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Old 01-15-2022, 07:39 AM   #1
Adenitz
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Default Learning to repair monitor's PSU

Hello guys,
I'm learning to repair monitors. I don't have much experience, but I managed to repair a few monitors by changing bad caps.

However, I'd like to learn how the PSU is working since, most of the time, there is a fault on PSU board.
Attached, there leis a picture of board PSU (Samsung SyncMaster), There is no problem on the board. I'd like to learn what some components is doing.

First of all, there is a AC power input (230 VAC), and after input EMI filter, there is a bridge rectifier (1) and a big cap (2). On this cap (2), there is DC voltage about 300 V. Ehat is the purpose of MOSFET (3) and transformer (4). I know that from this rectified input voltage, lower part of the board with its components produce 5V and 12 V (this can be 14 or even 16V).
I also know there is a high voltage transformer needed to supply backlight

Ca you please tell me, what is the purpose of transformer (4) and element (5).

I guess diodes (6) are used to rectify output from transformer (4) which is AC to DC voltage 12 V?
What caps (7) are doing? DO they filter this rectified voltage?

Thank you for your time.
I'm looking forward to your explanations.
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Old 01-19-2022, 02:19 AM   #2
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Default Re: Learning to repair monitor's PSU

LCD monitor power supplies operate the same as any other Switch Mode Power Supply (SMPS.)

Component (1) [bridge rectifier]: turns wall AC voltage to DC voltage
Component (2) [capacitor]: stores DC charge from (1) above
Component (3) [MOSFET (or other transistor)]: takes charge from capacitor in (2) above and pulses On and Off with high frequency, creating a high-frequency square wave AC voltage.
Component (4) [transformer]: is fed the high-frequency square wave AC voltage from (3) above on its primary side. On its secondary side, the transformer produces high-frequency, but low voltage... again, square wave AC.
Component (6) [output diodes / rectifier]: changes the low-voltage high-frequency AC voltage from the transformer (4) above and turns it into a "raw" DC voltage again (not smooth.)
Component (7) [output filter capacitors]: take DC voltage from the output diodes / rectifier in (6) above and produce a smooth DC voltage --> this is what generates the 5V and 12V/15V/16V/etc. DC for the monitor to use.
Component (5) [optocoupler]; used for sending feedback from the secondary side of the PSU to the primary side's controller. Basically, it tells the primary side (MOSFET) if it needs to switch more or less in order to maintain those 5V/12V/15V/etc. outputs constant.

And that's about it, in terms of basics.

Most monitor SMPSes tend to be of the discontinuous type, i.e. "flyback".
See page 20 of this technical seminar:
https://www.microchip.com/stellent/g...s/en528010.pdf
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Old 01-19-2022, 06:40 AM   #3
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Default Re: Learning to repair monitor's PSU

Thank you very much momaka, this is the explanation I was looking for.

Just one more question regarding the Component (6) output diodes / rectifier. On the attached image, on the left side there are also two diodes. I suspect that one pair of diodes produces rectified DC voltage 12V, while other pair produce 5V. I 'm pretty sure this transformer (4) has two separate secondary circuits.
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Old 01-21-2022, 12:12 AM   #4
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Default Re: Learning to repair monitor's PSU

Hello guys,

I have found a schematics of LG monitor power board and I'm currently analyzing what different parts of the board are doing.

The schematics is attached. I can understand what is the function of most of components, except for the BJT Q901 on the 12 V line. Such transistor doesn't exist on 5 V line. It seems to me that this is additional control element, because if the voltage on this 12V line is high enough, it will turn ON the Q901, and there will be additional current path via 4 resistors in parallel to the ground. This also sound like some kind of protection.

Can someone please clarify this?

Thank you.
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Old 01-21-2022, 01:21 AM   #5
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Default Re: Learning to repair monitor's PSU

Quote:
Originally Posted by Adenitz View Post
Just one more question regarding the Component (6) output diodes / rectifier. On the attached image, on the left side there are also two diodes. I suspect that one pair of diodes produces rectified DC voltage 12V, while other pair produce 5V. I 'm pretty sure this transformer (4) has two separate secondary circuits.
Correct.

Most monitor power supplies indeed usually have a 5V power supply and then also a 12-16V supply (varies with monitors.) Some will also have a 24V supply, in addition to 5V and 12V.

Quote:
Originally Posted by Adenitz View Post
I have found a schematics of LG monitor power board and I'm currently analyzing what different parts of the board are doing.

The schematics is attached. I can understand what is the function of most of components, except for the BJT Q901 on the 12 V line. Such transistor doesn't exist on 5 V line. It seems to me that this is additional control element, because if the voltage on this 12V line is high enough, it will turn ON the Q901, and there will be additional current path via 4 resistors in parallel to the ground. This also sound like some kind of protection.

Can someone please clarify this?
Correct again.
The whole point of Q901 and the 4 resistors in this case is to present additional load to the 12V rail if it goes too high. It's not so much for protection, but for voltage regulation when the monitor is in standby mode. Because in standby mode, the back lights will be off, and generally the back lights are the ones that use power from the 12V rail. But with them off, there will be little to no load on the 12V rail, and that can make it go high. Since the 5V and the 12V rails come from the same transformer, the power supply control circuitry has to keep a balance between voltage regulation on both the 5V and the 12V rail at the same time. So this circuit will basically help the control circuit by loading down the 12V a little when it is too high. That way, the 5V rail won't dip too low.

Of course, I will note here that generally you don't often see a circuit like that on most monitor boards. In fact, I've rarely seen it. So don't worry too much about this circuit.
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Old 01-21-2022, 02:19 AM   #6
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Default Re: Learning to repair monitor's PSU

Thank you for the help momaka, much appreciated.
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