P channel mosfet in series

[jomm]

Hi,


can i connect a p channel mosfet in series with both the source and gate tied?

how does the biasing of the circuit happen ? the circuit is shown in the attachment.



thank you

 

[alexan_e]

What you show resembles a solid state relay which uses Nmosfets
https://en.wikipedia.org/wiki/Solid-state_relay

 

[jomm]

i am using a controller to control the gate of a p channel mosfet . but when the gate is off.the diode inside it conducts and draws current from the supply,this is when i used a single p channel mosfet, to over come that i used another p fet in series as show in the attachment.

 

[alexan_e]

Why don't you just use a diode in place of the lower mosfet, that will have the same effect (but will increase the voltage drop).

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The sources of the two mosfets will have a voltage close to the power supply because of the diode in the lower mosfet, as soon as you apply a low voltage to the gate (gnd) then they both turn on (gate much lower than the source) if you apply about 8v to the gate then the gate voltage equals the voltage of the sources so the mosfets turn off.

 

[jomm]

Why don't you just use a diode in place of the lower mosfet, that will have the same effect (but will increase the voltage drop).

- - - Updated - - -

The sources of the two mosfets will have a voltage close to the power supply because of the diode in the lower mosfet, as soon as you apply a low voltage to the gate (gnd) then they both turn on (gate much lower than the source) if you apply about 8v to the gate then the gate voltage equals the voltage of the sources so the mosfets turn off.

actually i had to disconnect the load when the battery goes low which is monitored by a comparator which sends a high signal to the CTL pin of 4412, but when this happens the mosfet F2 diode conducted so to overcome that i used F1 in series,

can i use it like this ?? i have attached the schematic.

 

[jomm]

Why don't you just use a diode in place of the lower mosfet, that will have the same effect (but will increase the voltage drop).

- - - Updated - - -

The sources of the two mosfets will have a voltage close to the power supply because of the diode in the lower mosfet, as soon as you apply a low voltage to the gate (gnd) then they both turn on (gate much lower than the source) if you apply about 8v to the gate then the gate voltage equals the voltage of the sources so the mosfets turn off.

will the upper mosfet F1 turn ON when the lower one is ON ??

 

[alexan_e]

The P mosfet turns on when the gate has a lower voltage compared to the source , both mosfets have their sources tied together and as long as the 8v supply is on the mosfet sources have 8v (almost) so when you apply a low voltage to the gated they both turn on

 

[jomm]

The P mosfet turns on when the gate has a lower voltage compared to the source , both mosfets have their sources tied together and as long as the 8v supply is on the mosfet sources have 8v (almost) so when you apply a low voltage to the gated they both turn on

i did try using the online schematic editor and simulation using circuitlab.com ,when the battery supplied 9v and the gate s wer at 7v then only one mosfet on the left turned ON , the simulation result is shown on the left side of the attachment, the OUT node is zero even after the first mosfet is ON .In my design i use a p channel mosfet FDN306P, the symbol used in that editor is for P channel only.

Thanks

 

[alexan_e]

Your output is shorted to groung, there is no load so how can you see any voltage apart from 0v there?

 

[jomm]

Your output is shorted to groung, there is no load so how can you see any voltage apart from 0v there?

yes..... thnks

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as i lowered the gate voltage it conducted well ... here is what i did .

 

[alexan_e]

Yes, you could use a lower load that reflects the real circuit , I assume it consumes about 20 or 50ma so a resistor like 470ohm or lower would be more appropriate.

 

[jomm]

Yes, you could use a lower load that reflects the real circuit , I assume it consumes about 20 or 50ma so a resistor like 470ohm or lower would be more appropriate.

the schematic for my design is attached ..is it correct ??