Hisense LED driver output way too high!

[gojunnyo]

I have this TV power board named Hisense rsag7.820.5535 I can't find the schematics anywhere so I am confused where to start, the specifications said that the LED driver output should be 140V at 400mA. As I tested the output rails, it shows 232V at the output!, that's why the backlight of the said unit, was burnt into pieces.

I am looking for the TL431, but I can't find it anywhere due to the lack of schematic as a reference. Maybe it is on SMD package (Will try to look for it again) But is my suspicion correct?

Any tips where to start? Enlighten me with your knowledge and experience

[Chalcogenide]

The LED driver is current-controlled, so if the LED strip is broken (i.e. behaves as an open circuit) then the voltage will shoot MUCH higher than the nominal voltage, and is only stopped by over-voltage "open circuit" protection, so measuring 200+ V on a nominal 140 V output is relatively normal. I don't expect such a circuit to use a TL431, but a dedicated IC with current sensing.

[gojunnyo]

The LED driver is current-controlled, so if the LED strip is broken (i.e. behaves as an open circuit) then the voltage will shoot MUCH higher than the nominal voltage, and is only stopped by over-voltage "open circuit" protection, so measuring 200+ V on a nominal 140 V output is relatively normal. I don't expect such a circuit to use a TL431, but a dedicated IC with current sensing.

Thank you for such wonderful insight! and for the new knowledge. So the thing I should do is just replace this burnt parts and not expect it to happen again?

[Zero999]

How are the LEDs connected up? If they're all in series, simply replacing the bad ones will do. The problem is, if some of them are in parallel, then forward voltage matching will be requited for good current sharing, otherwise you'll end up with a patch of brighter or dimmer LEDs, with the other carrying more or less current.

[gojunnyo]

The LED's are connected in series, majority of them are shorted

[Tom45]

As already stated, the LED driver is a current source and the open circuit voltage tells you nothing.

However, the LED driver could still be faulty and the culprit. If it is, that could explain why some of the LEDs are bad.

Try connecting it to an incandescent light bulb of at least 60 watts. Also, put a 1 ohm resistor in series with the bulb. Then turn on the power and measure the voltage across the 1 ohm resistor. You should see around 0.4 volts across the resistor if the driver is OK.

[TheMG]

Most LED drivers in TVs will turn off after a very short amount of time if an open circuit is detected (voltage rises above a certain threshold without reaching the target current).

The reason for this is precisely to avoid the kind of carnage that happened - things burning up is an LED goes open-circuit and begins to arc!

So it's somewhat odd that this TV's driver doesn't have such a feature. What is the part number of the LED driver IC (it will be near the boost regulator for the LED output)?

If you have a 150 ohm 25W resistor (or something close) handy, hook that up and see what happens. The current should not exceed 400mA if the circuit is working correctly. If it exceeds it by a significant amount then there's a fault in the driver circuit and it's not current regulating like it should.

[gojunnyo]

Most LED drivers in TVs will turn off after a very short amount of time if an open circuit is detected (voltage rises above a certain threshold without reaching the target current).

The reason for this is precisely to avoid the kind of carnage that happened - things burning up is an LED goes open-circuit and begins to arc!

So it's somewhat odd that this TV's driver doesn't have such a feature. What is the part number of the LED driver IC (it will be near the boost regulator for the LED output)?

If you have a 150 ohm 25W resistor (or something close) handy, hook that up and see what happens. The current should not exceed 400mA if the circuit is working correctly. If it exceeds it by a significant amount then there's a fault in the driver circuit and it's not current regulating like it should.

The LED driver chip is selc2010m, I do not know where in the particular datasheet would mention such feature
here's the link btw https://www.datasheetq.com/datasheet-download/882166/1/Magnachip/SELC2010M

[gojunnyo]

Btw, how can I avoid such problems to happen again? as I was told the problem happened after a power surge due to lightning. But I was complete surprised that only the backlight was damaged, (I do not know yet on the LED driver) but the rest is working fine.

[gojunnyo]

UPDATE: I connected the replaced LED strips to the LED driver, I am confused, when verifying on the multimeter. The output of the driver should be on DC right?

Scenario 1: Connecting the + multimeter lead to + rail and - multimeter lead to - rail
the output is more or less 30VDC

Scenario 2: Connecting the - multimeterlead to + rail and + multimeter lead to - rail or in simpler terms connecting in reverse
the output is neagtive more or less 120VDC

Scenario 3: AC mode and Connecting the + multimeter lead to + rail and - multimeter lead to - rail
the output is more or less 110VAC

is it my multimeter bricked or the led power board?

[Chalcogenide]

Probably PWM is used to set the LED brightness. If you have the chance, set the range on your multimeter to something higher than 200 V and try again. If your multimeter is not true RMS, the values could be all over the place, as you are measuring now. You could also try to measure the voltage after manually setting the highest possible backlight value on your TV, which should increase PWM duty cycle to 100% or quite close to that value.

[gojunnyo]

Probably PWM is used to set the LED brightness. If you have the chance, set the range on your multimeter to something higher than 200 V and try again. If your multimeter is not true RMS, the values could be all over the place, as you are measuring now. You could also try to measure the voltage after manually setting the highest possible backlight value on your TV, which should increase PWM duty cycle to 100% or quite close to that value.

How can I manually change the pwm without having the hassle to mount the LCD panel again and illuminate it with a flashlight.

[gojunnyo]

UPDATE:

The LED strips now lights up but are quite dim, and for split seconds it would light up bright and dim again. More or less a fluctuation of voltages? Any thoughts are highly appreciated