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If the MOSFET breaks so that the 300V DC voltage leaks and goes through the gate of the MOSFET (please tell me if it's possible or not possible,) will this optocoupler protect V2, which is a control signal frm a microcontroller or if it doesn't will a combination of diode to block the reverse current and Zener to lower the spike voltage do?

Notes: V2 & V3 are control signals from a microcontroller (in this case, both are logic output from an Arduino.)

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  • \$\begingroup\$ To be clear, are you saying that V2 in your diagram is the Arduino's power supply? \$\endgroup\$ Jun 28, 2021 at 10:54
  • \$\begingroup\$ I'm not expert, but if the purpose of the optoisolator is to protect the Arduino, then it seems like maybe a bad idea to connect the Arduino to both sides of it. Why can't you get sufficient power to drive the MOSFET from the "hot" side of the circuit? \$\endgroup\$ Jun 28, 2021 at 10:55
  • \$\begingroup\$ I intend to connect the V2 to the digital arduino pin (~), can u elaborate your second comment? I doesn't get it, i need to connect the arduino pin because i want to control the switching time with microcontroller. \$\endgroup\$ Jun 28, 2021 at 10:57
  • \$\begingroup\$ So what's V3? Is V3 really just +5V? If the LED is always on, then what is the purpose of using an optoisolator at all? \$\endgroup\$ Jun 28, 2021 at 10:58
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    \$\begingroup\$ Yes. Not going to post an answer because I am out of time, and I don't want to draw a diagram right now. But they way to isolate your two circuits is to only connect the Arduino to the LED side of the opto part. If you want to use a general purpose output pin to "gate" the output from a PWM pin, then use digital logic (e.g., that AND gate that I mentioned) on the LED side for that. \$\endgroup\$ Jun 28, 2021 at 11:22

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The Opto is redundant ( protects nothing) since the input and output share the same grounds.

The protection in the FET gate insulation may get some coupling during flyback from Cdg. So add a 100 ~ 1k series R on the Gate with a Sch diode clamp to the Arduino Vdd on the gate.

You’ll have to use a logic level FET or a level shifter or logic gain IC (AC Coupled with Rfb CD4xxx series’s on 12V) for 12V on the gate for optimum RdsOn. I’d suggest the latter as logic level 600V FETs are $$$

Insert NPN common emitter with 12V pullup to drive your FET and protect the Arduino.

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  • \$\begingroup\$ If the initial input voltage to the gate is 5V if i add the resistor will it divide the voltage input to the gate? or the input voltage to the gate will still be ~5v and the resistor only limit the current? \$\endgroup\$ Jun 28, 2021 at 11:37
  • \$\begingroup\$ These FETs had a Vt of 4V max so 5V us NG. Just a series R to limit feedback current spikes and rely on internal diode clamps or add redundant Sch diode to 12V CMOS output. Rs 1k to 10k. 4xxx impedance is about 300 @ 12V or use an inverting NPN with pullup ! \$\endgroup\$ Jun 28, 2021 at 11:38
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    \$\begingroup\$ Thanks for your answer! \$\endgroup\$ Jun 28, 2021 at 11:49
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If the gate oxide were to get a hole then the gate voltage would rise until the mosfet turns on.

So you get about 5V on R2

Nothing else bad will happen.

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  • \$\begingroup\$ Typically a MOSFET fails as a three-way short, exposing the driver to a modest fraction of the load voltage, at whatever current is available from the load. \$\endgroup\$ May 31, 2023 at 23:00

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