LED backlight repair in Samsung and LG TVs

Many have long known that LED TVs of the fifth and sixth series of manufacturers Samsung and LG, where matrixes (LED panels) with direct illumination of the Direct LED display are used, LEDs can sometimes burn out already in the first or second year of operation.

The most popular Samsung models that often need backlight repairs: UE32F5000 ..., UE32F5020 ..., UE32F5300 ..., UE320F5500 ... and others where panels are installed, for example, HF320BGSV1V, HF320BGA-B1 with direct illumination.

Among LG TVs, the most often come in for repairs with faulty LED backlighting are models 32LN540 ..., 32LN541 ..., 32LN548 with LC320DUE and LC320DXE panels.

Symptoms of a malfunction - there is sound, no image. The LED driver is functional and produces the maximum output voltage due to the absence of current in the load. An open in the diode circuit can be checked by the current source (stabilizer).
When the panel is opened, one dangling LED is detected and usually a few more punched in the short-circuit.  
The fact is that protective Zener diodes mounted in the case and connected in parallel to the LED transitions in reverse polarity are subjected to avalanche and then thermal breakdown when the LED breaks. Usually, the zener diode breaks into a short circuit, and the current in the line is stabilized regardless of the number of remaining LEDs. The TV works in this case, the diffusers sometimes mask dark spots on the screen from the owner well.
This can happen with all the remaining LEDs until one of the Zener diodes in the avalanche mode breaks through not a full (C / S) but a partial breakdown, for example, several ohms or tens of ohms. Then, warming up with the standard driver current, this remainder of the PN junction will simply burn to dust from excessive temperature exposure. As a result, we get a complete breakdown of the LED and the lack of current in the line, as well as the result that will force the user to contact the repair service - the image has disappeared.
Often, after 2 years of operation, more than half of the broken LEDs have to be observed. 

The work of replacing LED rulers (strings) or individual LEDs in strings has become commonplace for repairmen, but not all of the masters are guessing to reduce the current of LEDs and, in these cases, TV owners will soon have to turn to them again for the help.
Moreover, manufacturers (or their dealers) by default set the backlight level to the maximum position in all modes, most likely for advertising purposes, which significantly accelerates the failure of LEDs. And the user enjoys the quality of the contrast image and does not notice the catch. For the time being.

Manufacturers, for example, Samsung, send newsletters to their authorized service centers with recommendations on modifying the modules, which describe ways to limit the current in diodes by about 10%, which allows the TV to work out at least the warranty period. To finalize the popular Samsung BN44-00605 ..., BN44-00615 ..., BN44-00620 ... blocks,
In any case, the current of the LEDs from the LED driver can be slightly reduced based on the driver circuitry, in particular, the organization of the Negative Current Feedback of the driver PWM controller. Usually, in such cases it is enough to slightly increase the value of the resistor - the current sensor in the circuit of the series-connected LEDs.
The resistor value in ohms must be changed inversely with the current in the diodes.

Consider the operation of the LED driver implemented on the basis of the PWM controller, in our case, in the picture SEM5027, which controls the width of the unlocking pulses in the gate of the transistor switch Q ₁ .

The operation of such a Step-Up boost converter can be briefly described as follows:

 During the open state of the key, the current from the output  V _in through the inductor L , the key Q ₁ and the resistor R _i goes to the ground and increases linearly, and the core of the inductor at this time stores magnetic energy. When the rising current reaches a critical value for the transistor, the transistor is closed by a comparator, which will work on the voltage at the measuring resistor R _i , increasing in proportion to the current in the inductor and the open key.
After the key is closed, the magnetic energy stored by the core of the inductor will generate an induction current in the winding, which will continue its path in the same direction, now through the diode Discharging capacitor c .
The self-induction current charging the capacitor depends on the amount of stored magnetic energy, therefore, on the open time of the key, that is, on the width of the pulses that unlock the key. In the startup mode, they are maximum and limited only by the voltage across the resistor R _i .
After several such start pulses, the voltage across the capacitor ( V _out ) will exceed the input V _in and then, as soon as the LEDs open, the current in the load will appear and the voltage across the resistor R _d proportional to it. This voltage is supplied to the control input of the PWM controller, inverted inside and fed to the second input of the comparator, which will now close the transistor taking into account the OOS voltage on the resistor  R _d . Thus, the stabilization mode is established.

In other words, the open time of the switch Q ₁ within the period that determines the voltage and current in the load is stabilized by the negative feedback voltage (OOS) on the resistor R _d proportional to the current in the LED bar.
In fact, this current stabilizer is a flyback DC / DC voltage converter with current stabilization in the load. 
Transistor Q ₂ performs the function of the On / Off key - turns on and off the backlight and is not involved in stabilization, and the resistor value R _d, within certain limits allowed by the input parameters, will determine the current in the load inversely with the resistance of the resistor.

