Other Parts Discussed in Thread: UCC256404
Tool/software:
I'm currently designing a 100W, 24V power supply and using the UCC28056 as a PFC-Boost controller to take a 120-277VAC input and put out a 450V DC Bus, which then goes through a DC/DC stage with a UCC256404 controller.
At 120VAC in, everything works great, but as I increase the input voltage, the switching waveforms of the Boost FET becomes a bit erratic, leading to poor input current waveforms and high power dissipation. Attached are my schematic and excel design file, as well as scope plots.
It looks like, at 277VAC, the voltage at the drain never rings close to 0V, like it does at 120VAC. Is that expected? The duty cycle and/or Fsw also seem to vary quite a bit at 277VAC. Probing across the current sense resistor doesn't show anything odd. I've tried probing the ZCD pin, too, but the converter doesn't like that and will declare a fault, so I can't get a good waveform for that.
Any help would be appreciated. Thanks.
Boost PFC Sept3 20025.pdfUCC28056X_Design Calculator Aug18 2025.xls
In the plots below, the green trace is the output current of my 24V rail (mostly irrelevant for this issue - it's just there as an indicator of how much power I'm pulling), the yellow is the input current from the AC line, the cyan is the drain-source voltage of the Boost FET, and the purple is the gate of the boost FET.
120VAC and 50W of output:
120VAC and 100W of output:
277VAC and 50W of output:
277VAC and 100W of output
