Bidirectional Mosfet switch - how to design.

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#1
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Hi

I need your help with designing switch using MOSFET transistors. This should be an analouge switch. I cannot find anywhere a design which would show how to make a mechanical switch. Switch should be opened or closed acording to the gate signal. Below I am enclosing an idea what I would like to obtain. It is only a drawing for an explanation what I want to achive.

and yes switch should have small Ron.

Thanks in advance for any replies. I appreciate your help. :)

Adam

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Quote:
This should be an analouge switch.
The picture doesn't show but how much voltage and current does it need to switch? Would an old fashioned 4016 or 4066 etc. do the trick?

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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I wanted to use 4066 but Ron of this CMOS equals 100 ohm when you give 15V on gate voltage. So this is the best situation in which you receive Ron = 100 ohm as far as I remember. I would like to use Mosfet transistors which can have Ron of 0,5 ohm or even less.

Dedication for switches is to switch capacitors.

Adam

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Quote:
Ron = 100 ohm
Newer chips like 74HC4066 can go down to 30 Ohms at 12V.
What else do you have along with the caps? Can R-on be used somehow?
In any case you can see the diagram for the analog switch in the data sheet if you want to copy it.

I'm sure you can find lower R-on solid state relays, again depending on the application. Also you can parallel the switches for lower R-on.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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ADI makes analogue switches with 0.25 ohm Ron.

Leon

Leon Heller G1HSM

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Quote:
What else do you have along with the caps? Can R-on be used somehow?

I am designig AD logarithmic converter and Ron should be small. Also such switch shuld have relatively small capacitance.
I found at farnell switches which have 5mohm. That`s really good.

Anyway by the heck of curiosity how to design such switch using Mosfets from store. ?
Drain is always conected to body and such switch seems to be impossible to build. I remember some years ago here in Poland there were Mosfets with 4 pins. Body was not connected. What was the reason to start connecting body with drain.
Having such transistors could be easy to copy for example 4066.

Adam

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Have you looked at an analog transmission gate (a nmos in parallel with a pmos)?