For example, in the popular Samsung BN44-00605A power supply  with the PWM SEM5027 described above, the value of the resistor (current sensor) R9110 is usually increased from 3.6 Ohms to 4.3 - 4.7 Ohms, while reducing the current in the diodes by about 25%, which does not critically affect the brightness of the backlight , but allows us to hope that the LEDs will not fail again soon.

There is a second way to reduce the backlight current in this block, which is described in the bulletin, by setting the VR9530 trimming resistor to set the voltage value 1.044V on the R9110 resistor. To do this, you must first add the 18 KOhm resistor, parallel to the R9509 resistor to increase the adjustment range.

With lateral illumination (Edge LED), the Samsung LE320BGM-C1 panel installed in SAMSUNG ES55 series ultra-thin TVs, for example, in the UE32ES5500, UE32ES5507, UE32ES5530, UE32ES5537, UE32ES5550, UE32ES5557, which use the LED 2012 plans,  can be classified as unsuccessful options  . 7032NNB 44 2D with LEDs  7032 6V 120mA (180mA max) type TS732A . 
Replacing such LEDs is more complicated and requires appropriate practical skills from the master. Moreover, the design and method of turning on LEDs in groups usually provoke a breakdown of the entire group of 11 series-connected LEDs. That is, at least 11, and more often 22 LEDs are detected in a breakdown state.

The current limitation in the BN44-00501A power supply should be made by increasing the value of the resistor R9131 (current sensor), which is installed from pin 8 (Sense) of the SLC2012M PWM controller to the chassis. In this embodiment, the driver ON / Off key is inside the PWM.
Increasing the nominal 3.5 ohms to 4.3 ohms will reduce the current in the load by about 20%.

To reduce the backlight current in LG TVs , for example, the popular  LGP32-13PL1 (EAX64905001), it is necessary to increase the resistance of the current sensor, which consists of a set of pairs of low-resistance R822-R829 resistors, only 4 pairs. Having repelled one pair, we increase the nominal value of the sensor by 25%, which increases the depth of the PWM OOS and proportionally reduces the current in the LEDs. From 400 mA to 300 mA. If you remove two pairs of resistors, the current will decrease by half (up to 200 mA), which affects the brightness of the screen.

Blocks LGP32-13PL1 power installed in LG TVs Direct LED backlit LED models to have trouble 2013 issue, for example, 32LN5400, 32LN540V, 32LN541V, 32LN541B, 32LN541U, 32LN542V, 32LN548C, 32LN570V, 32LN575S, 32LA615V ...
Used panel LC320DUE (SF ) (R1), LC320DUE (SF) (U2), LC320DXE (SF) (R1) ...

For each LED driver, with a technically competent analysis of circuitry, another individual approach can be found to limit the current in the LED lines.

In complex cases and in multichannel drivers, current reduction is practiced by changing the level at the controlled inputs of the PWM controller, but in this case the current limitation in abnormal modes is not guaranteed, for example, when the signal is turned on or off, moreover, these inputs can already be used to user backlight control from the menu.

The mass failure of the next generation of LEDs - dual (6 Volt 2W) are more often found in LG panels, for example, NC320DXN VSBP1, LC320EUN (SE) (F3) ... LC420DUE (FG) (P2) and others with similar LED strips. Twice as much power is dissipated in the same case as the previous 3 Volt 1W. Overheating in maximum mode is noticeable under the LEDs on the back of the strips. In order to increase the duration of further operation, it is advisable to replace LEDs with signs of overheating prophylactically, because their PN junctions may already be partially damaged. The technology for replacing LEDs and limiting the current of the LED driver in these cases remains the same. To replace LG LG 3 Volt 1W panels, you can use the Chinese analogs of LATWT470RELZK (3528), and 6 Volt 2W can be changed to dual LATWT391RZLZK (3535).

Consider one of the options to reduce the backlight current in LG 32-inch TVs with LGP32-14PL1 power supplies (EAX65391401) for six-volt (dual) LEDs.

The current sensor consists of six parallel-connected resistors R816-R821 - 1 Ohm, 1 Ohm, 1 Ohm, 1 Ohm, 2 Ohm, 8.2 Ohm. The total resistance, in this case, is calculated inversely with their conductivity. It will be 0.22 Ohm. If you remove one 1 Ohm resistor, the total resistance increases to 0.28 Ohm, the current decreases proportionally from 250 mA to about 190 mA.

