Need help to understand this circuit

[Fr3shGy]

Hi, i am having a hard time understanding this circuit.

"The optocoupler isolates the primary from the secondary side."

1. Does that mean that whenever there is an issue, lets say a big short on the primary side, the secondary won't get damaged?

2. Why does this circuit has 3 optocouplers?

[petehall347]

Re: Need help to understand this circuit

follow the lines back to icm801s and look at the datasheet for it then you should see what they do . as an example the top opto goes to fb .

[Fr3shGy]

Re: Need help to understand this circuit

That one of them is for the feedback circuit makes sense, the second one could be for over current protection, maybe?... but the third one?

the sample circuit of icm801s only shows 1 optocoupler for the feedback

[R_J]

Re: Need help to understand this circuit

PCM803S is for feedback regulation of the power supply when in standby, Burst mode
PCM802S is the isolator that tells the power supply to go from standby to full ON.
PCM801S is for feedback/regulation of the power supply when turned ON and operating at full power.
This is an always on power supply that has no separate standby section, so it operates at low power burst mode when the tv is in standby and does not require much current to operate.

[Fr3shGy]

Re: Need help to understand this circuit

PCM803S is for feedback regulation of the power supply when in standby, Burst mode
PCM802S is the isolator that tells the power supply to go from standby to full ON.
PCM801S is for feedback/regulation of the power supply when turned ON and operating at full power.
This is an always on power supply that has no separate standby section, so it operates at low power burst mode when the tv is in standby and does not require much current to operate.

Thank you for clarification.

1. If anyone of those PCM's had a problem on the cold side, the IC would always stay in burst mode?
2. When the IC operates in burst mode, output voltages would always fluctuate?

[R_J]

Re: Need help to understand this circuit

Thank you for clarification.

1. If anyone of those PCM's had a problem on the cold side, the IC would always stay in burst mode?

depends on the problem and which optocoupler had the problem

2. When the IC operates in burst mode, output voltages would always fluctuate?

The A13v could fluctuate and could be low, maybe around 9v, The vdr voltage would likely be quite low.

For example if there was a short on the A13v line, PCM803S could shut down the ic after 36 oscillations (over current detection)
If PCM802S failed, QM808 would not turn on which supplies the vcc to the pfc controller ic etc.
If PCM803 failed, the ic may detect an overload and also shut down. It's a complex ic
What problem are you having with this tv?

[Fr3shGy]

Re: Need help to understand this circuit

What problem are you having with this tv?

I have a problem, not with that exact board but a fairly similar one (BN44-00786A), just couldn't find the exact schematic.

Both mosfets went out and took the IC with em. I replaced both FET's the IC and a couple of resistors.

After that the IC would not switch on not even in burst mode. So i checked the FB line and found out that both optocouplers PC801S and PC802S were shorted.

Replaced both opto's and now the IC is sitting in burst mode and the Output (A13V) gives me 5,2 - 5,9V fluctuating.

I then went ahead and lifted all optocouplers output legs up and soldered 3 diodes to the board. After that the IC fully turned on and i got 8.9V stable without switching. Which should suggest that the issue is on the secondary side of the board?

I am stuck now without a general direction to go.

[R_J]

Re: Need help to understand this circuit

You can't install diodes in place of an optocoupler, what made you do that? that will never work. The optocoupler that turns ON the board will need to be turned on, How are you turning ON the power supply?

The power supply has a PFC circuit, does the pfc controller have vcc voltage? What is the voltage ACROSS the main filter cap in the primary? if it is 165v, pfc is not operating, if it is about 385v the pfc IS working.

[Fr3shGy]

Re: Need help to understand this circuit

My bad, i got it mixed up. That other thread isn't updated.

So the current state is:
I replaced the LLC IC, both mosfets, some resistors and both optocouplers

-Now i get 5,2 - 5,9V fluctuating at the output.
-Main filter cap is sitting at 330V
-VCC for the PFC IC is fluctuating 11~13,5V (not turning on)
-FB pin sitting at around 0.6V

On the secondary side, i checked the bigger electrolytic capacitors and the rectifiers.

I noticed that while measuring the resistances on the secondary side, the resistance of every resistor slowly rises, until it hits the correct value.

When i measure a diode in diode mode, the value slowly rises, until it hits the correct value.

Does that mean something?