Voltage error through Analog Load Switch

Thread Starter

BKI

Joined Nov 13, 2014
2
Hi,

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?
 

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crutschow

Joined Mar 14, 2008
34,280
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.
 

Thread Starter

BKI

Joined Nov 13, 2014
2
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?
 
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