LG LCD power supply

[budm]

Re: LG LCD power supply

Seeme to me there is drive signals to drive those two MOSFETs, you need the scope or just check to see if there are at least some action at the Gates of the MOSFET.

[vinceroger69]

Re: LG LCD power supply

can i check the gate with a multi meter or do i need a scope? what can cause no gate activity? as i do have a parts board but not sure what parts too change over cheers budm.

edit on the other section of the board testing the other two same ics that are used ther on the gates i do get a voltage reading one has 4 volts one has about 8 volts this section uses the same three ics if that makes sence.

[budm]

Re: LG LCD power supply

There is not much for that self running buck regulator. You may want to check the resistance between GND and all the pins, then the DCV of all the pins. We know that we have 5V on Vcc and the 12V at Vc but the IC is still not running.
It is going to be something we are not seeing since the circuit is pretty simple.

[vinceroger69]

Re: LG LCD power supply

u200 readings
1 0.78
2 5.12
3 1.88
4 0
5 3.64
6 9.80
7 2.00
8 2.92
low voltage on the vc line?
i have just changed this ic for another one diffrent order as ordered spares.

[budm]

Re: LG LCD power supply

Vc: This pin is connected to a voltage that must be at least 4V higher than the bus voltage of the switcher (assuming 5V threshold MOSFET) and powers the high side output driver. A
minimum of 1μF, high frequency capacitor must be connected from this pin to ground to
provide peak drive current capability.
Boost Supply Vc
To drive the high-side switch it is necessary to supply a
gate voltage at least 4V greater than the bus voltage.
This is achieved by using a charge pump configuration
as shown in Figure 18. The capacitor is charged up to
approximately twice the bus voltage. A capacitor in the
range of 0.1μF to 1μF is generally adequate for most
applications. In application, when a separate voltage
source is available the boost circuit can be avoided as
shown in Figure 1.
So you have 9.8V which meets the requirement since the Vcc is 5V.
The output drives is not at 0V but on the low side especially the High drive.
It looks like you are going to need the scope to look for activity of the IC.

[vinceroger69]

Re: LG LCD power supply

its a odd one as thats the second ic with the same readings on both so that should rule the ics out i guess.

[budm]

Re: LG LCD power supply

Something is stopping the IC from working, and we know that we do no have shorts or heavy load on the 3.3V output, and there is no command line to turn the IC on and off that I can see from the pictures of the board.
[B]Pin 8 SS/SD:
This pin provides soft-start for the switching regulator. An internal current source charges
an external capacitor that is connected from this pin to ground which ramps up the output
of the switching regulator, preventing it from overshooting as well as limiting the input
current. The converter can be shutdown by pulling this pin below 0.5V.[/B]
You have 2.92V so that is not it either.
I am running out of Ideas.

[vinceroger69]

Re: LG LCD power supply

this is a very common problem on these psus either low or no 3.4 volt line theres a lot of posts regarding it i know tom66 had this issue too but he didnt have time too debug the board.

[budm]

Re: LG LCD power supply

I saw those threads too. I wish I have the board in front of me or at least the diagram of the board.

[vinceroger69]

Re: LG LCD power supply

Not much information about these boards online these psu supplies i guess there isnt many ccfl sets using these as seem too used in older hdmi sets probably most have been recycled by now (this was a set found dumped in a bin shed does have stand and a remote) i will look on ebay i guess for a really cheap psu as not worth spending too much on it really but hey if can get another year or so out of it alls good and ive had practice using my hot air station too.Thanks again budm for your help i do have a scope only a 20 mghz one to learn how to use one really, but still havnt learnt the basics on it yet too set it up etc and test basic waveforms and understand what they mean etc but in time i will soon learn how too use it.

[budm]

Re: LG LCD power supply

20MHz is good enough to troubleshooting this buck regulator section, and it is on the cold side so no need to worrying about the isolation.

[vinceroger69]

Re: LG LCD power supply

Well this is the scope i have ive got the probes now and have set the trimmer pot on them as per the manual that came with them i just need too learn how too use the scope now
https://www.badcaps.net/forum/showthread.php?t=54421

[momaka]

Re: LG LCD power supply

this is a very common problem on these psus either low or no 3.4 volt line theres a lot of posts regarding it i know tom66 had this issue too but he didnt have time too debug the board.

Sorry if I sound a bit ignorant as I only quickly glanced through the last several pages. But it appears from your pictures (particularly this one) that the 3.3V rail is generated from 5VST. Now, from what I've seen in most TVs and monitors, the 5V standby supply is typically only rated for 2 to 4 Amps (and sometimes less). That means the 3.3V line in your PSU probably isn't rated for more than 3-4 Amps. So if the buck regulator circuit for the 3.3V line is not working... then you can just get a 1085 LDO linear regulator and use that to convert the 5VST to 3.3V. A 1085 regulator is good for up to 3 Amps. Another option would be the EZ1580 and EZ1581 LDO linear regulators, if you need more than 3 Amps. The only downside to using an LDO is that you will have more excess heat to get rid off. So you will probably need to make a separate conversion board with a small heatsink.

