PSU with FAN8754 won't torn on

[rievax_60]

Re: PSU with FAN8754 won't torn on

The data sheet for the FAN8754 mentions that it has built in protection for open regulation loop. You can't depend on it to save the power supply every time.

[dick_barton]

Re: PSU with FAN8754 won't torn on

I've simulated the opto-isolator circuit attached. The opto-isolator is not activated until the 24V output supply comes up at which point it is switched on. Anything less than 24V at the output and the opto-isolator is off.

Edit
D3 acting as the internal opto-isolator led.

[rievax_60]

Re: PSU with FAN8754 won't torn on

I'm suspecting that there is a fault causing the opto to turn on when the output is only 3v.

[budm]

Re: PSU with FAN8754 won't torn on

Resistance between pin 1 and 2 of IC U101, with no power applied, is 100 Ohms.
Voltage drop on optcoupler pins 1 and 2 is 1.05V
With power on, led voltage of the optocoupler between pin 1 and 2 is 1.05V .

So NO power applied to the board and you measured the resistance between pin 2 (FB) and GND and you get 100 Ohms reading which is not normal, it should be in thousands of Ohms.
As you can see in the diagram, there is 100 Ohms resistor R103 connected to pin 2, the other end of the resistor is connected to the MOSFET Q101 the Source of the MOSFET is then connected to GND, so you should check and see if that MOSFET is shorted out or not.

[dick_barton]

Re: PSU with FAN8754 won't torn on

Have you disconnected the main board from the power supply and used a 1K resistor to connect 3.3V (standby voltage) to P_On and then checked the 24V supply?

[fejlong]

Re: PSU with FAN8754 won't torn on

Hey, I'm back. I just had a day off and had time to play with this PSU.

I did played with the whole day,and I was really frustrated, but the success arrived after about 12 hours of thinkering and trial, but at the end worth it.
I tried finding the 431 problem, with a circuit built on a proto board, similar to the one which regulates the voltage. No luck
Inspecting, measuring resistors, caps, opto's, either removing them, and with new spare parts. Not working.
At the end I measures the voltage coming out from the 431 cathode, and driving it with an external laboratory PSU adjusting from 0-32V. The voltage on the cathode was cutting at 2.8V . After trying again, it was cutting at 4.5V . Finally I decided to desolder the 431 SMD type and solder a new one back. And voila, it works.

Thanks for all of you guys trying to help, especially for mr. rievax 60 "I'm suspecting that there is a fault causing the opto to turn on when the output is only 3v."
you where right, the 431 was faulty and was turning on randomly.
Another TV saved from being a waste for the planet.