TV Switched mode power supply repair

[Deckert]

Hi guys,

I recently repaired a SMPS of an older LCD TV set. I documented the process in a 15 minute video below.

Something's been nagging at the back of my mind a bit. Close to the end, when I turn on the PSU and measure the DC voltage over the smoothing cap, it initially settles on 326V, which is expected with an AC feed voltage of around 230V AC.

However, when I turn on the set, the DC voltage increases to 400VDC and remains at that level. Since the cap itself is rated at 450V I'm not too worried, but is there any explanation as to why the DC voltage rises so much while in operation? Some distortion of the DC voltage, perhaps?



The set works fine though, and has been for at least a month, without any high temperatures on any of the components.

--deckert

[ambrosia heart]

For any TVs with large panel, they will have a PFC circuit.
This explains why 400V is measured.

My TV is 24 inch. No such circuit. I don‘t want a TV as
large as a coffin 

[Picuino]

A PFC (Power Factor Correction) circuit is a Boost circuit.

https://toshiba.semicon-storage.com/info/docget.jsp?did=68570

Good video 

[Deckert]

@Picuino: ha! thanks for that initial bit of info. the TLCD-32FHDA has a 32" screen, so no that big, but I get your point! 

@ambrosia heart: thanks for the linked document - really interesting reading, although I'm not fully grasping all of the concepts just yet. Saved for future reference.

As a matter of interest, I could not locate and of the more reputable capacitor brands that have the same specs and fit into the same form-factor, which tells me the original spec may have been inflated by the original capacitor manufacturer.

--deckert

[wraper]

As a matter of interest, I could not locate and of the more reputable capacitor brands that have the same specs and fit into the same form-factor, which tells me the original spec may have been inflated by the original capacitor manufacturer.

Those capacitors are not some very small size type which may be troublesome to buy. And Samxon is a decent brand.

[Deckert]

Indeed, Samxon is not too bad per se, but as in many of these cases, the thermal design of the PSU is not great. It sits vertically in the TV with the capacitor at the top (i.e. the heatsink below it "dumps" a lot of heat onto the cap.

I guess the standby mode also does not help, with the cap under a constant 325V charge (does that even make a difference to the capacitor's lifetime?)

--deckert

[andy3055]

Great clip. Just to the point with no stupid music! Thanks.

[TheMG]

The PFC boost converter is turned off in standby mode to reduce the standby power consumption, that is why the voltage goes up when you turn the TV on.

I guess the standby mode also does not help, with the cap under a constant 325V charge (does that even make a difference to the capacitor's lifetime?)

Not really. Keeping a cap charged, with only a very tiny 120Hz ripple current, is not stressful on the capacitor at all.

The capacitor is under more stress when the PFC is active and the TV is turned on (since the PFC operated at higher frequency, and the ripple current of course increases as the TV uses power). Heat, as you've guessed, really isn't good for capacitors, and probably the primary cause. The bulk capacitors in power supplies are usually not the the main cause of failure.

[wraper]

It this case capacitor is in between of 3 heatsinks, so it's not surprising it failed. Also guy in the video mistakenly says that the cap is a smoothing cap after a rectifier bridge. Which is totally wrong, There is no smoothing cap after the diode bridge at all. And this cap is connected to the output of PFC boost converter.

[Deckert]

It this case capacitor is in between of 3 heatsinks, so it's not surprising it failed.

Agreed.

Also guy in the video mistakenly says that the cap is a smoothing cap after a rectifier bridge. Which is totally wrong, There is no smoothing cap after the diode bridge at all. And this cap is connected to the output of PFC boost converter.

The guy in the video is me :-)

But you're right, of course. I was more referring to a bulk capacitor.

--deckert

[wraper]

The guy in the video is me :-)

But you're right, of course. I was more referring to a bulk capacitor.

Then TV you repaired probably won't last too long. This cap experiences quite high ripple current and receives a lot of heat from it's surroundings. So placing a cap with lower value, and especially lower temperature rating (85^oC instead of 105^oC) is not the best idea.

[Deckert]

Then TV you repaired probably won't last too long. This cap experiences quite high ripple current and receives a lot of heat from it's surroundings. So placing a cap with lower value, and especially lower temperature rating (85^oC instead of 105^oC) is not the best idea.

Like the old saying goes: the best camera is the one you have with you.

Gotta work with what you have - I'd gladly order the correct components, but the supply chain in my country does not make provision for it. All of the components have been repurposed and the TV is working again instead of ending up in a landfill. That's a win for me. If it breaks in a year or two, that's fine. :-)

--deckert