410 in a high power audio
    amplifiers as a driver. The purchasing dept found the RCA410 'equivalent' at a
    lower price and initial samples worked fine so we used them.

    Some months later we started getting stability problems with *some* of the
    RCA410s. An alert test engineer noticed that the problem parts had a specific
    code stamped into the TO-3 header. It turned out that were 3 such codes in
    total; C, CN, and CNX.

    I sawed the lid off one of each and even a quick inspection by eye showed that
    each one had a different die fitted.

    RCA initially denied everything but I eventually got an admission form their
    Belgian European Headquarters.

    Motorola's spec on the MJ410 was quite minimal. It didn't matter to them because
    they made the part all from the same line. RCA however looked at the skimpy spec
    and reckoned they could fit lots of devices that failed to meet better and more
    financailly worthwhile specs into their eqivalent RCA410 device spec. So they
    made RCA410s from the fall-outs from 3 other product lines with widely varying
    other specs (that weren't on the data sheet). Those varying 'other specs' were
    what was giving us trouble.

    Graham
     
  10. Rudge

    Rudge Guest

    I want an NPN T039 or T05 driver transistor for use in an audio amplifier.
    The supply rail is 75volt. Quiesent current is about 10ma I think. 1 Watt
    dissipation at 25 C. The original was an RCA 39252 (which I have no data on)
    or a 2N2102. They may have been selected to to work at 75volt.

    Many thanks,
    Rudge
     
  11. Arfa Daily

    Arfa Daily Guest

    Interesting story. Just goes to show that no matter how 'reputable' a
    company, you really never can tell exactly what they are up to ...

    Arfa
     
  12. mike

    mike Guest

    You need to give more details on the circuit. If this is the first
    stage of the amplifier, it's a lot easier than if it's the "driver"
    for the output stage.
    If this thing goes between ground and 75V, you want WAY more than an 80V
    transistor.
    If it goes between +-75V, you need way more than a 150V transistor.
    Unless this is a special transistor, you can't expect to get anywhere
    the maximum power dissipation at anywhere near the breakdown voltage.
    Take a serious look at the dynamic characteristics, peak voltages
    and currents driving the capacitive load and the secondary
    breakdown curves. The RCA Power Circuits manual SP-51 has
    a whole chapter on this.

    And worry about the power numbers too. That 25C number is a metric
    that gives you some idea about what's going on. What you really
    care about is the die temperature. And it's WAY more than ambient
    in a small package like that.
    mike
     
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