Dell 2007FPb with 4 buck step-down regulators

[seweryn]

I have monitor Dell 2007FPb. It was working good. Some day it was shut off and stay dead.
I checked some voltages on the boards. I read this thread:
https://www.badcaps.net/forum/showthread.php?p=335335
and very usefull budm posts ( thanks buddy, it was great work ... ).
My damage was typical: 3A fuse burned and ZD701 diode was shorten.
So my board is in few points different than other described in citing thread. On my board are 4 buck converter regulators. Apart regulators 3.3V (Q6), 5V (Q14), 12V (Q9), is ther fourth, build on MOSFET Q2. Two TL1451 has together 4 PWM controllers. I drawed some circuits from that board on my schematic picture.
I checked capacitors, resistors on output monitoring circuits, MOSFET's, shotky diodes - all was ok.
To solve problem I tried connect only Buck Converters Board to my external power supply DC 19V with current limitation. I changed fuse, and zener diode ( I got 3V6 zener SMD diode, cause it think, it worked as transil for
overvoltage ). After all that, my regulators for 3V3 and 5V works. No voltages on two remaining.
After analysis of schema I found that transistors in DTC circuit blocked this two regulators, when pins
2 and 4 was not connected to Logic board. It must be some "power ON" button circuit. When I was connect this to Uref ( pin 16 from TL1451 ), both stay to work. Q2 regulator makes about 4.8 V.
So I assembled this boards together and try turn on monitor. It didn't work. After measuring I found, that
5V regulator (Q14) which worked fine, where board was alone, now haven't any voltage.
It must be, that connecting
to Logic board makes short and turn off this regulator.
I don't know how I can investigate this further. Any help will be welcome.

[keeney123]

Re: Dell 2007FPb with 4 buck step-down regulators

I have monitor Dell 2007FPb. It was working good. Some day it was shut off and stay dead.
I checked some voltages on the boards. I read this thread:
https://www.badcaps.net/forum/showthread.php?p=335335
and very usefull budm posts ( thanks buddy, it was great work ... ).
My damage was typical: 3A fuse burned and ZD701 diode was shorten.
So my board is in few points different than other described in citing thread. On my board are 4 buck converter regulators. Apart regulators 3.3V (Q6), 5V (Q14), 12V (Q9), is ther fourth, build on MOSFET Q2. Two TL1451 has together 4 PWM controllers. I drawed some circuits from that board on my schematic picture.
I checked capacitors, resistors on output monitoring circuits, MOSFET's, shotky diodes - all was ok.
To solve problem I tried connect only Buck Converters Board to my external power supply DC 19V with current limitation. I changed fuse, and zener diode ( I got 3V6 zener SMD diode, cause it think, it worked as transil for
overvoltage ). After all that, my regulators for 3V3 and 5V works. No voltages on two remaining.
After analysis of schema I found that transistors in DTC circuit blocked this two regulators, when pins
2 and 4 was not connected to Logic board. It must be some "power ON" button circuit. When I was connect this to Uref ( pin 16 from TL1451 ), both stay to work. Q2 regulator makes about 4.8 V.
So I assembled this boards together and try turn on monitor. It didn't work. After measuring I found, that
5V regulator (Q14) which worked fine, where board was alone, now haven't any voltage.
It must be, that connecting
to Logic board makes short and turn off this regulator.
I don't know how I can investigate this further. Any help will be welcome.

Here is the data sheet for the TL1451 perhaps that will help you.

https://pdf1.alldatasheet.com/datash...TI/TL1451.html

[seweryn]

Re: Dell 2007FPb with 4 buck step-down regulators

Here is the data sheet for the TL1451 perhaps that will help you.

https://pdf1.alldatasheet.com/datash...TI/TL1451.html

Thank You. Its helpfull. I read this to understand Dead Time Control function.

[seweryn]

Re: Dell 2007FPb with 4 buck step-down regulators

I did some experiments. The measuring environment is shown in the attached figure. Two boards "Buck Convertes" and "VGA / DVI Logic" connected together, powered by a laboratory power supply with current limiting.

1. Current limit set to 1A.
After being turned on, the power supply works in C.C. mode, gives the permissible 1 A from the power supply, the voltage instead of 19V is about 5.5 V. Output voltages are too low. It behaves as if VGA/DVI Logic board has a short circuit.

2. Current limit set to 1,5A.
After switching on, the current drawn from the power supply remains at the level of 0.36 A. The 3.3V buck converter is at the correct voltage. There is 0V on the buck converter to 5V. There is no short circuit. Current on the +3,3V wire between board is about 1,8 A.

I don't understand that very much.

[sam_sam_sam]

Re: Dell 2007FPb with 4 buck step-down regulators

Here is a test that you can do and it does not require the VGA board

Take your converter board and use some incandescent light bulbs that are the correct voltage and see if it will work on the 3.3 volt rail and on the 5.0 volt rail if that works correctly then your issue is with the VGA board now if you have two power supply you test this board as well but use an automotive incandescent light bulb which is used in series with board and the power supply and see what your current on each power supply rail is then you will know if you have any issues on either voltage rail

Put an incandescent light bulb one on each power supply rail if you have a short on a power supply rail that light bulb will be at full brightness [ at the voltage level for example 3 volt will be one brightness and at 5 volts will be at different brightness this is normal results ] ( one note use some incandescent light bulb that have a current draw of about an amp or so { at 12 volts } for best results 1156 or 1157 type of bulb or something similar )

One other note if you do not have two separate power supply if you have tested the converter board and it functions correctly you can do the same exact test with it put the incandescent light bulb in series with the converter power supply rails and do the same test I referred to above ( one note about this type of test it would not tell you how much current you are drawing on a certain power supply rail unless you have two DVM set on current to measure the current on each rail )

I hope this helps you with testing these boards

[seweryn]

Re: Dell 2007FPb with 4 buck step-down regulators

Thanks sam_sam_sam. I used a similar method and I already know that almost all of this current goes through the 3.3V supply. Current value is about 1,9 A. Just connecting 3.3V power to the VGA board looks the same. On the VGA / DVI Logic board, the voltage of 3.3 V enters two linear stabilizers. You can see them in the photo.
950T65 works fine. Its output is 2,5 V. But 952T43 not works properly. On his output is only 0,4 V ( Should be 1,8 V ).
Static measurement with an ohmmeter showed a short circuit on its output line to ground.
Looks, like I will have to check the items on this path one by one. This means that some SMD components need to be desoldered. It is a difficult task for me, but I will try.

[kenke]

Re: Dell 2007FPb with 4 buck step-down regulators

hello guys,

I need the diagram of the video card, do you have it? I was already able to lift the short that I had in the buck converter, the typical one of the zd701 and the 3A fuse..
but I have a short on the 3.3V rail of the video card, I have already lifted most of the components. I just need to lift the video processor but I would like to confirm if I missed any component