Why there is this discussion?
There are so many questions in the forum related to the startup of UCC2870X or other TI PSR controllers/switchers. We know the anxiety when customer met startup issues. So we develop to try to aim customer on their issues.
Meeting troubles in the debugging process is very common for power supply designers. The troubles may include smoke when you first power the supply or it doesn’t start up at all without any light or noise. The fundamentals of debugging should be to make sure the components are assembled right as the schematic and no wrong connections in the PCB. In this troubleshooting guide, I will assume that you have checked the board and have repaired the failure components. We will focus on the design issues in the remainder of the application note. This application guide can be used for TI’s primary-side regulated (PSR) controllers and switchers such as UCC2870X, UCC2871X, UCC2872X, and UCC2891X.
How to use it?
In the attached document, we talk about the potential reasons which may cuase the startup problem.
So if you are not sure what causes the startup issue you met, we encourage you check the lists on your board item by item first.
If none match your board or the suggestions doesn't solve your problem, please post your check results(include the items you checked, and the waveforms you captured ) and your problem, we can help more.
Issue : The power converter cannot startup or shutdown unexpected
Cause 1: VDD UVLO
Cause 2: VDD Clamp Current exceeding rating (only for UCC2891X)
Cause 3: VIN UVLO:
Improper BJT selection
Cause 4: On-time Detection:
-Too long on time at first startup cycle
- Too long on time cause shutdown
Cause 5: CS short circuit (1.5V) protection
CS Noise caused by layout
Cause 6: AUX winding detection (OVP)
Output voltage trigger OVP at zero load
The shape on VS pin affect the detection
You are welcomed to input more causes for the troubleshooting.