The LGP32-14PL1 power supply unit is installed in the 2014 models 32LB551U, 32LB552U, 32LB5610, 32LB560U, 32LB563U, 32LB563V, 32LB565U, 32LB5BU, 32LB5BU, 32LB5BU, 32LB5FU, 32LB5FU, 32LB5FU, 32LB5FU, 32LB5FU, 32LF5FU, 32LF5FU, 32LF5FU, 32LF5FU, 32LF5FU,
32LF5FU (FG) (A3), LC320DUH (FG) (P1), LC320DUH (FG) (P2), LC320DUE (VG) (M1), LC320DXE (FG) (A5), NC320DXN VSBP1, NC320DXN VSBP2, NC320DXN VSBP2, NC320DXN VSBP5, NC320DXN ...

TVs of the 32LBxxxx series (2014) are the most popular in repair. Currently, they enter the workshops repeatedly, or in the third round and need to replace the complete set of LEDs (18 pieces). Cracks on the radiating surface of the ice and dark spots (traces of overheating) from the bottom of the bar are eloquent confirmation of this.

Consider another popular LGP32D-15CH1 power supply for LG 32-inch TVs in 2015 with direct illumination and dual (6V) LEDs.  Improvement of the backlight is also achieved by increasing the resistance of the current sensor R816 2.4 Ohm to 3 or 3.3 Ohm. The backlight current will proportionally decrease from the standard 250mA to 200 or 180 mA, respectively.

The LGP32D-15CH1 power supply is used in 32LF560U, 32LF560V, 32LF562V, 32LF653V, 32LF564U, 32LF564V ...
TVs LC320DUH (MG) (P1), LC320DXE (MG) (A3), NC320DUN-VBBPBBBPP ...

Large diagonal panels for 42, 47, 50, 55 inch TVs with 3V and 6V diodes are being repaired using the same technologies. The trims they have are composed of two halves, and the LEDs are the same as in 32 inch TVs.

In most cases, they have two channels, that is, two identical series-connected LED circuits and two current sensors. 
We will briefly review the development of power supplies to limit the current of the LED driver in them, and for the two-channel options we use one general picture below. 

LGP42-13R2  (EAX64905401), LED controller BD9483F with two channels in the load and two current sensors, is used to power the LC420DUE (SF) (U1), LC420DUE (SF) (R4) panels in LG 2013 42LA620V, 42LA621V, 42LN613V. Backlight current sensors R811-R818 - channel 1 and R829-R836 - channel 2. All resistors in pairs of 4.3 Ohm. To reduce the backlight current by 25%, one pair of resistors in each channel should be removed.

LGP3942-14PL1  (EAX65423701) with BD9486F LED PWM controller is used to power the LC420DUE (FG) (P2) panels in LG 2014 TVs of 42LB620V, 42LB629V. In this case, one channel. Backlight current sensor - resistor block R811-R818 in pairs of 4.3 Ohm. You can reduce the backlight current by 25% by removing one pair of resistors.

LGP3942D-15CH1 (EAX66203001) with LC5901 LED backlight controller , used to power the LC420DUE (MG) (A3), LC420DUE (MG) (A6) panels on LG 2015 TVs 42LF550V, 42LF551C, 42LF560V, 42LF562V, 42LF664V. LED current 250 mA. Current sensor - resistor R816 rated 3 Ohm. If you increase the rating to 4.3 Ohm, the current decreases to 175 mA.

LGP4750-13PL2 (EAX64905501) - power supply unit using a BD9483F LED PWM controller  , used to power the LC470DUE (SF) (R1), LC470DUE (SF) (R4), LC470DUE (SF) (U1), LC470DUE (SF) ( U2), LC500DUE (SF) on LG TVs of 2013 of release 47LN540V, 47LN570Y, 47LN613V, 47LA615V, 47LA620V, 47LA621V, 50LA620V. Current sensors in the R811-R818 LEDs - channel 1 and R829-R836 - channel 2. Resistors in the channels in pairs of 4.3 + 3.9 Ohm. You can reduce the backlight current by 25% by removing one pair of resistors in each channel.

LGP55-13PL2 (EAX64905601) is a power supply unit with a BD9483F LED controller for powering the LA62M55T120V12 panels in LG 2013 TVs 55LA620V. Current sensors in LEDs R811-R818 - channel 1 and R829-R836 - channel 2. Resistors in pairs of 4.7 Ohm, four pairs in the channel. To reduce the backlight current by 25%, you need to remove one pair of resistors in each channel.

LGP55-14PL2 (EAX65423801) is a power supply unit with an LED driver and a BL0202B LED backlight controller for powering the LC550DUH (FG) (P2) panels in LG 2014 TVs 55LB650V, 55LB652V. Current sensors in the R811-R818 LEDs - channel 1 and R829-R836 - channel 2. Resistors in pairs, four pairs in a channel. To reduce the backlight current by 25%, you need to disable one pair of resistors in each channel.

Based on the experience gained in operating and repairing modern LED panels, the information will be updated.