Our customers, when testing the DCDC isolated power supply with the UCC21750 EVM, found that the input voltage on the isolated side is not a fixed value under different input voltages, and the output is abnormal under low voltage, resulting in the gate drive VDD-COM voltage being lower than the expected value. What is the cause of this? Is it directly related to the construction of the external control circuit of the open-loop control? If so, what specific nodes in the external control circuit (capacitors, resistors) would cause this effect?

Thank you!

Best regards