Mosfet switch problem

mosfet switch problem

Hi everyone,

I want to use some mosfets as switches. I will control them with a microcontroller (3.3V signals).

The problem is that in some mosfets the Source is not connected to ground so I am not sure if the Vgs signal will work.



In my circuit I have two pairs of switches. Mosfets M2,M4 and M1,M3. I want to change the polarity of current by closing each pair seperately.

What do you think of my circuit? Is it ok for M3 and M4 not to be connected to Ground?

Thanks

It looks like you are trying to build an H-Bridge.See this


You need to pull an N-Fet gate above the source by at least the data sheet threshold voltage (Vths) usually with a safety margin as well, and you also have to be sure that at the Vgs you are supplying that the Mosfet will be able to pass the amount of current you need (this can be found in the data sheet under the Vgs,Ids curves). Then you have to make sure your drive circuit can supply the current you need to charge the capacitance at the gate to switch your mosfet on fast enough (source current) as well as sink current discharge the capacitance quickly enough for your switching speed. What you need is a high side driver either an IC or using discrete components. If practical you could use a P-FET Vgs pulled below the source. There are more considerations but you can read about them on the links provided.

H-bridge using P and N-FETS

You can also search the forum or google for “ chopper” “ H-bridge” “ Doublers” high side drivers etc.

If you’re interested in getting a better understanding on Mosfets the best place I could recommend is IR.
Check there archives to, well worth the time.