We use a LNK623 together with a custom EE13 Transformer (prim. 2mH) for a 1W 3.8V from 230V/50Hz supply and are struggling with two issues.
First: The feedback voltage is settling quickly (<2us) and the regulation is stable, but the on time of the fet is never below 1.6us. Even for no or very little load. The datasheet says it should go down to 0.7us. How is this possible?
Second: We would like to have the supply run continuously on 100kHz to prevent all audible noise (transformer is tip varnished), but need very high load (4W) to achieve this mode. How do we have to modify the transformer to get continuous pulses at 1W (not only at 4W)? Increase or decrease primary inductance? Change air gap? Something else?
Looking forward to get your reply.