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  1. #1
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    Pulse power dissipation in a BJT

    Colleagues,

    I'm making a pulsed current sink. It will be a low side constant current sink, which will consist of an OpAmp, a current sense resistor, an NPN transistor.
    The output current will be up to 16mA. The voltage drop across the the transistor will be up to 100V. Pulse duration will be up to 10ms. Duty cycle will be up to 10% .
    Space is at a premium in my instrument. It will be hard to fit anything larger than a SOT-23.

    NPN transistors in SOT-23 are usually rated for around 200mW dissipation. Am I going to burn the NPN transistor if I have a 1.6W peak power dissipation but keep the average power dissipation under 200mW ?

    Any suggestion, insight or reference is really appreciated!

    - Nick

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  2. #2
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    Re: Pulse power dissipation in a BJT

    In addition to the average power dissipation, in a bipolar transistor you must also observe the SOA curves.
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    Re: Pulse power dissipation in a BJT

    Hi,

    Yes, I agree with schmitt trigger.
    Most transistor datasheets show an SOA chart.
    I expect no big problem... but refer to the datasheet.

    Klaus
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    Re: Pulse power dissipation in a BJT

    10 ms is much too long for the small thermal capacitance of low power SOT23 transistor, e.g. BC847. A medium power transistor in SOT23 with larger chip might work.



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    Re: Pulse power dissipation in a BJT

    It's hard to find thermal time constant data for discrete
    devices (let alone ICs).

    Now when you say that space is constrained, does that
    mean you have no plan for a heat sink? What is thetaJA
    times 200mW, and what's your probably-still-air penalty?

    I much prefer switched-resistor loads (power DAC using
    MOSFETs of very low Rds(on) and binary weighted burden
    resistors) as they have no control loop to meddle with
    the output and -its- stability sensitivities. Have had trouble
    with E-loads' transient behaviors which led me to create
    my own load jigs.



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    Re: Pulse power dissipation in a BJT

    I see that medium power transistors like FMMT495, MMBT5550 or BCX41 have apparently sufficient thermal capacity to handle the requested pulse load, under the prerequisite that the average power can be dissipated on your board.



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    Re: Pulse power dissipation in a BJT

    Sot-223 ...



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