analog switch with mosfet

I want to replace a bistable relay with a analog siwtch with mosfet.

Because the mosfet are more cheap.

The voltage between IN and OUT is from 0 to 30VDC.

I want to turn on and off the switch i will not do PWM.

The R20 and R21 are very big because the consumption is very crusial. The current must be less than 30uA.

Will i have a problem from ESD, EMC and the switch will turn on or off by itself because the R20 and R21 is very big?

Thanks in advance.

With such high resistances, you indeed risk random behavior of the switch when, for example, someone touches the node between R20 and R21 on the board. But aside from that it won't give you much trouble if you keep the traces sufficiently short (i.e. don't give it the chance to pick up noise).

If you can afford a slow switching speed, I'd consider making R20 even higher, 2M2 for example, and omitting R21 since in the current setup you're making a 1:2 voltage divider which is prone to weird behavior. If Q1 is on then the current will be Vin / 2M2, if Q1 is off then it will be negligibly small and R20 will passively pull up the line to keep Q7 and Q8 turned off.

What is the purpose of R23?

xxtigerxx said:

I want to replace a bistable relay with a analog siwtch with mosfet.

Because the mosfet are more cheap.

The voltage between IN and OUT is from 0 to 30VDC.

I want to turn on and off the switch i will not do PWM.

The R20 and R21 are very big because the consumption is very crusial. The current must be less than 30uA.

Will i have a problem from ESD, EMC and the switch will turn on or off by itself because the R20 and R21 is very big?

Thanks in advance.

Click to expand...

If the gate transition time from RgCiss is long, the MOSFET will burn out. Your requirements must use a low impedance push pull driver and rely on low gate leakage of selected MOSFET.

Hi ArticCynda,
R23 is for turn on and off Q1, so i turn on and off the switch. This is controlled buy a mcu.
But if i omit R21 the VGS of the Q6 and Q7 wil be like 30V. the limit for the mosfet is +-20 VDC.

Hi SunnySkyguy,
So what do you propose?

You must add a zener diode parallel to R20, e.g. 12 V. And accept that the switch can't work for input voltages below the MOSFET threshold voltage. I don't believe that your specification can be achieved with this circuit.

If the "30 µA" limit refers to leakage current drawn from the switched voltage rather than total power consumption (not clearly specified), a photovoltaic opto coupler may be the best solution to drive the gates.

FvM said:

You must add a zener diode parallel to R20, e.g. 12 V. And accept that the switch can't work for input voltages below the MOSFET threshold voltage. I don't believe that your specification can be achieved with this circuit.

If the "30 µA" limit refers to leakage current drawn from the switched voltage rather than total power consumption (not clearly specified), a photovoltaic opto coupler may be the best solution to drive the gates.

Click to expand...

Hi Fvm,
Thank for the answer. The limit 30uA is the total consumption not the leakage.
This device is powered by the line 30VDC.
I do not know the photovoltaic opto coupler. Do you have a tutorial link for them?