Hi, I picked up a free Best Buy / Sharp LC-32LB261U LED TV from Craigslist. The LED back light had failed. I found that the set uses 3 LED strips which have 7 LEDs per strip, and the 3 strips are driven in parallel by the Power Forest PF7903B boost converter. During troubleshooting, I found that the boost converter would startup but be shut down by its over voltage protection circuit as the output voltage spiked to around 42V.
From the schematics and product brief, the feedback voltage on the PF7903B is 240mV and the 0.192489 ohm current sense resistance in the LED return path (5 resistors in parallel; 2 @ 0.91 ohm & 3 @ 1.0 ohm in parallel = 0.192389 ohms) indicated that the full load current should be 1.247 amps.
To test the converter, I used a 27.5 ohm 100 watt resistive load and that allowed the converter to run and produce 33.3 volts across the load and 1.246 amps through the load. That confirmed that the boost converter / current regulator was operating correctly.
Next, I used a bench power supply to test and make sure the LED back light assembly was bad before removing the panel. With 35 volts and the current limit set to 500 mA, I could get no current flow through either of the VLED+ to LED- connections. That confirmed that the LED back light strips were bad.
After disassembly, I found 3 strips with 7 LED per strip, labeled as Evertop LBM320P0701-FC-2. I tested each LED individually with bench supply @ 2.7 volts and current limit set to 400 mA. I found a total of 5 failed LEDs; 3 on one strip and 1 on each of the other 2 strips. Replacement strips are available on EBay, but before ordering, I wanted to try and replace the bad LEDs on each strip. (As an aside, the reflector in the set is paper and glued to the chassis which makes removing it without damage almost impossible. Very poor design with regard to serviceability.)
Does anyone know what the LED vendor and part number used on the LBM320P0701-FC-2 strips might be? These appear to be around 1.25 watts based on the 1.247A full load current into the parallel connected strips. This means each strip should have a current flow of 416mA and assuming a forward voltage drop of 3V, that would be 1.247 watts per LED.
If you know what vendor & part number the LEDs might be, please let me know.
Thanks,
From the schematics and product brief, the feedback voltage on the PF7903B is 240mV and the 0.192489 ohm current sense resistance in the LED return path (5 resistors in parallel; 2 @ 0.91 ohm & 3 @ 1.0 ohm in parallel = 0.192389 ohms) indicated that the full load current should be 1.247 amps.
To test the converter, I used a 27.5 ohm 100 watt resistive load and that allowed the converter to run and produce 33.3 volts across the load and 1.246 amps through the load. That confirmed that the boost converter / current regulator was operating correctly.
Next, I used a bench power supply to test and make sure the LED back light assembly was bad before removing the panel. With 35 volts and the current limit set to 500 mA, I could get no current flow through either of the VLED+ to LED- connections. That confirmed that the LED back light strips were bad.
After disassembly, I found 3 strips with 7 LED per strip, labeled as Evertop LBM320P0701-FC-2. I tested each LED individually with bench supply @ 2.7 volts and current limit set to 400 mA. I found a total of 5 failed LEDs; 3 on one strip and 1 on each of the other 2 strips. Replacement strips are available on EBay, but before ordering, I wanted to try and replace the bad LEDs on each strip. (As an aside, the reflector in the set is paper and glued to the chassis which makes removing it without damage almost impossible. Very poor design with regard to serviceability.)
Does anyone know what the LED vendor and part number used on the LBM320P0701-FC-2 strips might be? These appear to be around 1.25 watts based on the 1.247A full load current into the parallel connected strips. This means each strip should have a current flow of 416mA and assuming a forward voltage drop of 3V, that would be 1.247 watts per LED.
If you know what vendor & part number the LEDs might be, please let me know.
Thanks,
Comment