UCC28740 power up and power down queries

Mahendra, see responses below.

1. Minimum start up voltage of 80VAC : transformer primary inductance and turns ratio will determine this threshold or there is some additional component?

Startup voltage is dictated by the current out of the Vs pin during the primary on time. Once this current is greater  than 225uA the IC will start switching. The turns ratio and value of Rs1 control this voltage per section .51 of the datasheet.

2. During power down power supply should switch till Bulk capacitor voltage on the input fall to 50V or 25ms of output hold up time:

The turn off threshold is also dictated by the current from the Vs pin, again as described in section 5.1 of the datasheet. There is a ratio of ~2.8 between the two, so for and 80Vac startup the IC will switch down to ~40V on the bulk. Ensuring that this equates to 25ms requires selecting the appropriate bulk cap as you say.

Apart from choosing correct input and output capacitor, is there a way to configure this?

3. What parameters affect input to output delay or stat up time of the power supply? Customer requirement is 1.5Sec power up time, once the AC input is applied. 

The time required to charge the bias cap via the HV pin and the time required to charge the output cap at the required load set the turn on time. The unit will operate in CC mode on startup until the output cap charges, so both times can be calculated as t = C . dV / i , where

• C is the bias cap, dV is the Vcc UVLO voltage and i is IHV from the datasheet, for the bias cap charging time
• C is the output cap, dV is Vout, and i is the I_current_limit - I_load for the output cap charging time.

Regards,

Billy