offline SMPS charging up 30mF capacitor

[eem2am]

Hello

I am designing an isolated offline flyback for 28W output power.

The 36V output has 30mF capacitor on it.
-this is needed to provide some power to the output in the event of a mains failure.

The overall spec is.

Flyback
Fsw = 100KHz
Pout = 28W
Vin = 85-265VAC
Vout = 36V
Feedback by opto

I wish to use the NCP1271 PWM Controller

*** NCP1271 datasheet….
https://www.onsemi.com/pub_link/Collateral/NCP1271-D.PDF

However, on page 9 (bottom right, under “fault protection” it is apparent that “when no feedback signal is received for 130ms”……the IC goes into fault mode and basicallt cycles itself repeatedly on and off.
-the second diagram down in figure 29 on page 12 also details this.

The point is, that during start-up, it would take 1 second to charge up the 30mF cap to 36V…………….and since this exceeds the 130ms fault timer, the power supply will just never get running.

Do you know of any fixes for this situation?
-Also, do you know of any PWM controller ICs which have features built in to handle this situation.
-I am finding that the vast majority of PWM controller ICs have this ~130ms “time out” feature and so would be no good in this case.

 

[FvM]

Try to add a soft-start circuit that provides a FB signal during startup.

 

[eem2am]

hi
The problem will be that this soft start will slow up the charging process