Why MOSFET is called current bidirectional switch ???

crutschow

Joined Mar 14, 2008
25,632
When the Vgs is biased so the MOSFET is fully ON, it will conduct current equally in both directions with the same ON resistance between source and drain.
But it can only block in one direction due to the inherent substrate diode between source and drain.
But if you use two MOSFETs in series with the sources back-to-back, then the combination will block current in both directions when OFF and conduct current in both directions when ON.
 

Thread Starter

mishra87

Joined Jan 17, 2016
854
When the Vgs is biased so the MOSFET is fully ON, it will conduct current equally in both directions with the same ON resistance between source and drain.
But it can only block in one direction due to the inherent substrate diode between source and drain.
But if you use two MOSFETs in series with the sources back-to-back, then the combination will block current in both directions when OFF and conduct current in both directions when ON.
Thank you so much for your reply !!!
Below schematic doesn't have body diode even if it conducting in positive and negative current in ON state why ???
I wanted to know exactly this ???
upload_2018-9-17_23-51-9.png
 

bertus

Joined Apr 5, 2008
20,866
Hello,

Many circuits do not show the body diode, but is inherent to the construction of the mosfet and is always there.

The circuit with two mosfets that @crutschow mentioned is used in the AQV21 mosfet optocoupler:

AQV21_mosfet_opto.png

Bertus
 

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Thread Starter

mishra87

Joined Jan 17, 2016
854
Hello,

Many circuits do not show the body diode, but is inherent to the construction of the mosfet and is always there.

The circuit with two mosfets that @crutschow mentioned is used in the AQV21 mosfet optocoupler:

View attachment 160065

Bertus
Yes, I agree with you and its true.
But if you ideal mosfet Which is having on state current in both direction i.e. 1st and 4th quadrant,
my question was what is reverse conduction in that.
 

crutschow

Joined Mar 14, 2008
25,632
But if you ideal mosfet Which is having on state current in both direction i.e. 1st and 4th quadrant,
my question was what is reverse conduction in that.
Not sure I understand the question.
The substrate diode reverse direction is drain-to-source for an N-MOSFET and source-to drain for a P-MOSFET.
So, when OFF, an N-MOSFET blocks a positive voltage from drain to source, and a P-MOSFET blocks a negative voltage from drain to source.
For the opposite voltage drain-source the substrate diode will conduct when the MOSFET is off.
 
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