SMPS start up problem

[whatty]

Hi,

Hi i have attached the schematic as shown in schematic i have connected all components in bread board.

What issue i have faced: When R5 start-up resistor was 18k the controller starts up and there was output at the secondary but the resistor R5 heats up after some time it was burned. The measured voltage across vcc of IC is 8.4v above which the voltage doesn't goes up

Then i changed the resistor R5 to 100K but there was no output at the secondary and measured the voltage across vcc of ic it was 4v above that it was not going.

After i took out startup resistor and ran it under external lab bench power supply the output and everything works well there is no output voltage across auxiliary does this is due to improper flux linkage between primary and auxiliary....?

I measured the leakage inductance between primary and tie all secondary and auxiliary together it was 130uh primary inductance is 840uh so this gives a coupling factor of 0.92 .

I have given image of winding order which is done in transformer.

I have attached both the image and schematic pdf.

(Previously i had started a thread start-up resistor yes it was me please pardon me i didn't find any response hence started new)

[Eraldo]

First of all what is the output voltage of the transformer since assuming it is 30v the power dissipation would be only 0.05w

Next can you check the current consumption of the ic when connectet to the bench psu? From the datasheet of the ic, it is capable of outputting a max of 1 amp and with a 18k resistor it would never come even close to that (again assuming the 30v transformer it would be less than 2 mA)

In this case i would try to use a 12v (or 9) linear regulator. Higher current capabilities and more stable voltages.

Also just in case, check that C3 in case it's making a short circuit.

Good luck

[capacitor_explosion]

Hello Whatty , I think your issue lies in the fact that LT1246-7 has a very specific Start up threshold voltage which I don't think you have accounted for . Even if you have choosen the correct resistor (R5 should  be above 100Ohms 10W ) the device starts , you need to design the auxiliary winding to be above the minimum operating voltage stated in the datasheet (is not the same as UVLO) .
Let us know the progress .

Simo