Hi, so I am making an isolated power supply for a gate drive circuit. I'm using UCC21732 for my gate drive, attaching its pin diagram for reference.

The purpose is to generate the DC inputs VDD and VCC w.r.t the high power side ground, Vcom. Vcom is the potential I give to the source terminal of my MOSFET switch. I'm trying to achieve the isolation by using transformers. I'm able to generate the DC outputs by giving a +-5 square pulse to primary winding of the transformer, and using appropriate turns ratio to generate +-18V and +-5V at the two secondary windings, followed by a diode bridge rectifier to get +18V and -5V.
But there are three problems:
i) LTspice throws an error that
WARNING: Node VDC_LO is floating.
WARNING: Node VCOM2 is floating.
which gets solved only when I connect grounds at NODE1 and NODE2, but I feel that defeats the purpose, and if I could do it without having to ground them, it would be a lot better.
ii) On grounding NODE1 and NODE2, the DC outputs are not exactly 18V and -5V but 16.4 and -3.4, which I get is due to drops in the diodes, but if someone could help me with a way to maximize the output, it would be great!
iii) Biggest problem is that, these DC output voltages are generated w.r.t Vcom1 and Vcom2, but they need to have a common reference, Vcom. So, I tried tying up these two but that just messes everything up. The output voltages are greatly reduced, even the primary and secondary winding voltages change. I really need some guide on how to generate the output voltages w.r.t a common isolated ground, Vcom.
I have seen some research papers using the same method to generate isolated ground DC voltages, but they were just generating one voltage, so I can't find any solution to generate two voltages with a common reference.

This is the circuit I've designed (with NODE1 and NODE2 grounded):


And the output waveforms:


But on tying up Vcom1 and Vcom2 together, this is what I get:


Now I don't know if I'm doing something wrong or this is not the correct circuit to generate what I want. Any help would be appreciated!

 

Hi WJ,
Please post your asc file and any model files used in the simulation
E

 

Hi WJ,
Please post your asc file and any model files used in the simulation
E

Certainly, here you go

 

hi Wj,
This is what I see running your asc file.
What changes do you make when it will not run?
E

 

hi Wj,
This is what I see running your asc file.
What changes do you make when it will not run?
E
View attachment 323182

Hi, I actually want the potential "difference" (Vdc_hi,Vcom1) and (Vdc_lo,Vcom2) to be 18 and -5, which I am getting (minus the drop in diodes). But, I want to generate 18V and -5V w.r.t a common reference and not different ones, like Vcom1 and Vcom2 that I'm getting here. Basically I want to tie up Vcom1 and Vcom2 to make a common reference for Vdc_hi and Vdc_lo; Vcom.
But that changes my output. I just want a way to generate voltages (Vdc_hi,Vcom) and (Vdc_lo,Vcom) equal to 18V and -5V respectively.

 

Hi WJ,
Does this option help?
E

 

Hi WJ,
Does this option help?
E
View attachment 323184

Hey it actually throws this error when I try to run it. Same error (Node COM floating) when I delete R4 as well

 

Hi WJ,
Try this, note the .options on the bottom of the asc file circuit.
E
Attached asc file

 

Now I don't know if I'm doing something wrong or this is not the correct circuit to generate what I want. Any help would be appreciated!

1. I've added C2,C4 to provide the floating grounds.
2. Only one "K" statement is required.
3. I replaced the ideal diodes with a randomly chosen, fully defined, shockley diode.

 

1. I've added C2,C4 to provide the floating grounds.
2. Only one "K" statement is required.
3. I replaced the ideal diodes with a randomly chosen, fully defined, shockley diode.

View attachment 323196

Oh great! It is working even if I short the two nodes Vcom1 and Vcom2. That was helpful, thanks a lot!

 

Hi WJ,
Try this, note the .options on the bottom of the asc file circuit.
E
Attached asc file

View attachment 323190

This is working. Thank you!!

 

Oh great! It is working even if I short the two nodes Vcom1 and Vcom2. That was helpful, thanks a lot!

1. I've added C2,C4 to provide the floating grounds.
2. Only one "K" statement is required.
3. I replaced the ideal diodes with a randomly chosen, fully defined, shockley diode.

View attachment 323196

Hi, I have one question. What's the purpose of such a big resistor in parallel to the capacitor? It works just fine without it too, right

 

Hi, I have to design a power supply to provide constant dc voltages of 18V and -5V w.r.t an "isolated ground" Vcom, meaning (Vdc_HI,Vcom) and (Vdc_lo,Vcom) in the attached screenshot.

I have to generate these isolated voltages to provide input at the high power side of IC UCC21732, in which there's an isolation between the low power ground (0) and high power ground (Vcom). That is why I need the secondary side of transformer to have no connection to ground. However, the simulation does not work without ground connection.

It's some or the other node floating, if not NODE2.
I got an idea from some amazing people first that I should connect Vcom to ground through a capacitor and large shut resistance, and that worked; but I later realized I cannot provide any electrical connection between Vcom and ground, as that provides a path for common mode current to flow.
If anyone can provide me with any way to achieve what I am trying to without any ground connection after the secondary windings, it would be great!
Attaching the .asc file for reference.

 

Hi, I have to design a power supply to provide constant dc voltages of 18V and -5V w.r.t an "isolated ground" Vcom, meaning (Vdc_HI,Vcom) and (Vdc_lo,Vcom) in the attached screenshot.
View attachment 323309
I have to generate these isolated voltages to provide input at the high power side of IC UCC21732, in which there's an isolation between the low power ground (0) and high power ground (Vcom). That is why I need the secondary side of transformer to have no connection to ground. However, the simulation does not work without ground connection.
View attachment 323310
It's some or the other node floating, if not NODE2.
I got an idea from some amazing people first that I should connect Vcom to ground through a capacitor and large shut resistance, and that worked; but I later realized I cannot provide any electrical connection between Vcom and ground, as that provides a path for common mode current to flow.
If anyone can provide me with any way to achieve what I am trying to without any ground connection after the secondary windings, it would be great!
Attaching the .asc file for reference.

Read post #9 again.