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When I connect two PMOS in series and let both of them working in cut-off region, it seems the voltage drops of those MOS are not evenly distributed: for example, when I apply 24V (higher than the breakdown voltage of a single MOS), one of them has a voltage of almost 20V, and the other takes 4V; and when I apply 22V, one of them has 20V and the other takes 2V.... this also applys for the simluation (LTspice,not here strangely lol). P.S. the breakdown voltage of a single MOS is 20V, I have a application which requires 27V, so I want to 'increase' it a bit, I only need them to work in cut-off.
the circuit can be seen below:

schematic

simulate this circuit – Schematic created using CircuitLab

schematic

simulate this circuit

VDD/V V1/V V2/V

15 15 0

20 19 1

21 19.1 1.9

23 19.3 3.7

25 19.5 5.5

my current soultion is to connect a 1M resistor to each of those MOSs parallelly, but I wonder why this happens: why are the voltage not evenly distributed? or is there any better ways to even the voltage (I tried using Cap instead of resistors but not working)?

Thx for your time and looking forward to anyone's suggestions!

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    \$\begingroup\$ Different gate to source voltages? \$\endgroup\$
    – MadHatter
    Apr 30, 2020 at 3:55
  • \$\begingroup\$ The gates are connected to the same potential but the sources are connected to different points, so the gate-source voltage of each MOSFET is different. Also, the MOSFETs are in different packages and on different dice so they are not identical either. \$\endgroup\$
    – DKNguyen
    Apr 30, 2020 at 4:20
  • \$\begingroup\$ yeah,seems the problem is there, forget to count in this parameter, cheers guys! \$\endgroup\$
    – Charles Zhang
    Apr 30, 2020 at 7:11
  • \$\begingroup\$ But this does not explain why one of the MOS stay on the breakdown voltage, any idea? \$\endgroup\$
    – Charles Zhang
    Apr 30, 2020 at 10:23

1 Answer 1

Reset to default
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The drain--bulk leakages may be causing this. Examine those models.

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  • \$\begingroup\$ yeah, it seems Vgs is different which result in this problem, but this does not explain why one of the MOS stay on the breakdown voltage... \$\endgroup\$
    – Charles Zhang
    Apr 30, 2020 at 10:24

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