drive mosfet

siavashs said:

what do you mean by return supply?

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ronsimpson said:

There are two wires coming from the 300V supply. It really feels like the black one is not connected. One reasons why we ask for pictures.
There is a loop from +300 to resistor, to MOSFET, back to 300V return. (negative) It feels like that loop is broken.

Or there is 150 volts of ripple on the supply.
That is why we want to see the scope picture for the +300V. Or for the Source.
--edited--

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ronsimpson said:

OK. I do not know what is happening.
With good voltage on the G-S the D-S is not turning on. (or something else is happening)
It appears that you have about 1/2 of the supply voltage across the Drain resistor. So something is only allowing uAs to flow in the D-S of the MOSFET.

Lets think of what could have happened:
1) Bad MOSFET likely the Gate is broken. (you replaced the MOSET)
2) Bad 300V supply. Checked.
3) Gate and Source reversed. (pin out wrong?)
4) Bad solder connection. (replaced FET so maybe OK)
5) Damaged scope probe or damaged front end of the scope. We know that low voltage signals are ok. But high voltage signals are bad. Maybe the "volts/division" knob is bad. (open resistor in the voltage divider in the scope) Could be some of the settings on the V/Div. knob do not work (highest setting?)

Hopefully some one will add to the list.

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