UCC28880: UCC28880 CCM operation

Hello Someno-san,

The UCC28880 can be operated in forced-CCM only at maximum output power/current and low input voltage. As the load reduces the operation will move out of CCM into DCM.  In general, high input voltage will also move it from CCM to DCM, but your input range is relatively narrow so at maximum load, it may still stay in CCM.   Due to the on/off control method, this IC cannot be prevented from going to DCM at lighter loads.  

To force the CCM at maximum load, low-line, the flyback transformer primary inductance must be chosen at a value high enough such that the primary current reaches the current limit I_LIMIT by the end of the maximum on time, t_{ON_MIN}.  Also, the secondary current cannot reach zero before the end of the minimum off time, t_{OFF_MIN}.  This is a tricky situation to manage because of the various tolerances of the on and off times and the primary inductance and the I_LIMIT threshold varying with junction temperature. This situation can be sustained only at the lowest line voltage and with V_FB always below 1V (V_{FB_TH}), which means that the output voltage must be allowed to drop below nominal regulation at the maximum load current.   This may be successful at the nominal values of all of the components, but may not stay in CCM with components whose values are closer to their min/max limits.

In terms of the power level, 12V x 0.15A = 1.8W which is within the UCC28880 capability. However, the higher power level at 3.6W is above the UCC28880 capability.  In this case, the UCC28881 is better suited to the 3.6W load level.  Once the output current is finally decided somewhere between 150mA and 300mA, the choice of UCC28880 or UCC28881 can be made based on current limit and temperature rise considerations.

I found a few reference designs using the UCC28880 and UCC28881 in flyback configurations by searching the TI website “Applications and designs” page. The reference design PMP20106 is an example of a flyback at ~2.18W from 270Vdc to 410Vdc input. This is different from your requirements, however the concept can be modified to your input range.  The outputs can be consolidated into a single 12-V output. Follow this link http://www.ti.com/tool/PMP20106?keyMatch=UCC28880&tisearch=tidesigns  to view the files.

If your power level settles up to 3.6W, then TIDA-00708 using UCC28881 may be a better reference to follow. It is also a multiple-output design, but can be adapted to a single output by eliminating the un-needed parts and redesigning the transformer.   http://www.ti.com/tool/TIDA-00708?keyMatch=UCC28881&tisearch=tidesigns  

I could not find any designs that matched to your parameters more closely.  I hope this helps you.

Regards,

Ulrich