Problem with Analog Switches

[BKI]

Yes, still simulation. Well, its not so easy to measure why it occurs.
I realize, when i make the transistor bigger to decrease the Rds,on the behavior is getting worse. So i can guess that it is not a matter of Rds,on but maybe a matter of Cds ?! When i decrease the control current 1 to 10uA and control current 2 to 100uA it shows best behavior....so probably it is a matter of charge injection?

It would be nice to get some input! This issue is still not resolved.

Thanks!

 

[BKI]

Body Diode Problem in Load Switch?

I have two rails (-60V/+200V) and i want to connect those rails to the load through integrated PMOS switches.
I have limited Vgs to around -2V through zener diodes. leakage currents are flowing through a resistor.



What the circuit does is: the output voltage starts at -50V and goes down to -120V, then it goes up to +120V
What it should do is: the output voltage should start at 0V and goes down to -60V, then goes up to 200V.

The question is, why does it start at -50V instead of 0V. Can it be a problem of the PMOS parasitic body diode?

If yes, how can i resolve it? i only have PMOS transistors available. So, i can not connect two PMOS in series, because then one Vds is positive (which is not allowed).

I need to solve this problem urgently!!! Please help!!!

 

[dick_freebird]

Re: Body Diode Problem in Load Switch?

This is an old, old problem and parts like analog multiplexers
that solve it, are still around because it's not easy. The
approach I have seen involves diode-blocking the body.
This needs a VT(P) >> Vf to avoid low level leakage problems,
and it needs the body to be free of the substrate (dielectric
isolation) or close enough (triple well - maybe - depending
whether the thing needs to be well behaved with power off,
on and everywhere in between).

Now, integrated HV MOS processes are often not architected
for high side PMOS switching and more particularly, bidirectional
and abnormal-conditions blocking. We do not know yours. A
bit of time spent with cross sections and challenging the
device with the various normal, startup and fault conditions
(pen and paper will do) could lead you to more or less
confidence in the technology you have picked, or define
what attributes you do require. Having a body control scheme
in mind, would be good beforehand.

Look at the old Harris HS-1040 multiplexer datasheet and
one bit of interesting schematic detail is shown. There are
of course some nuances you will have to polish.

I have also seen some other companies using NMOS-PMOS-NMOS
(Oreo cookie style) switches to get a good wide-range blocking
behavior. I am not aware of what they do as far as body
control, there.