Burnt drain resistors on switching regulator

[Robyn]

Hi All!

I just scavenged a TV from the rubbish. People sometimes throw out perfectly good things but this find doesn't fit the category. After trying to change the cable fuse and checking the internal one I proceeded to a visual inspection of the power board and found two burnt resistors. The are connected in parallel and both connect the drain of two voltage regulators (and the anode of a diode) to ground.

- I want to change these two resistors but I am also wondering why this happened and how I can prevent it from happening again. The main fuse did not burn which would be surprising in case of a surge.
- Also, when looking for replacement parts I have found quite different looking resistors and I am wondering if the value and the wattage are the only things that matter or if the technology (say wire wound vs. silicon) is also important.

Thank you for your brain energy

The crime scene:



The schematic:


And for indexation in case anyone is looking for this schematic it is the power board for a LG 42LD450

 

[Alec_t]

For those resistors to fry, the MOSFET above them would almost certainly have passed excess current and died too. The possible reasons for MOSFET failure include load impedance too low, high-frequency spurious oscillation, incorrect feedback to the control circuit driving the MOSFET, and failure of the control IC.