When starting up a prototype flyback converter some engineers use a separate bias supply for powering VDD of the flyback controller. Some of these designers have mentioned that there are no gate driver pulses observed and the flyback controller appears to be not functioning. Most of the time the issue is when power was applied to VDD there was no input voltage applied to the flyback converter. The flyback converter had already sampled the input voltage and because there was none present the controller entered input UVLO fault protection. The gate driver stopped switching and the external bias supply is prevents cycling of the VDD pin between VVDD(off), VVDD(on) and VVDD(off) to reset the fault.
To resolve this issue apply the input voltage to VIN that is greater than the UVLO trip point. Then bring the bias voltage to VDD above VVDD(on). The other option is adjust the bias voltage at VDD below VVDD(off) and then above VVDD(on) to reset and clear the UVLO fault.