SMPS theroy question

[gobo207]

Hi all, electronic theory question ahead,

Quick background - I'm repairing a SMPS in an Dick Smith LCD TV. It looks like the MOSFET had failed dramatically so I replaced all the affected components. It still wont switch so I now suspect the PWM controller got a taste of 240v where it shouldn't have.

While diagnosing the IC I pulled up the datasheet and found the attached diagram and had a question. With reference to the application circuit in the datasheet, how is it that the VDD voltage is generated since there is no switching on the transformer?

The IC is a OB5269, datasheet attached.



Regards,
GoBo

[keymaster]

the chip draws a small power from the pin 8 HV to start the operation , then it continuous to operate  with power from the transformer.

edit: At page 8 of the pdf is the theory of operation .

[gobo207]

Thank you, that makes sense! Probably should've read all the datasheet....

Would something like this be a suitable replacement? It's getting all the inputs it needs and still not switching.

https://au.rs-online.com/web/p/pwm-current-mode-controllers/1784578/

Datasheet: https://docs.rs-online.com/074d/0900766b8167ad34.pdf

Regards,
GoBo

[PKTKS]

If you succeed  report the steps..

I have a similar issue with a similar OBxxx chipset.

Ordered a batch of 5 of them...
replaced  the chip - it works kinda strange
for 15 minutes and went dead by itself

Still thinking if I will shot the next 4 pieces
or just  brick those crappy chipsets.


Paul

[gobo207]

I ended up replacing it with the ON Semiconductor NCP1239JD65R2G which looked like a good fit given the datasheet. Worked out of the box! Has it running all day yesterday to "burn it in" and no issues. Taped up all the breather ports to get it warmer.

GoBo207