UCC256304: Inquire UCC28056 + UCC256301/UCC256304 design question

Hi Edward,

Nice hearing from you.

2A. PMP21251 is able to achieve 90mW standby power. An additional 10mW could be saved if the VOSNS and BLK dividers are combined.

2B. Typically the burst mode threshold is set based on what load is the application desires to exit burst mode. In practice, I have found the easiest way to do this is depopulate the top LL/SS resistor and set the output current to the desired set point for entry/exit to burst mode. Measure the VCR waveform peak to peak when the high side and low side gates are turning off. This peak to peak voltage is what you need to set the burst mode threshold to. You can then populate the upper and lower LL/SS resistors to give the appropriate burst mode threshold. 

3. BLK voltage start threshold is different and the BLK overvoltage threshold is different. At a board level, the main difference is the UCC256301 needs to have the PFC start up once before the LLC will begin switching. After the PFC has started up once, it can then be shut down and the LLC will continue running. For UCC256304, you can either set up the BLK divider to require the PFC to be always on or set up the BLK divider for the LLC to start switching as soon as AC is applied. This UCC25630x selection guide also might be helpful:http://www.ti.com/lit/sg/slyt728/slyt728.pdf

4. I'll make a diagram and email it to you

5.For UCC256304 BLK Vov, the threshold will be 5.03V multiplied by the BLK resistor divider ratio. I would say most designs using UCC256304 are not relying on BLK for overvoltage protection, they rely on a function inside the PFC instead (UCC28056 has two independent methods of OVP for example).

6. The power stage would need to be designed to support full output power from 120V or else it will hit overcurrent or ZCS protection. Most AC/DC designs that have the LLC start up first need the requirement of starting into light load where the gain curve is much steeper. Please note this is not a limitation of UCC25630x, this is simply one of the challenges with the LLC topology. I would also like to note that UCC256304 can be configured to start up after the PFC. This approach is more common if startup into full load is needed.

7. For a design that requires both AC and DC input, I would suggest using UCC256302. The X cap discharge function inside UCC256301 and UCC256304 are not meant to be paired with DC inputs and they will continuously try to discharge.

Best Regards,

Ben Lough