MOSFET Vgs

[dark]

Hello;
I post a schematic here My question is how to bias mosfet Vgs for such configuration . I need to shut the Line for anything in between 25 to 200 VDC . I know Vgs cant take more than 15V?


Thank you

 

[Ubergeek63]

well the simplest way is 2 resistors and a zener, but that tends to be slow. What are you trying to do? You do not need 2 FETs for DC.

Dan

 

[dark]

well the simplest way is 2 resistors and a zener, but that tends to be slow. What are you trying to do? You do not need 2 FETs for DC.

Dan

I am trying to avoid over voltage condition , by shutting off the supply in case it appears from the output or the right hand side of the schematic , anything entering from 24V to 200V is a threat and the MOS is required to put off the line untill the overvoltage condition is gone.

Thanks

 

[Ubergeek63]

use your transistor to interrupt a 1mA current sink ( base to 5V, emitter to 5K, 5K to your collector)

current sink collector to 10K and 15V zener across PFET gate source

 

[dark]

use your transistor to interrupt a 1mA current sink ( base to 5V, emitter to 5K, 5K to your collector)

current sink collector to 10K and 15V zener across PFET gate source

I didnt get you ? a 15V zener will not handel 200V DC in case its the over voltage condition . I try to turn on the transistor when there is enough current at 25V Dc to produce a Vbe of .6V .

 

[Ubergeek63]

I didnt get you ? a 15V zener will not handel 200V DC in case its the over voltage condition . I try to turn on the transistor when there is enough current at 25V Dc to produce a Vbe of .6V .

Sorry ... that is the job of the current source and I forgot tell you to put a 24V zener to your transistor base for the actual voltage sense

 

[dark]

Sorry ... that is the job of the current source and I forgot tell you to put a 24V zener to your transistor base for the actual voltage sense

Is this that you want me to do? to bias the MOSFETS? . I want to know when the voltage is 200V will it not destroy the 15V zenner tied between Gate and source, till the BJT comes into action.
When the Vlolts are below 24 say 20V MOS are required to ne ON as automatically the BJT is OFF , but it doesnt seems to conducting?

 

[Ubergeek63]

no, like this

 

[phildog]

ok...i think u should have a gate drive circuit to drive the gate of each mosfet. Gate drive can be opto-coupler and transistors joining together. Adding zener diode and resistor for overvoltage protection or adding clamping diode and resistor for faster switching turn on and off time.

 

[Ubergeek63]

ok...i think u should have a gate drive circuit to drive the gate of each mosfet. Gate drive can be opto-coupler and transistors joining together. Adding zener diode and resistor for overvoltage protection or adding clamping diode and resistor for faster switching turn on and off time.

Gate drive would require gate power and is only needed for high speed circuits. This is simple and there was no indication that it needed great response time, just an overvoltage switch.

 

[phildog]

yup...this is for high speed and high voltage mosfets..

 

[Ubergeek63]

yup...this is for high speed and high voltage mosfets..

you are not the OPer. It is the application requirement and not the part capability that determines if you need a driver or not.

 

[dark]

no, like this

Ubergeek , thanks for the explanation . Actually I thought I have provided some less information actually I attach the schematic where My remote SMPS is getting Overvolted as shown. Please advise if to use N-MOS opr P-MOS ? . How the transistor BJT should be biased in such a situation and from where . The combination is confusing me if I use a crowbar is it feasable to protect the supply? . I add your basic circuit to the scheme please advise the turn on time if you can guess actually I havnt used MOSFET for such .

Thanks for your time

-Adi

 

[Ubergeek63]

Now I am confused. I was thinking you meant that the supply to the SMPS was going high and you needed to disconnect it.

Crowbars are pretty easy, the simplest being a zener firing an SCR to blow the fuse.

Dan

 

[dark]

Now I am confused. I was thinking you meant that the supply to the SMPS was going high and you needed to disconnect it.

Crowbars are pretty easy, the simplest being a zener firing an SCR to blow the fuse.

Dan

Apologies for the confusion actually I am confused too , Please have alook at the second attached image which I have added a little later above. It is to be noted that the Output is getting overvolted and not the input.

-Adi

 

[Ubergeek63]

you are applying 200 to the output of a 12V switcher?

 

[dark]

you are applying 200 to the output of a 12V switcher?

Not Me but its often happening as an accidental application at the remote sensor(25feet away) because of wiring mistake! when ever it happens. The sensor gets its supply from the mainboard. Now the Sensor is saved because of Crowbar but the supply blows off, any trick to save the supply.

-Adi

 

[phildog]

wut is OPer?

 

[Ubergeek63]

wut is OPer?

Original Poster

 

[Ubergeek63]

Not Me but its often happening as an accidental application at the remote sensor(25feet away) because of wiring mistake! when ever it happens. The sensor gets its supply from the mainboard. Now the Sensor is saved because of Crowbar but the supply blows off, any trick to save the supply.

-Adi

Well that makes more sense. What is the current requirement? If it does not need much all you need is a series resistor.

Dan