Author Topic: Mosfet Not turning Off fast  (Read 972 times)

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Offline ZeroResistanceTopic starter

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Mosfet Not turning Off fast
« on: April 21, 2019, 03:46:00 pm »
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,
 

Offline ZeroResistanceTopic starter

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Re: Mosfet Not turning Off fast
« Reply #1 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.
 

Offline ogden

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Re: Mosfet Not turning Off fast
« Reply #2 on: April 21, 2019, 05:28:18 pm »
Ok, I put a stronger load like 150ohm and the turn off now seems sharper not razor sharp though.

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.
 
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Offline David Hess

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Re: Mosfet Not turning Off fast
« Reply #3 on: April 21, 2019, 06:48:37 pm »
Exactly, the load impedance is too high.  Either use a lower load impedance or use a transistor with lower capacitance.
 
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