PSU with FAN8754 won't torn on

[fejlong]

Re: PSU with FAN8754 won't torn on

At the other end of the series resistor-diode I measured 102.4V DC regarding to IC ground pin 1

[budm]

Re: PSU with FAN8754 won't torn on

Looking at the pictures:
https://www.badcaps.net/forum/attach...6&d=1474884739

So what happen to the diode and the two resistors connected to pin 4 for the HV pin, some one replace those parts? D101 R102, R???

[fejlong]

Re: PSU with FAN8754 won't torn on

Yes, replaced them. But no change

[budm]

Re: PSU with FAN8754 won't torn on

Yes, replaced them. But no change

What ACV are you getting at the Anode of D101 ref to GND pin of the IC?

[fejlong]

Re: PSU with FAN8754 won't torn on

At D101 anode is 119V AC regarding to ground.

[dick_barton]

Re: PSU with FAN8754 won't torn on

I'm wondering if the mains input supply which should be 230V is low. In post #1 it was reported that there was 300V DC across the HV capacitors but I think it should be about 325V which may be why the HV voltage at pin 4 is just below the minimum Brownin voltage.

[rievax_60]

Re: PSU with FAN8754 won't torn on

The 2v on the FB pin 2 is close to the possible zero duty threshold. There could be a leaky component on pin 2 or the optocoupler is being turned on. What is the LED voltage?

[fejlong]

Re: PSU with FAN8754 won't torn on

Leaky component, you mean caps ?
The only cap on that line is C112 - 4u7 / 50V I don't know his role.
The multimeter cannot measure resistance on this cap, nor capacitance. I will desolder it to check it more thouroughly.

[rievax_60]

Re: PSU with FAN8754 won't torn on

I was talking about pin 2 of the IC, U101. Do a resistance measurement to live ground (pin 1) first. Do not try to apply power with any pin of the optocoupler disconnected.
What is the voltage across the opto's LED pins?

[fejlong]

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 .

[rievax_60]

Re: PSU with FAN8754 won't torn on

The bit that doesn't make sense is the 1.05v across the opto's pins 1 and 2. Is that with no power applied also? Do you measure any voltage at the 24v output?
I should have mentioned that we don't normally make resistance measurements with power applied. It is not safe for you and the TV. You must have had power applied to get the 100 ohm reading.

[fejlong]

Re: PSU with FAN8754 won't torn on

I was not fully understanding you question in post nr 29, so I measured voltage between opto's pin 1 and 2. It results 1.05V with power applied and Pson.
And also desoldered pin 1 and 2 of the opto, and with the meter set to diode test, it shows 1.05V voltage drop. I also measured other opto's wich I have as spare, they all read about the same 1V.

At the 24V output I can measure 3V.

There is also a ratling sound from the transormer, it's about 6-10 Hertz.

[dick_barton]

Re: PSU with FAN8754 won't torn on

From the information supplied the voltage across the capacitor indicates that the PFC circuit is not operating.

The voltage at the HV input (94.5V) is below the minimum threshold (104V) to maintain the PWM output drive of the FAN6754. However, these figures are given in the data sheet for a 200K resistor/diode combination and not 156K/diode combination as in this circuit.

The VCC (pin 7) is at 18.5V. D103 connected to the VCC input to ground is an 18V Zener so that voltage looks correct. This voltage is generated by U201 (VIPER17) which also supplies the 3.3V standby which is also present.

You could lift out diode DS1 to isolate any problems with the 24V supply side and check if voltage at Vb rises > 300V.

Can’t think of anything else at this time.

[fejlong]

Re: PSU with FAN8754 won't torn on

PFC ? You mean Power Factor Corrector ? I don't know if this has any PFC

[rievax_60]

Re: PSU with FAN8754 won't torn on

The 1.05v across the LED indicates that it is passing forward bias current and emitting light causing the opto's internal transistor to turn on, in turn causing U101 to throttle almost fully down. There will be bursts of very brief pulses applied to he MOSFET's gate causing the rattling and very little power flow through the transformer.
The circuitry involving the TL431 and the opto's LED isn't supposed to turn on the LED until the output has climbed to 24v.
Difficult to figure out the reason without an understanding of how the TL431 works. Do some research on the TL431. Never apply power while the regulation loop is open. The output will go over voltage and cause more damage. That capacitor you circled can possibly be the cause if leaky.

[dick_barton]

Re: PSU with FAN8754 won't torn on

OK, with DS1 removed what is the ac voltage at Pins 9 & 10 of TS1

[fejlong]

Re: PSU with FAN8754 won't torn on

Pins 9 and 10 are joined. if you mean output of the transformer coils, is zero volts. And the rattling disappeared, the feedback killed the supply totally. I will try study TL431 on this board to see what causes the high feedback. Thank you rievax_60 , i will be back with the feedback

[rievax_60]

Re: PSU with FAN8754 won't torn on

OK, with DS1 removed what is the ac voltage at Pins 9 & 10 of TS1

Doing that will likely cause the MOSFET to fail.

[rievax_60]

Re: PSU with FAN8754 won't torn on

Pins 9 and 10 are joined. if you mean output of the transformer coils, is zero volts. And the rattling disappeared, the feedback killed the supply totally. I will try study TL431 on this board to see what causes the high feedback. Thank you rievax_60 , i will be back with the feedback

I hope that you did not remove DS1 and apply power?

[fejlong]

Re: PSU with FAN8754 won't torn on

Actually I removed only 1 pin, the cathode. Nothing happened as i can see. Soldered it back, and still rattles.