Hello, I want to simulate the effect of UCC21220AD_TRANS? floating ground in LTSPICE
My gate drive needs to create floating voltage for irf7854pbf MOSFET.
However i know that in LTSPICE floating ground is not possible.
I have attached bellow all symbols and simulation file.
How should make a floating ground in LTSPICE?
Is it possible to create two different types of ground in LTSPICE?
Thanks.


/g/LTspice/files/Temp/john23/UCC21220AD_TRANS.asy
/g/LTspice/files/Temp/john23/irf7854pbf_test.asy
/g/LTspice/files/Temp/john23/UCC5304_Simplis_Test5.asc

/g/LTspice/photo/294510/3786158?p=Created%2C%2C%2C20%2C2%2C0%2C0

开云体育

I would suggest you place an appropriate resistor||capacitor pair between the real "ground" and your floating circuit's reference level net, which you can name as you wish. The resistor||capacitor combination should approximate the actual isolation characteristics you might find in the real world. It could be megohms and picofarads or gigaohms and nanofarads, but see below - those values may not even matter.

The reality is unless your floating reference net has a significantly different voltage (especially dV/dt) from LTspice's (note the spelling, LTspice - uppercase LT; lowercase spice) "ground" reference level, the isolation characteristics don't really matter much. If it does have a significantly different voltage, then modelling it might matter.

It would also be nice of you to zip those files together so that they remain associated. Somewhere, somewhen down the road, someone will want to learn from your work and all these separate files make finding the right bits to use more difficult for them.

Donald.

开云体育

On 25/05/2024 10:58, john23 wrote:

Hello, I want to simulate the effect of UCC21220AD_TRANS? floating ground in LTSPICE
My gate drive needs to create floating voltage for irf7854pbf MOSFET.
However i know that in LTSPICE floating ground is not possible.
I have attached bellow all symbols and simulation file.
How should make a floating ground in LTSPICE?
Is it possible to create two different types of ground in LTSPICE?
Thanks.

You didn't include "UCC21220AD_TRANS.LIB" in the list of required files. Neither is it in the folder you created.

And please, when you upload a schematic with models and symbols, put them all in a zip!

Hi.
Generally speaking I don't think floating land is necessary. It is possible to calculate circuits without it.
But especially for stubborn people I have created a model of floating ground and a symbol for it. Everything is in my large collection of models.
Here are the contents of the model:
.subckt GNDFLOAT 1 Cp=1pF Rp=1G
c 1 0 {Cp} Rpar={Rp}
.ends GNDFLOAT
Make your own symbol for this model.
There can be many such floating grounds in the schematic.
Real floating grounds have this equivalent.
With galvanic isolation, there is always a small capacitance between windings. When optocouplers are used, they also have parasitic capacitance in them.
My floating ground is better than the commonly used high impedance resistors for simulation.

john23 wrote, "I want to simulate the effect of UCC21220AD_TRANS? floating ground in LTSPICE"

I write about this many times, and I thought I did already with you, but many not.

The simple answer is, you can't.? No part of any circuit in SPICE can float.? Every part must have a path capable of conducting DC that leads back to SPICE's Ground.? Must.

What to do about that?

(1)? Connect it to ground anyway.? Just do it!? Why not?? If the circuit works when the bottom of your R6 goes to Ground, then simulate it that way.? No circuit (well, VERY few circuits) won't work when it is grounded, so just ground it, and let the simulation run.

(2)? Ground it through a resistor.? (A capacitor is unnecessary, and probably not what you want.)? I suggest this only to appease people who are easily made uncomfortable by doing it right.? The resistor accomplishes nothing, in fact it might make it simulate poorly, but if you like it that way, I won't stop you.? The resistor "allows" the floating ground to have a non-zero voltage on it - but it can't, because that part of the circuit is floating.? If it does end up with a non-zero voltage on it, then something is seriously wrong with your schematic or the models.? And if that happens, well everything is pointless anyway.

(3)? Connect the "floating" ground (the bottom of your resistor R6) to every possible signal voltage.? That's what you are trying to do anyway, isn't it - to prove that the circuit works no matter what that voltage is, right?? So do exactly that.? Every DC voltage.? Every sinusoidal voltage.? Every pulse and every spiky voltage.? Obviously, you will be running those simulations for the rest of your life and it will never be over.

(3a)? Connect the "floating" ground to a slowly variable voltage (perhaps a ramp) that covers the expected range of voltages you expect to encounter.? For example, if the "floating" ground might be between -300 and +500 V, then connect the "floating" ground to a voltage source with a slowly ramping (PULSE) waveform that ramps from -300 to +500 V.

Check what Frank Muenchow used in his "UCC5304_model_test.zip" upload in the "Temp" folder for an example.? Don't follow his lead exactly, but set up your simulation with parameters that fit your needs.? (Probably a slower ramp that covers a wider voltage range.)

Andy