Electronics > Beginners

Mosfet Not turning Off fast

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ZeroResistance:
I am trying to PWM a Mosfet STP55NF06, gate resistor is 2.2ohm and drain voltage is 12V.
The gate is being driven by a driver MIC4422.
The driver has a supply voltage of 12V too. And the pulse frequency is 250Khz with duty cycle 50%.
I want to eventually drive a inductive load but currently testing out with a 1K resistor.
While probing the drain voltage with a scope I find that the turn on is razor sharp but turn off is like i ramp.
I have attached a picture. I hope someone can shed some light on this.
Regards,

ZeroResistance:
Ok, I put a stronger load like 150ohm and the turn off now seems sharper not razor sharp though.

ogden:

--- Quote from: ZeroResistance on April 21, 2019, 04:10:15 pm ---Ok, I put a stronger load like 150ohm and the turn off now seems sharper not razor sharp though.

--- End quote ---

MOSFET has significant Drain-Source capacitance - that's why you see sawtooth waveform. Waveform quite well agree with t=RC for 1KOhm and 1300pF (input capacitance of mosfet you use). Even with 150 Ohms RC constant (65% of 12V reached) is quite long 0.195 microseconds. Test with 5A current or so - then capacitance of transistor itself does not play significant role anymore.

David Hess:
Exactly, the load impedance is too high.  Either use a lower load impedance or use a transistor with lower capacitance.

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