VIZIO L32 HDTV, No Backlight, Repaired

A VIZIO L32 HDTV came into the shop that would power on, the standby/power LED would go from amber to green and TV had audio but no picture. The back lights were not coming on, not even briefly. I opened the TV and powered it on. Next I checked all my secondary voltages on the SMPS to see if they were all present and within tolerance. Immediately it was apparent one of my voltages was missing, and it was the voltage to the inverter board. I disconnected the cable that connects the SMPS board to the inverter board to see if the voltage returned with the inverter board disconnected, so that I could verify it was the SMPS and not the inverter board that was failing. After disconnecting the cable and turning the TV back on the secondary output was still measuring 0V. I removed the power supply and checked the corresponding components for that output on the SMPS. I found the .027uF 630V MKP "bootstrap" capacitor location C9 was swollen and obviously bad. I check the rest of the surrounding components, for instance secondary diodes and filter capacitors and the Power MOSFETs on the primary side that are connected to the bootstrap capacitor. Everything was fine so I replaced the MKP capacitor with a .022uF 630V MKP from another board I had and put the TV back together. Upon turning the TV on it powered up the backlights came on and the TV is working great.

Bootstrap Capacitor Defined:

A N-MOSFET/IGBT needs a significantly positive charge (VGS > Vth) applied to the gate in order to turn on. Using only N-channel MOSFET/IGBT devices is a common cost reduction method due largely to die size reduction (there are other benefits as well). However, using nMOS devices means that a voltage higher than the power rail supply (V+) is needed in order to saturate the transistor and thus avoid significant heat loss.

A bootstrap capacitor is connected from the supply rail (V+) to the output voltage. If the capacitor is polarized then the orientation of the capacitor is as follows: Anode(marked with ‘+’)→(V+) and Cathode (marked with ‘-’)→Output. In other words, the capacitor should be between the output (source of an N-MOSFET) and (V+). Usually the source terminal of the N-MOSFET is connected to the cathode of a recirculation diode allowing for efficient management of stored energy in the typically inductive load (See Flyback diode). Due to the charge storage characteristics of a capacitor, the bootstrap voltage will rise above (V+) providing the needed gate drive voltage.

Hope you enjoyed this repair tip if you have any questions you can always email me john@preher-tech.com. Have a great day.

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