I am creating a pulse generator. The requirement calls for a switch to be closed for 100ns, and open for for the remainder of a 500us cycle. The sampling rate of the transducer being driven by the pulse generator is 2kHz. hence 500us. The capacitor should discharge within 4 tau, which gives a 25ns time constant.
The requirements call for a microcontroller to interface with the switch, in order to control exactly when the pulses are sent. I've planned on using the Arduino as it has analog input pins which will simplify the design. These pins will only be receiving a very low mV range signal.
In order to explain better I've posted a picture of the circuit. When the switch is closed, there should be a very low current flowing through the 560kohm, so the 100ns window the switch is closed shouldn't damage the transducer. The transducer is a dual element probe, so the side connected to the HV pulse is isolated from the side interfaced with the arduino.
Perhaps this part would work as the switch?IPC302N25N3It is rated for Vds = 250V, within requirements.It's Vgs is 2 to 4V, so the Arduino output pins could handle activating the gate of the mosfet.
If it is activated for 100ns, that would require at least 2 instructions to turn the pin high and back to low. (Probably much more if using digitalWrite() ). Unfortunately, if the Arduino only process it's instructions in increments of 62.5ns, I would only be able to get 125ns minimum, which would be optimistic and still not be within requirements. However the clock rate of 1/16MHz should allow up to 8,000 instructions per 500us cycle for processing purposes of the reflected signal and display of information on an output display.Are there other microcontrollers that have analog input pins, with potentially higher resolution clock rates?
Any advice is appreciated. Thank you.https://www.infineon.com/cms/en/pro...fet/30v-250v-n-channel-power-mosfet-bare-die/
(I suspect only MLCC ceramic can be fast enough).
So you'll need a very fast opamp too
The 560k resistor is not part of the switched circuit at all, thats just the capacitor and load, so the currentscould be several amps easily - just do the math from the stored charge and discharge timescale...No, that means it definitely cannot work at logic level.
Vthr is the definitely-off voltage. That MOSFETrequires 10V or more gate drive as the Rds(on) specification indicates
You must at these voltages use a gate driver IC anyway, so it doesn't matter at gate drivers use 12V.
You can certainly interface a high speed SPI ADC to the Arduino, microsecond or so sample times aren't too difficult/expensive.
My main concern is your desired narrow pulse width - 25ns or so with dV/dt of around 10GV/s anddI/dt around 100MA/s
I had arbitrarily used 4 time constants as the amount of time the capacitor would be fully discharged. Each time constant the cap should lose 37% of the previous voltage. Are you still saying that the capacitor will create an enormous voltage? This does not coincide with some of the other advice I've gotten from allaboutcircuits.com which helped me formulate the schematic shown.
If the gate driver will act as a buffer between the microcontroller sending a pulse, what would be the parameters necessary to ensure it doesn't fail? The mosfet would have the 220V across its S/D.
And you have the problem of finding a MOSFET that can switch that fast (ie very low Crss figure), andthen switching it high-side.
I would suggest parallelling several SMT MLCC's across a wide PCB trace (wide = low inductance), which will allow faster discharge
No, the Vgs must be kept within spec, preferrably just in the range 0V to 12V. This is why your high sideswitching is problematic.So why not switch low-side?
I am confused by the terms high-side and low-side switching. If I control the on/off state of the mosfet with the gate, wouldn't it just be "middle switching"?
Rather than be confused, you can lookup what it means, its not difficult and its nothing to do with MOSFETs, its to do with switching in general, you can either switch on the high side or the low side.(or both, even)
The time constant is 25ns, not 100ns, so the initial slope is 220V/25ns = 8.8Gv/sThere is about 100nC in the capacitor which has to drain on timescale of 25ns, ie about 4A,and it has to rise to that sort of value in that timescale roughly, 4A / 25ns ~ several 100MA/s