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    350V BJT cannot stand overvoltage of VCE=400V

    Hi,

    The attached is a HV linear regulator based on a BSP19 BJT. This is rated 350V. At 100ms in the simulation, the VCE of BSP19 suffers a transient of 400V……our contractor says that because of the 5k1 resistor in series with the collector, the effect of this overvoltage will be survivable by the BJT. We don’t agree…do you?

    LTspice sim and pdf schem attached

    BSP19 datasheet
    https://assets.nexperia.com/document...t/BSP19_20.pdf

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    Re: 350V BJT cannot stand overvoltage of VCE=400V

    I'm not familiar with high voltage BJT breakdown characteristics. A fet with a proper voltage and avalanche rating would do the job well.

    For these applications I like depletion fets because they eliminate the high voltage/power requirement on R2 (R2 would go to the drain instead). They're very common at 400V+.

    See LND150


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    Re: 350V BJT cannot stand overvoltage of VCE=400V

    Yes and no.. There should always be a safety margin in engineering.Using a component at its' limits is not a proper idea.Because max. ratings given for any component are "nominal" values so harsh environment,high temperature,semiconductor manufacturing differences,unpredictable events will play a very important limiting roles in a design.I suggest you to use a transistor with a higher Vce limits like 600 V or more..



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    Re: 350V BJT cannot stand overvoltage of VCE=400V

    the current is limited to 400 / 5100 = 80mA as long as the peak die temp stays below 150 degC the xtor will survive .....



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    Re: 350V BJT cannot stand overvoltage of VCE=400V

    Thanks, so it will survive, but will be slightly damaged, and then when the overvoltage repeats again anfd again, the cumulative damage will build up and the device will fail?
    The 400V on VCE, is , after all, above the max VCEO of 350V for BSP19



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    Re: 350V BJT cannot stand overvoltage of VCE=400V

    Your mileage may vary but personally I wouldn't trust it at any voltage higher than its rating. It is followed by a 33uF capacitor to ground and I assume R3 is actually some kind of load so the time constant at the emitter is quite long. Why not just add a small capacitor from collector to ground so that in conjunction with R1 you eliminate the offending spike altogether.

    Brian.
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    Re: 350V BJT cannot stand overvoltage of VCE=400V

    There are a handful of high voltage regulators too. This comes in the same package as your fet. Eliminates a couple components but the 500uA minimum might be annoying.

    https://www.digikey.com/product-deta...GCT-ND/4918747


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    Re: 350V BJT cannot stand overvoltage of VCE=400V

    The peak power in the device is Vbreakover-xtor x current. Vbreakover is Vin minus the volt drop in the 5k1.

    say the xtor breaks over at 350V for 400 Vin the current is therefore 50/5k1 = 9.8mA power in the xtor is then 3.4W

    depending on heatsinking etc a pulse 0.5mS wide every half cycle ( 170mW ave ) will be unlikely to kill the device - unless it breaks down to a voltage of half the Vin ( max power point ) even then this is only 39mA at 200V = 7.7 watts x 0.5/10 (ms) = 0.4 watts ave ...

    - - - Updated - - -

    as the current is limited - you will not "kill" the xtor for just a few pulses ...



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    Re: 350V BJT cannot stand overvoltage of VCE=400V

    Regardless of whether Q1 is damaged or not, when it breaks down you're going to see a rise in the output voltage. I assume that's going to damage the load, so you will need some sort of mitigation. Otherwise you might as well put a 350V clamping device across Q1. At least then Q1 won't be damaged...


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    Re: 350V BJT cannot stand overvoltage of VCE=400V

    Yes, a higher voltage xtor or 5W 16V zener on the o/p - or both ...



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