Hi,
We have a design based on PMP11185 using the UCC28710. It is targeted for 5V / 2A output to power a small BLDC inverter. Our power supply design is almost identical to PMP11185, including the Wurth transformer.
We notice that randomly we loose our 5V output for a few milliseconds, which of-course causes a reset in the inverter. We don't think the issue is noise or layout, as we have had previous generations of the UCC28710 device with 5V/1A running fine.
The image below shows the yellow trace which is VDD (pin 1 of UCC28170). At about 1.5ms it fires out one pulse to the FET and then enters shutdown, and re-starts when VDD is below 8.7V. VDD has a 1uF cap on it and it is clearly entering shutdown mode, allowing VDD to decay to 8.7V before restarting.
The other channels are Green (CS), Blue (VAux Winding), Purple (VS)
We have checked almost every condition that we can think of to determine why a fault occurs in the UCC28710.
For example, the image below shows Green (CS), Blue (Aux Winding), Yellow (VDD) and we note that there is little noise on the CS line, no spikes and little ringing.
We have varied many parameters, including the size of the VDD capacitor (currently 1uF) up to 10uF as well as the VS resistor pairs. We have small 4.7pF caps on CS and VS pins (incase there was any noise).
We have followed in detail SLUA783.pdf and checked almost everything we can, for example the case below shows purple (VS) and Blue (CS) lines and we feel that we are not viloating conditions like the skipped valley and our T_on value is low.
The image below shows Purple (VAux), Blue (CS), Yellow (VDD), Green is the current being sourced from the 5V rail.
The only possible condition we see here is that we might violate SLUA783 clause 6.2 "Very quick resonant decay with TVS Snubber". In the purple trace below (VAux) there is a small blip that aligns with Clause 6.2, but that should be very insignificant.
Can anyone from TI help out and possibly suggest why the device randomly enters shutdown mode and then restarts. We know there has to be a reason (for example some EMI or inverter noise suddenly aligning with a sampling window), but otherwise we are at a loss to determine.
Any help or ideas or things we can check would be great. We are happy to take more detailed scope captures of specific test cases if that can also help.
Thanks!