Why do the television manufacturers (Samsung, Sony, LG, so on) exceed the voltage capacity on LED strips (backlight) on TVs?

Question

I don't understand why manufacturers always exceed voltage to backlight TV (LED strips.) For example, I'm fixing a TV with some LEDs burned out.

This is the backlight:

There are two LED strips. Each strips has 9 LEDs, and each LED consumes 3 volts. All LEDs and strips are in series.

It would be: 3 volts * 9 LEDs * 2 strips = 3 volts * 18 LEDs (2 strips are in series) = 54 volts.

Just 54 volts is enough to "on" the backlights.

Why does the SMPS of the TV deliver 133 volts D when I test the voltage to the backlight?

It's 80 volts above capacity of all LEDs together. Why don't the LEDs burn out instantaneously?

Answer 1

Isn't 133V the open-circuit voltage? When LEDs are working, voltage should be lower.

LEDs are driven by constant current sources, it's far better than using voltage regulation, because LEDs have widely varying forward voltages and they tend to have negative temperature coefficient : Overheating reduces voltage which raises current with a constant voltage regulator which leads to even more overheating until one LED in the chain die.

Here is for example the datasheet of a LED driver used in a LG TV I've repaired :

https://fscdn.rohm.com/en/products/databook/datasheet/ic/power/led_driver/bd9285f-e.pdf

"Current mode DCDC converter"

Answer 2

Whoa! you're totally misunderstanding how this works.

Each LED will drop a slightly different voltage, so they are wired in series and driven at a contant current which flows through ALL of them. The open-circuit voltage of the supply will always be higher than the on-load figure, because that is the way the current source needs to work. I haven't looked at the data sheet which is mentioned in the original post, but 450mA as the drive current sounds like a lot to me, but the LEDs the tv manufacturers use are heavy duty units. If it is correct, taking 3V as the drop in a diode, 3 V x 0.45 A = 1.35 Watts of dissipation per diode. You can compare 18 x 1.35 to the sort of rating of a filament bulb replacement, so you would be looking at the light output from a 24 Watt LED light bulb - around the same as the amount you would get from a 150 Watt Halogen bulb, so quite bright. What tends to happen in these TVs is that if some of the diodes fail and go short-circuit, the voltage across the chain goes down, raising the voltage drop in the current source. That overheats and shuts down, protecting the rest of the set, at the expense of you losing the picture. If you are really lucky, you could replace the individual LEDs and make things work again, but getting hold of the correct LEDS is not easy, so the best option is to replace the damaged strips.