UCC28911: Intermittent Operation

Hello,

The VS pin is sensitive to capacitance and will cause the device to think it is failing input under voltage.  So you cannot probe it directly.  You need to probe the the aux voltage across the VS divider.    More than likely you are shutting down do to an OVP, OCP or input UVLO fault.The following will bring you an application note the describes primary side fault sensing, most common issues and how to resolve them.  https://www.ti.com/lit/pdf/sluaac5

When it comes to selecting Lp you should choose a switching frequency greater than 54 kHz to get the full range of the control law.  This will determine what your Lp should be.

Your bulk input voltage should not be spiking when the energy starts to be delivered to the secondary.  This would indicate that your design does not have enough input bulk capacitance.

Your turns ratio Np/Ns seem to high it should be based on volt seconds and Dmax.  The Na/Ns should be sized to deliver 12 V to VDD.  If you were charging a battery you would probably design for a VDD of 20. You have a Na/Ns = 1.26.  So with a 14 V output you will have 17.8V.  So that is O.K.

I reviewed the schematic and functionally it is correct. On first review the input capacitance and transformer turns ratio for this design is not correct. You can use the Webench tool that can be found on the following web page to size the transformer turns ratio correctly and primary magnetizing inductance.  I know this is for a single output but you can use it to calculate the primary magnetizing inductance, proper turns ratio, input and output capacitance.

https://www.ti.com/product/UCC28911 , search on Webench:)

Just need to do the following.

1. Do a design for full input power.  This will give Lp, Np:Na:Ns and input capacitance, Rcs and VS resistor divider.

2. Do the design for 1/3 of the power.  This will give you the output capacitance for each bank.

Regards,