UCC28780: Hard Limitation of minimum Vin

Domingo

You are correct, UCC28780 can operate at lower input voltages.  The absolute minimum is slightly higher than the UVLO turn on threshold (18.2V abs max) since the startup needs to go thru the depletion FET used to sense the switch node.  We generally use 25V to 30V as a minimum voltage.  That being said, the operation of UCC28780 is designed for a higher input, such as an AC input from 85-265VAC, so the performance at such low voltages will be limited.

Both the GaN and Si FET based SIMPLIS models that are on the UCC28780 page here http://www.ti.com/product/UCC28780/toolssoftware are designed to run from 110VDC to 380VDC.  When I run the Si based model (http://www.ti.com/lit/zip/slum643) at 110VDC it runs and provides a 20V output as expected.  I recommend you redownload the SIMPLIS model to make sure that no other parameter has been changed which is interfering with your simulation model.  Note that the default circuit has a UVLO threshold set at 107VDC, if you need to operate with a lower voltage we recommend that you modify the design to support the input range you desire.

Best Regards,

Eric

Hi Eric,

Thank you for your feedback.

My design is slightly different than the one published by TI but I followed your spreadsheet to adjusted all values per my requirement.

I am also using the same Simulation file you recommended.

What I am observing is actually an Start-up issue.

The unit will not start @Vin lower than ~130-135V.

But if started with Vin= 140V and then Vin is changed to 100V it will regulate as expected.

So far the only thing I find is that VCST swings down at first PWM activity and perhaps it is the cause of RUN being terminated.

At this moment I do not fully understand all the protections built in this IC.

Can you tell me what are the signals that I should watch?

I am running the circuit with VDD generated by the IC not by the Pri Bias Kick circuit.

Thank you,

Domingo

Domingo

There have been some errors identified in the UCC28780 excel calculator tool that's currently on ti.com.  We are close to updating it with a fixed version, we expect to have it uploaded within the next week.  I will reply to this thread with a link to the corrected tool once it's available on ti.com.

There are several fault protections that UCC28780 has that could prevent it from properly starting up.  Section 7.4.10 (page 30) of the datasheet gives an overview of them.  We recommend that you review the datasheet to see if any of these fault responses is causing your simulation model to not run.  In addition to the datasheet, we are currently working on creating an application note to help explain how to debug a design with UCC28780 to startup.  I have attached a guide that we currently have that you can use to help determine what is preventing your simulation from starting up properly.

3060.UCC28780 Debug Guide.pdf

Best Regards,

Eric

Domingo

The UCC28780 excel tool has been corrected, the errors you identified have been resolved.  An updated rev B is now available at the same location.  Below is a direct link.

http://www.ti.com/lit/zip/sluc664

Best Regards,

Eric