Hello, I am making a simple BJT and MOSFET switch to control the state of a IN-3 nixie tube with a microcontroller. Currently, I have the simulation in LTSpice running, although with my IRF820, the 2N3904 and it's current circuitry can't seem to switch on the MOSFET fully. Even with 11.7 volts going to the gate there are still some issues arising. What could I adjust to get the MOSFET to switch on fully?

I can attach my LTSpice files, https://drive.google.com/drive/folders/1h7pwr3ey3wk0qwx6KgAaaUyo-0ome40x?usp=sharing

 

Please label the node you are plotting with a Net Name, so we know what we are looking at, and then plot the gate and drain voltages of the M1.
V(n003) tells us nothing about what's being plotted.

Where did you get that MOSFET model?

 

Please label the node you are plotting with a Net Name, so we know what we are looking at, and then plot the gate and drain voltages of the M1.
V(n003) tells us nothing about what's being plotted.

Where did you get that MOSFET model?

The library .MODEL came from a manual input since Vishay does not have the .LIB file on their website anymore.

 

The library .MODEL came from a manual input

Input from where to where?

There appears to be a problem with that model.
The sim with a different MOSFET works fine (below):

(Note that by going to Plot Settings and selecting Add Plot Pane, you can simultaneously plot as many different nodes as you want).

 

Input from where to where?

There appears to be a problem with that model.
The sim with a different MOSFET works fine (below):

(Note that by going to Plot Settings and selecting Add Plot Pane, you can simultaneously plot as many different nodes as you want).

View attachment 354117

I made a separate .lib file that contains the .model code.

In real life with these same components I am still having this issue with the MOSFET not being able to switch on or off. By this point do you feel that I should just switch to another MOSFET from the LTSpice simulation that worked?

 

In real life with these same components I am still having this issue with the MOSFET not being able to switch on or off.

Must not be connected correctly if not working or a defective component.

 

Must not be connected correctly if not working or a defective component.

The IRF820 is brand new and connected correctly. However, I will get some measurements later today from the physical board that this circuit is attached to.

 

In real life with these same components I am still having this issue with the MOSFET not being able to switch on or off.

I see no reason for that.
The IRF820 should switch just as well as the MOSFET in my sim.
I would double check your wiring, especially the MOSFET pinout (below):



But I did find the problem with your IRF820 model.
The RDS (shunt drain-source resistance when off) has a value of 3, whereas it should have a very high value such as 3E6.
My sim below with that value for the model included as a SPICE Directive:

 

I see no reason for that.
The IRF820 should switch just as well as the MOSFET in my sim.
I would double check your wiring, especially the MOSFET pinout (below):

View attachment 354122

But I did find the problem with your IRF820 model.
The RDS (shunt drain-source resistance when off) has a value of 3, whereas it should have a very high value such as 3E6.
My sim below with that value for the model included as a SPICE Directive:

View attachment 354134

Thank you for seeing the issue with the model file, I will update my .model accordingly.

As far as the physical circuit goes, once I get back from work I will make some measurements and test my MOSFET to narrow down what the issue could be. Everything is wired up correctly so it has to be a faulty component.

 

Sorry, I just noticed another problem with that model.
It's drain voltage is only going to about 13V instead 180V when off.
I haven't been able to determine what model parameter is causing that anomaly.

 

Sorry, I just noticed another problem with that model.
It's drain voltage is only going to about 13V instead 180V when off.
I haven't been able to determine what model parameter is causing that anomaly.

Thank you for helping out! This forum has always been a big help

 

Okay, I found there was a problem with the values for W and L.
I didn't know what they should be so I removed them (below):

 

Okay, I found there was a problem with the values for W and L.
I didn't know what they should be so I removed them (below):

View attachment 354140

Perfect!

 

MOSFET Test:

Drain-Source Diode Test: good, forward 0.6V backwards open



Gate Voltage: 3 volts



Now it seems like the BJT is the issue here. Could it be because the Arduino Pro Micro can’t supply enough current to the BJT?

 

Possibly but R2 can be increased to 4.7K

 

I’ll try to take my microcontroller out of the pcb to manually apply the 5V and 0 volts to the BJT. Just to rule that possibility out

 

The gate voltage should be appx 12 volts when the transistor is OFF or zero output from the micro.
Are you trying to measure the gate voltage while it's being pulsed?

 

The gate voltage should be appx 12 volts when the transistor is OFF or zero output from the micro.
Are you trying to measure the gate voltage while it's being pulsed?

It’s being measured at pulses that are multiple seconds long. So like on for 4 seconds off for 4 seconds.

 

What voltage reading do you get on the collector when the pulse output is zero volts?

 

What voltage reading do you get on the collector when the pulse output is zero volts?

When the pulse output is 0 volts, I am getting 7.5V across the collector of the BJT.