I have to make a pulse generator square wave for ultrasonic transducer with frequency 1 MHz and 70V for the amplitude, the input is function signal generator with 10V VPP output and 1 MHz. I have made it and it works, but there is a problem. There is always a ringing or overshoot in turn on or rise time but not for the fall. It happens for all frequency, not just in 1 MHz, in 100 KHz for an example, the overshoot is so tight or narrow. this is the circuit that i made
and this is the result of the output
what i have done
1. try to add resistor 1M or 10K in gate of MOSFET, the result : no output signal
2. Add Decoupling capacitor, the result : no effect
UPDATE
how i add up the resistor
So, how to fix the overshoot or the ringing of the output?
Thank you
Best Answer
You've done that what you shouldn't do! If you check the datasheet then you'll see that the input capacitance, \$C_{iss}\$, is quite high: 2.16nF. A 1M gate stopper resistor plus this capacitor will form a nice low-pass filter having a cut-off frequency of \$f_C=1000/(2\pi \cdot 2.16 \cdot 1) = 73.7Hz\$, so the input pulses will be totally chopped off thus the MOSFET will never turn on. That's why you get no output.
Cure: Remove 100k and change 1M to a resistance so that it forms a LPF having a cut-off frequency of at least \$3 \cdot f_{SW} = 3MHz\$.
Of course! Any inductance causes ringing. Where do you have inductances? Answer: Cables and internal drain inductance of the MOSFET.
Cure: A snubber network across Drain and Source/GND. You can find a lot of info about snubber design on Internet.
PS: Your MOSFET does not seem to be suitable for switching at 1 MHz. You should use another like this one (I picked this one randomly. You select according to your voltage and current needs).