I'm currently designing a Phase Shift DCDC using the UCC28950 controller. Below are my specs:
VIn : 400V (typical) / Vout : 12V / Pout :3.8kW / Application : Datacenter
(I'm using UCC28070 as PFC preregulator controller)
The controller is configured as:
Control : Voltage Mode / Switching freq = 75kHz
What we're seeing now is that there is C-D mismatch or imbalance, and gets worse as we increase our load. I'm familiar with the UCC3895 CD Asymmetry issue having used this part in several projects already, but UCC38950 seems to be a different part. Our application is also different.
What's strange is we've already set ADEL = GND to fix the deadtimes BUT we're still seeing this imbalance. For reference:
Pout "C" Duty "D" Duty
0W 46.8% 46.8%
75W 44.2% 48.8%
150W 42% 48.8%
360W 38.9% 48.8% (Waveform attached)
WE stopped here because of this mismatch we're seeing.
For A-B outputs, they are matched.
Here are some parameters for reference (during DEBUG):
RAB = 20k, RCD = 20k, ADEL = GND, KA (ADELEF) = 0.5, VDD = 12V, VREF decoupling = 1UF ceramic , RTMIN = 49.9K, DCM = VREF (always DCM). WE use internal error amp with non-inverting input = 2.5V
The local FAE seems not useful in this issue, so I'm trying my luck with this forum.
Please let me know if there is some key item that we need to understand on the part which is not found in the datasheet. Thank you.
Dennis Martin
