Need help with high frequency switching solution

[mbferguson]

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 have tried diligently, but cannot find a switch that meets these requirements. 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.

 

[mbferguson]

Perhaps this part would work?
IPC302N25N3

It 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 LCD.

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/

 

[Harald Kapp]

It is rated for Vds = 250V, within requirements.

Only barely. Z1 is meant to limit the voltage to 220 V which leaves not much headroom (safety margin) for the transistor. Better use a 400 V or 600 V transistor. If you require a higher Vgs, insert a driver stage between the controller and the MOSFET. See also below.

If it is activated for 100ns, that would require at least 2 instructions to turn the pin high and back to low.

You could use an external monostable multivibrator to generate the 100 ns pulse. The trigger at an interval of 500µs can easily be generated by (almost) any microcontroller. The multivibrator can be built to operate from a higher voltage thus providing a higher gate drive voltage if required.

 

[mbferguson]

Only barely. Z1 is meant to limit the voltage to 220 V which leaves not much headroom (safety margin) for the transistor. Better use a 400 V or 600 V transistor. If you require a higher Vgs, insert a driver stage between the controller and the MOSFET. See also below.

Someone else has suggested I use a gate driver to interface with the microcontroller. I have never used something like this before. Are there any other things you could tell me about that might not seem obvious to an amateur?

You could use an external monostable multivibrator to generate the 100 ns pulse. The trigger at an interval of 500µs can easily be generated by (almost) any microcontroller. The multivibrator can be built to operate from a higher voltage thus providing a higher gate drive voltage if required.

I must be honest, a circuit of that complexity is beyond my knowledge at this point in time. I don't think I'd be able to figure out how to use that without a lot of help.

I realize now that there is no way to interface a microcontroller with a switch without there being some capacitance that would bleed back to the board, hence the need for the gate driver.

 

[Harald Kapp]

Someone else has suggested I use a gate driver to interface with the microcontroller. I have never used something like this before. Are there any other things you could tell me about that might not seem obvious to an amateur?

A gate driver is the 'best' solution as it is designed to turn the MOSFET on and off quite fast. Here is an example for such a chip. The idea is to drive a high current into the gat (and out of) to quickly charge (discharge) the gate to switch the transistor on (off).

I must be honest, a circuit of that complexity is beyond my knowledge at this point in time.

If a monostable multivibrator is too complex, the you should reconsider your whole project.
Do not despair, however. There are ics that can do the job, e.g. a 74HC4538A. The datasheet gives all the information on how to use this ic. Figure 14 on page 12 shows the setup for a non-retriggerable monostable multivibrator, just what you need. Figure 3 on page 7 shows which timing elements (R, C) to use for a certain pulse width.

Note that there are 2 units in one case. You need to connect the unused unit as shown in figure 15.
100 ns is at the lower limit of this chip. You will not be able to generate shorter pulses than that.