I have a case that I need to build a pulser switching for my ultrasonic transducer which operate in 180 V and 1 MHz. I use mosfet IRF250 for the switching, and below is the circuit:
This is pretty good with response like this:
The green is input signal and the blue is output signal in the load, the problem is R1 resistor just 100 ohm, and the power needed will be 324 Watt, I want to reduce the power since the transducer only uses the voltage not the current, so I raise the resistance of R1 to 10K so the power will be 3.24 Watt, but the response is like this:
The falling time is increased drastically, what causes this ? Is it the capacitor between drain and gate inside of mosfet? How can I change the R1 to 10K but with same response as 100 ohm?
Best Answer
The large fall-time is most likely caused by the drain-source capacitance of the mosfet (700pF in datasheet). When turning off you have essentially a 10k resistor in series with 700pF of capacitance which takes time to charge up.
If you only need round about 20mA I would choose a smaller mosfet with less capacitance.