Pathway resistor for internal capacitance of the mosfet gate

[tom911]

Hi,

I'm struggling with the aspect of internal capacitance of the mosfet, the pathway to remove the charge is required and thus I've added a resistor in parallel with the capacitor (colored in red) but I've no clue how to determine its size therefore would you be able to provide some feedback on that issue ?

**broken link removed**

Thanks,
Jack

 

[audioguru]

Your circuit will not work.
The load is connected to the source of the Mosfet instead of to the drain. Then the gate must be +191V (10V higher than the source pin's voltage) for the Mosfet to turn on.
The opto transistor does not need the emitter resistor. The resistor from the gate of the Mosfet to ground is also not needed because the transistor conducts to ground to turn off the Mosfet.

If you correct the wiring (put the load between the drain and +181V, and connect the source to ground) then the logic will be reversed.
It can have the correct logic if the Mosfet wiring is corrected and the opto transistor is changed to an emitter-follower like this:

 

[tom911]

Thank you audioguru, I will mention you in acknowledgement section of the report

 

[MrAl]

Hi,

You also have to think about how fast you need the MOSFET to switch. Resistors and opto couplers alone usually do not charge/discharge the gate fast enough so a special MOSFET driver IC chip has to be used. It does depend how fast you need it to switch though.
To drive a 100 ohm resistor plus a mosfet gate you'd need a pretty special opto coupler anyway.

 

[tom911]

I'm currently at the stage of researching the high-side mosfet design consideration as what I've learnt yesterday is that my circuit topology is in such format. Therefore I will definietly fix that circuit. The mosfet is of a very low speed (1050Hz) and the output resistor is to be 100Ω.

The optocoupler is a 4N35 which says in datasheet that 100Ω connected to the 10V supply would provide the fastest operation. What I believe is that the value of 100Ω provied an enough current to satisfy the project requirements and thus the next step is to define the highside resistor.