Voltage error through Analog Load Switch

Discussion in 'General Electronics Chat' started by BKI, Nov 13, 2014.

  1. BKI

    Thread Starter New Member

    Nov 13, 2014

    i have a problem with analog switches.

    I have two high voltage supply rails (-60V and +200V). I want to connect those rails to the load(5MOhm/10nF) through analog switches.
    I already simulated it in Cadence with external NMOS transistors. That worked.
    Now i try to integrate it with PMOS transistors (i only have integrated PMOS available).

    What the circuit should do is: at first the output voltage should go from 0V to -60V, then afterwards from -60V to +200V.
    What the circuit does in the simulation is: at first the output voltage goes from -50V to -110V, then afterwards from -110V to +120V.

    I don´t understand why the output voltage starts at -50V and not at 0V. Do you have any idea what the problem is? And how can i resolve it?
  2. crutschow


    Mar 14, 2008
    The problem is the parasitic body diode in MOSFETs. If you draw in that diode you will see the problem with your circuit.
    Since a MOSFET will conduct in both directions, one solution is to use two MOSFETs in series in a back-to-back connection (source to source). That way one of the body diodes is always blocking.
  3. BKI

    Thread Starter New Member

    Nov 13, 2014
    I only have integrated PMOS transistors available. If i use two PMOS in series connected source to source, i have the problem that one transistor has a positive drain-source voltage right?
    That is a problem because Vds has to be a negative voltage.
    Any other idea?