Hard to give specific advice, the failure is probably related to very specific implementations details, and you don't provide a schematic. Or any waveforms.
What exactly do you mean here? Does the LLC converter use a PWM controller chip, or are you directly defining the PWM waveforms to the LLC on startup? What provides the low voltage power supply to the PWM controller and gate drivers? In a real implementation it will have to be derived from the 390V bus, but it sounds like you're providing an external supply (otherwise how could the MOSFETs be driven before AC is applied).