Hello,
I designed a flyback converter using the UCC28711. The DC input voltage ranges from 150V-1000V. And the output voltage consists of 2 outputs of 17V each. Unfortunately, the circuit does not respond as expected. In particular, the constant voltage regulation does not stop on the desired voltage. Even though, the calculations of Rs1 & Rs2 are correct (the voltage on Vs is 4.05 when the output voltage is 17V), the IC keeps charging the output until 22V, then it stops and when the output discharges to 10V in starts charging again until it reaches 23V. At this point the overvoltage protection is activated and the IC swithces off, until Vdd reaches the Vdd turn-off threshold (8.1V). Then, the process starts again with the aforementioned pattern.
Is it possible that the problems occur because the NTC pin is left floating?
Can the ripple on the Vauxiliary create confusion to the Vs pin during the sampling time?
The following pdf files include the waveforms of the converter:
Blue=Vdd pin-Channel A
Red=DRV pin-Channel B
Green=Vauxiliary-Channel C
Yellow=Output Voltage-Channel D
The file ti1 presents the waveforms of the above measured signals. At 1.712ms is the point where the IC detects the overvoltage, even though it reaches the desired regulated voltage at 0.462ms
The file ti2 presents the same waveforms but zoomed in to the moment where the output voltage reaches 17V. I would liek to hear your opinion regarding the shape of Vauxiliary (Green) and how it can affect the sampling of the Vs pin.
Thanks in advance.