I guess this is not really a fix for the problem at hand, but rather a "workaround". Still, this might just be cheaper than getting a new power board (or if finding one is not possible). I'm sure there are many other circuits that can be built to convert 5V to 3.3V. It's just puzzling why the that buck controller on your board isn't working if you changed it and changed the FETs. Personally, I'd take out all of the parts that connect to that buck controller and test them out of circuit. There are at least several resistors, diodes, and ceramic capacitors around that buck controller IC. See if the resistors are open circuit, and test diodes and ceramic caps for short circuit.

[vinceroger69]

Re: LG LCD power supply

many thanks momaka for your input on the board connectors theres a pin marked 3.4 on i persume this maybe what turns that 3.4 volt section on? but why would it have that pin if its running of the 5v stanby line (i forgot too ask budm this the other day) im very new too electronics so still learning all time what does not help as budm says theres no schematics/diagrames for this psu.

edit!!
just re read budms post 107 so i doubt its the 3.4 on pin issue either momaka do you mean test all the parts around u200 ic?

[momaka]

Re: LG LCD power supply

on the board connectors theres a pin marked 3.4 on i persume this maybe what turns that 3.4 volt section on?

Hmmm. Could be that, or it could be a signal to tell the logic/video board that the 3.4V supply is operational (sort of like a Power Good signal on a PC PSU.) But you mentioned you have a working PSU board? If so, you could check that by either pulling the 3.4V ON pin low (to ground) or high (to 5VST).

It seems that the 3.4V ON pin goes through a resistive divider formed by SMD resistors R202 (470 Ohm) and R203 (4.7 KOhm). So you probably don't even need a resistor when pulling that line low or high.

I'm leaning towards that 3.4V ON pin to be the ON/OFF control for the 3.4V supply, because Q200 and Q201 appear to control the 5 VST voltage to pin 2 of U200 (Vcc).

But why would it have that pin if its running of the 5v stanby line

Good question.
Perhaps something on the video/logic board does need 3.3V in standby.

momaka do you mean test all the parts around u200 ic?

Yes.

I would also suggest removing the rubber sleeve from inductor L204 (the inductor for the buck circuit of the 3.4V line) and post a picture here. Unlikely to be burned, but it's better we check anyways.

**edit**
I can't seem to trace pin 6 on U200. Pin 6 is Vc and it should be, according to the datasheet, "at least 4V higher than the bus voltage of the switcher". Since the 5VST line is the bus voltage here, then that means pin 6 should have at least 9V. ... unless I am somehow mistaken here.

[budm]

Re: LG LCD power supply

To find out if the 3.3v buck regulator circuit is a switched power supply or not by running the know working board by itself without connection to other board and see if the 3.3V output is present, if 3.3V is present then it is not a switched supply.

[momaka]

Re: LG LCD power supply

To find out if the 3.3v buck regulator circuit is a switched power supply or not by running the know working board by itself without connection to other board and see if the 3.3V output is present, if 3.3V is present then it is not a switched supply.

I don't know about that, budm. We've always been thought not to leave logic pins floating. In this case, you'd have to be assuming that R202 and R203 are pulling the logic to the proper level - and that may not be the case. So I think pulling 3.4V ON low or high won't cause any trouble. R202 is 470 Ohms, which is high enough to prevent damage from direct connection of pin 3.4V ON to ground or 5 VST.

[budm]

Re: LG LCD power supply

According to the spec sheet of u200, to turn off the U200 you have bring pin 8 to below 0.5V, right now pin 8 is at 2.9V.
The question is, is the 3.3V buck regulator circuit is an always on power supply or not, hence running the good power supply board by itself.
All he has to do is to run the known good board by itself.
5VSTBY feed the 3.3V BUCK regulator, it is not controlled by the 3.3V regulator.
And for the output to be present at the out put of the Buck regulator, the Voltage (Vc PIN 6) for the high side MOSFET has to be present (9.8V) which it also has.
https://www.badcaps.net/forum/attach...6&d=1465580164

[momaka]

Re: LG LCD power supply

According to the spec sheet of u200, to turn off the U200 you have bring pin 8 to below 0.5V, right now pin 8 is at 2.9V.

Or you can turn Vcc off.
But if pin 8 shows some voltage, then perhaps the IC is getting power.

And for the output to be present at the out put of the Buck regulator, the Voltage (Vc PIN 6) for the high side MOSFET has to be present (9.8V) which it also has.
https://www.badcaps.net/forum/attach...6&d=1465580164

So you're saying that vinceroger69 has verified that U200 on his faulty board is getting both Vcc and Vc voltages? (Sorry if I missed it. I read very slow, and with multi-page threads, I sometimes get lost.)

[budm]

Re: LG LCD power supply

The VCC is fed by the 5VSTBY which he had found the the 5VSTBY output was showing shorted circuit (post 21) so I start helping him locate the shorts, we fond that the MOSFET for the buck regulator were shorted, then he changed the IC and now the 5VSTBY is back but the 3,3 V is at only 1.4V.

https://www.badcaps.net/forum/showpo...4&postcount=68

https://www.badcaps.net/forum/attach...8&d=1463421872

The wiring of the buck regulator looks to be matched with the circuit app notes of U200.

"So you're saying that vinceroger69 has verified that U200 on his faulty board is getting both Vcc and Vc voltages? (Sorry if I missed it. I read very slow, and with multi-page threads, I sometimes get lost.)" Read post 104, 105, 107.