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Splitting voltage across BJTs to avoid breakdown

Discussion in 'General Electronics Discussion' started by Knillinux, Aug 17, 2013.

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  1. Knillinux


    Aug 17, 2013

    I'm trying to use a BJT bipolar totem pole to drive a MOSFET gate to an arbitrary voltage, similar to this:


    However, I am working with +/- 240 V, driving gate capacitances of over 10nF at ~1 MHz, which requires up to 4.5A and hundreds of watts of power in the BJTs. I'm finding it difficult to get power BJTs, especially PNPs, which have a 450+V CE breakdown voltage and can dissipate hundreds of watts of power.

    So is it possible to split the voltage between chained BJTs? Here's a design I threw together to do this:


    The resistor ladder splits the voltage evenly between the BJTs so that none of them experience a Vce over 240V. The diodes are to compensate for the Vbe of the BJTs.

    It works well in SPICE, but I don't have time to do real life testing before inserting this into my project. The resistor values can be lowered if more current is needed to feed the BJTs.

    Are there any potential problems I'm overlooking?
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Vcc should probably not exceed 20V in this application (more typically 10 to 15V)

    Why do you need BJT transistors capable of switching such a large voltage?
  3. john monks

    john monks

    Mar 9, 2012
    I am very interested in your chained BJTs. I am tempted to try building this circuit and characterizing it. I've laddered JFETs this way successfully. But I don't see what this has to do with driving a MOSFET. Typically I use a circuit similar to your first schematic to drive a MOSFET. Sometimes I simply use other MOSFETs to drive a MOSFET. Typically you drive a MOSFET with 5 and 0 volts. Can you state your purpose?
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    As Steve and John have already said, your transistors should be switching fairly low voltages.

    MOSFETs have a maximum allowable gate-to-source voltage, Vgs(max), which is typically in the range 10~30V with either polarity. If you exceed that voltage, the gate insulation will be damaged and the MOSFET is history. This voltage is not related to the maximum drain-source voltage that the MOSFET will withstand.

    Standard MOSFETs typically saturate with a positive gate-source voltage in the range 5~10V and a +12V rail is often used as the supply for the drive circuit.
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