General PFC question.

[senndogg]

i may be posting pictures later because at the moment i am lost...
the TV is a Philips 42PFL3403/12 ( europe version) and the psu is a LIPS250PS02 3122 427 25041.
-The TV is dead because of an overvoltage, >400V
-the main fuse is OK. but there is NO voltage at the outputs, even at the standby section. so the tv led doesnt lit.
-voltage at the big caps is 300VDC.
-the visible damage is at the PFC circuitry.
the mosfet is shorted and the .68Ohms resistors are open. the PFC IC ( a ST L6562A) doesnt have visible signs of failure.

i SUPPOSE that the standby voltage must be there ALTHOUGH the pfc may be dead. right?
it looks like it wants to start, only a few ms i sthink, it kicks every 1-2seconds but sth stops it.

- i have repaired the PFC section, at the moment. but the VDC for the IC is not steady, it there only a those few ms when the standby wants to start.

Should i look for any error at the standby circuitry or it is wholly depended by the PFC?
i mean, should the standby 5V be there although the PFC may be dead?

[tom66]

Re: General PFC question.

Sometimes the standby supply will power the PFC control IC. The PFC IC could short. That would stop the standby and PFC from working.

[senndogg]

Re: General PFC question.

thanks Tom, that's what i thought in first place.
few minutes ago i was able to troubleshoot the standby circuit. it was a simple non-IC 3.3V supply. i swapped the 2.2uF cap and now the 3.3 is good.
-Now the tv lights the red led twice. maybe indicating that there is no 5V and 12V at the mainboard after it triggers the PS_ON.
when i press the power button, the voltage at the 5/12V ic supply and also at the pfc ic supply rises slowly from 0V to 1.6V and than falls again. i am going to cut the wire which feeds the PFC ic and see what happens.
question no2.: what if i delete the PFC? at the moment the DC at the big caps is steady 305VDC. is this sufficient?

[tom66]

Re: General PFC question.

No, you must run it with PFC. PFC based supplies can use smaller mains capacitors due to how they work. Without PFC, you will get 100Hz noise on the 24V, which will kill the filter caps in a few weeks or months, and could cause the inverter to flicker or cycle.

305V is much too low, it indicates mains voltage of 215VAC? Is this correct?

2 blinks is an error code. SM:

[senndogg]

Re: General PFC question.

finally i was able to get steady 20Vdc at the auxilary winding of the standby transformer. sth was shorted at the path to the PFC ic. after cutting the trace i turned the TV on just for few seconds and it looked quite well.
Now ill be working cautionsly at the PFC section.
Tom, i am trying to understand how much can the absence of PFC in the general operation of this tv. Since i dont have a decent camera to show you real photos i have made a simple schematic of the psu layout. hold a minute!

[tom66]

Re: General PFC question.

Without PFC the voltage on the caps will swing VERY approximately 65V per watt-uF. So if it uses say 220uF and uses 250W from mains, it will swing from 325V to around 260V. (A simple linear approximation.)

Now the PSU has transformers wound for operation at 390V +/- 10V for example (PFC is quite capable of holding the voltage that steady.) Do you think they will work at 260V?

The answer is... sort of. They can work at that but the output will sag as the mains cap voltage drops. And when the mains power comes back for the next cycle, the mains cap charges to 325V, the 24V rises again. So you get ripple on the output. Usually a few volts p-p. Ripple current is basically proportional to voltage ripple. So the output caps see a lot of ripple at 100Hz and they don't like that. This cooks them so even good caps will fail when the PFC dies.

And what about a low-line conditions -- for example, 180~200V mains brown outs?

[senndogg]

Re: General PFC question.

Without PFC the voltage on the caps will swing VERY approximately 65V per watt-uF. So if it uses say 220uF and uses 250W from mains, it will swing from 325V to around 260V. (A simple linear approximation.)

Now the PSU has transformers wound for operation at 390V +/- 10V for example (PFC is quite capable of holding the voltage that steady.) Do you think they will work at 260V?

The answer is... sort of. They can work at that but the output will sag as the mains cap voltage drops. And when the mains power comes back for the next cycle, the mains cap charges to 325V, the 24V rises again. So you get ripple on the output. Usually a few volts p-p. Ripple current is basically proportional to voltage ripple. So the output caps see a lot of ripple at 100Hz and they don't like that. This cooks them so even good caps will fail when the PFC dies.

And what about a low-line conditions -- for example, 180~200V mains brown outs?

i must agree with all of that!

[senndogg]

Re: General PFC question.

mains: 220,78V AC.
big caps on stand by: 305.6V DC
big caps when TV on: ~290,22V DC, +-2V.

it about 100V less than its ought to be.

[budm]

Re: General PFC question.

The PFC is not working. When MOSFGET failed, it will take out the Gate drive circuit of the PFC IC with it, you also to need to chake all the resistors and diode connected to the Gate.

[senndogg]

Re: General PFC question.

thanks Budm. i have traced so far 5-6 shorted/opened smd components and the main transistor and resistor. i have changed all of them, but i am really fearing that the PFC IC is partially broken and out of service. at the moment i am having trouble bringing the 20VDC to the Vcc Pin of the PFC IC. maybe a smd cap or transistor is shorted down the way from the standby aux 20V winding to the pfc ic. not exactly shorted (i have double-checked) but not-properly working. if connected it will overload and switch off the standby, so the entire tv. i have an out-of-focus camera that cant help

[tom66]

Re: General PFC question.

The PFC isn't working at all.

Voltage swing from 325V to 260V indicates average of 292V, again assuming linear drain (it's not quite linear but as long as it doesn't get near zero it's close enough.) That is basically what you are seeing.

[budm]

Re: General PFC question.

I would replace that PFC controller IC.