UCC28C42: Convergence error - Simulating UCC28C42 in buck mode

Hello Venkatesh,

Thank you for reaching out to us. We are reviewing your case and we will get back to you within 24 hours.

Regards,

Jonathan W

Hi jonathan,

Thanks for taking this up!

FYI. I referred PMP10783 reference design | TI.com for the buck regulator circuit reference.

Regards,

Venkatesh Naidu

Hi Venkatesh,

Thank you for letting me know about the PMP10783 reference design. I think you were missing a capacitor and diode from the reference design in your schematic. I have screenshots of the UCC28C42 Buck Converter schematic with the simulation settings and results. The capacitor C7 and the diode D2 should be added. Let me know if this works. 

Regards,

Jonathan Wong

Hi jonathan

Thanks for your reply!!

For this particular circuit the startup time itself is ~3s, the simulation goes into convergence error only after that time.  The Vout will not reach the required voltage before that that. Could you please repeat the your simulation by increasing the run time to >5s.

Also the Vin has to be a step input and not a pulse from 0V to 400V to replicate my system requirements.

Could you also probe V(VDD:SW) to get an indication of the startup of UCC28C42.

Capacitor C7 was not considered since we had already a high value 100uF capacitor,  Also the diode D2 is used only for the mosfet turn off. Didn't feel the need to have it in the initial simulation. 

Here are the results even after adding the Diode and capacitor- 

Circuit - 

Hello Venkatesh,

I was unable to replicate your convergence error with the circuit that I simulated in the screenshot. I have a screenshot of the simulation for 5s and 20ms. Convergence errors can sometimes occur when some components have a wide range of values (Ex. pairing a 2.5M resistor with a 10 resistor in parallel) or some parameter is set unusually. Is there a reason you chose 40 for R29? You can also try combining the nodes with wires instead of offscreen connectors or increasing the max step size in the simulation.

I will be referring you to our Electronic Design Automation team who specializes in PSPICE and I will come back with an answer as soon as possible. 

Regards,

Jonathan Wong

Hi Jonathan,

Thanks for taking it forward!

Just one point here. The expected VOUT for this circuit is 15V, The result in your output is different.

Regarding R29, I connected this to simulate the nominal load for our system.

Did you see any issues with the properties of components used in my simulation?? 

I will try some more permutations with the different setting and connection. Will put it here in case of a different result.

Regards,

Venkatesh Naidu

Hello Venkatesh,

Yes, you are right the output should be 15V. I'm not sure why mine does not have this behavior. I am still waiting on EDA. In the meantime, you can look at these similar E2E posts:

TINA-TI: https://e2e.ti.com/support/tools/simulation-hardware-system-design-tools-group/sim-hw-system-design/f/simulation-hardware-system-design-tools-forum/584533/tina-spice-ucc28c42-ucc28c42-convergence-problem?tisearch=e2e-sitesearch&keymatch=PMP10783%2525252520simulation# 

SIMPLIS: https://e2e.ti.com/support/tools/simulation-hardware-system-design-tools-group/sim-hw-system-design/f/simulation-hardware-system-design-tools-forum/576528/tina-spice-pmp10783-buck-converter---pmp10783?tisearch=e2e-sitesearch&keymatch=pmp10783%25252520simulation# 

TINA-TI is another free ECAD software from TI. You can download it and find tutorials here: https://www.ti.com/tool/TINA-TI 

Regards,

Jonathan Wong

Hi Jonathan,

Any update from the EDA team?

I saw both the posts, in one of the posts the simulation file that was shared by the TI representative consists of a different part number - UC3842.

For the TINA-TI simulation, I need to update the file from flyback to Buck. I need some time for that.

Regards.

Hello Venkatesh,

Sorry for the delay. I still do not have updates from EDA yet.

As for the schematics, the UCC28C42 and the UCC38C42 are the same device. The only difference is their application (UCC28C42 is industrial while UCC38C42 is commercial). The data sheets that are on both of the products are the actually the same since their behavior and simulations will be the same. Using the UCC38C42 model will be equivalent to using the UCC28C42 model.

Regards,

Jonathan Wong  

Hi Jonathan,

That's okay. Let me know as soon as the info is available. 

Now about the simplis solution, The part number is mentioned as UC3842 and not UCC38C42. Do you still call it a similar device?

Regards.