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| 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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