I currently have the following circuit shown below on a prototype PCB. The circuits intended purpose is to do the following:
- Receive two 5V PWM signals from a microcontroller.
- Boost the current a little using the FDS6912A N-channel MOSFET
- The signal then passes through a 4N25 optocoupler. Each optocoupler has its own independent battery, which isolated the microcontroller power from right hand side.
- The output of the 4N25 optocoupler is passed through to an ESC (Electronic Speed controller). To me the speed controller is a "black box", but it is used to drive a motor.
The problem I'm facing is that the FDS6912A N-channel MOSFET fails (for a reason I can't understand). The PWM signal at the input of the PWM_CH1 and PWM_CH2 is fine when I scope it. The output of both optocouplers remain at 0 V.
What I've found is that after replacing the FDS6912A mosfet the PWM tends to work fine again. My observations lead me to believe that the mosfet is blowing when I initially connect the batteries but I can't work out why. This is my theory but I can't be sure.
Because the circuit is already on a PCB I am severely restricted in what I can do to try and fix it. If I can put in a better replacement MOSFET that suits an SOIC-8 package or a suggestion can be made as to the cause it will greatly assist.
I've attached datasheets of the mosfet and Optocoupler in the links below and any help would be appreciated.
I Re-Examined the Gate voltage after another mosfet Failed and yes.. The PWM duty dropped from 5V to approximately half a volt (0.5v) after it failed. If I disconnect the mosfet it returns to 5V. Can anyone suggest a more resilient mosfet... Or will i have to resort to somehow modding the PCB to try and include a series gate resistor?