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I have an LED TV Samsung T22B350EW (Type No.: LT22B350) which can not power on. I'm trying to fix it, to learn more about such type of circuits. A capacitor in its power supply board is blown up.

The board is:

PD23A0Q
Samsung BN44-00505A
2012.03.14
Rev 1.3

Refer to this schematic (also available here):

enter image description here

The exploded capacitor is CM804 (221 / 1 kV).

I replaced it with a (hopefully) more robust capacitor F221K29S3NR63K7R Vishay 220pF ±10% 3kV cc, dielectric S3N, but it blew up as well.

I guess (according to the capacitor required features) that its terminals should receive a 220 V AC voltage. Is this correct?

  1. By using a multimeter and an oscilloscope, and connecting this board to the mains, is it possible to detect which block causes this problem?

  2. Maybe something before this capacitor should be investigated? For example, the STRW6052 is an integrated power MOSFET, which is the primary switcher, and I am not able to determine if it correctly works.

  3. Is this switching power supply circuit a common one? Is there some documentation available about it?

Ideally, I would need to determine the expected voltages at some relevant nodes in the schematics, so that I can compare them with the measurements obtained through a multimeter.

Despite many Google searches, I was not able to find anything more detailed than the above schematics about the power supply circuit. It seems that Samsung did not make it available.

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    \$\begingroup\$ A more complete schematic is required. \$\endgroup\$
    – Andy aka
    Aug 23, 2021 at 11:04
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    \$\begingroup\$ is it possible to detect which block causes this problem? Avoid asking a "is it possible" question. Someone with a multimeter and scope but having no clue about how this circuit (should) work is not going to find the problem. Someone with an understanding of this circuit might only need a multimeter to find the fault. The circuit looks like a flyback converter so I suggest learning about those. I hope you realize that it is quite dangerous to "repair" mains connected circuits with only little experience with such circuits. \$\endgroup\$
    – Bimpelrekkie
    Aug 23, 2021 at 11:04
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    \$\begingroup\$ I consider this circuit as a chance to learn something, starting from a very basic level. I would STRONGLY suggest to start learning electronics with low voltage circuits. i.e. not this circuit. I know it is tempting to start "learning electronics" to repair your device which has just failed but this generally ends in dissapointment and confusion. \$\endgroup\$
    – Bimpelrekkie
    Aug 23, 2021 at 11:34
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    \$\begingroup\$ Something else has failed and CM804 blowing up is a symptom of it. If it has failed, there is a good chance many other components have failed in addition so on many occasions it is simpler to buy a new power supply and swap the whole thing. But since CM804 blows up, it might mean diode DM801 or the cap and/or resistor above it are broken, since they form a RCD snubber for the flyback pulse and if the snubber does not work the flyback voltage can be so large it damages CM804. \$\endgroup\$
    – Justme
    Aug 23, 2021 at 12:51
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    \$\begingroup\$ I'd suggest you wear eye protection when working on these things live as they have a tendency to go pop and blast a piece of epoxy in your eye. A remember the capacitors hold a charge after power off - so handle the board carefully. You’ll certainly know when it zaps you! \$\endgroup\$
    – Kartman
    Aug 23, 2021 at 13:21

1 Answer 1

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I would never have guessed 220 pF 3kV part would blow up until I looked closer at the design. (CM804)

This partial schematic of flyback power supply uses a 6A FET (ICM801S) to pull down the coil connected to the high voltage DC input (PFC). The flyback is clamped thru the diode RC. There is over-current-protection at the Source (S\OCP) sensed by RC= 0R35 * 0.01u (104/50V)

Sanken is certainly a master of Flyback converter IC's but some faults may get added in the snubber/damper and layout design.

Have a look at the FET datasheet which has a complex OCP compensation curve but very little or no advice on the snubber /damper cap value, which is to control EMI but also the OCP must be able to allow enough power into the coils and not dissipate into the cap.

Even though it is an NTC 0.2% DF 2SC type a fast rate 65 to 100kHz of high slew rate current pulses can dissipate a lot of heat. The NTC allows it to reduce capacitance with temp. and thus reduce dissipation. But it appears not enough....

A snubber is for EMI. But the cap. across the OCP R boosts the surge current which seems like conflicting . So I would look at removing both to get it working thru the DCR snubber which is also to protect the FET , then improve the EMI.

It will take time to isolate the details in the datasheet to determine a safe way to test this. (Which is probably why there is no service info just for the board) type.

https://pdf1.alldatasheet.es/datasheet-pdf/view/803741/SANKEN/STR-A6052M.html

https://www.semicon.sanken-ele.co.jp/ctrl/en/product/category/AcDcConverter/detail/?product=STR-A6052M

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