# MOSFET switching speed

I´ll be clear and fast in what I´m asking once my last question did not have many views.

So, a couple days ago I was at the bench switching a IRLZ34N with a 555 timer, powered by a 15V, @ 50kHz, $$\33\Omega\$$ gate resistor. As I started changing the drain resistor values, I could the see the mosfet, when switched off, would increase its drain voltage in a speed which relied on the drain resistor. To bring some numbers, @ $$\R_{drain} = 47k\Omega\$$ it was really slow to build up that 15V again, @ $$\R_{drain} = 3k\Omega\$$ it was able to reach 15V within 2us. @ @ $$\R_{drain} = 330\Omega\$$ it reached 15V in about 400ns.

Although I´ve not uploaded any image to clarify, I searched for what could be the reason why it has such behavior and realized it has to do with drain-gate capacitance ($$\C_{DS}\$$) and gate-source capacitance ($$\C_{GS}\$$) though I really don´t get how it affects the switching like that.

I know $$\C_{gs}\$$ affects the circuit in increasing $$\V_{GS}\$$ and switching the transistor on and off but I don´t understand how to relate that to the drain voltage as I just mentioned.

Why does the mosfet behave like that after all?

• You have some formatting problems, here. Your math hasn't rendered.. – Reinderien Dec 8 '18 at 1:22
• Fixed. You need to escape your dollar signs on this board (which is opposite the mathematics Stackexchange). So to get $\Omega$ you need to write $\Omega$, not $\Omega$ – TimWescott Dec 8 '18 at 2:09
• Read up on Miller effect and Plateau. Also, if you switch 10amps in 10 nanoseconds, or 1 amp/1nS, thru 5 mm of wire (which is about 5 nanoHenries) you will see volts across that wire. If you grounds (the gate driver must share a GND with the FET, right?), then depending on how your GND is wired, you may experience negative feedback and the FET switching will be delayed; or you may get positive feedback, and the entire system becomes an oscillator. – analogsystemsrf Dec 8 '18 at 3:12