Some Flyback converter expertise?Rick Hille
6/29/2012 4:55 PM EDT
Bouyed by success in reviving an LCD monitor last year, I now find myself having spent $80 on an LCD TV with little hope of recovery. I'd like to learn a bit about a subject I believe is at the heart of the problem; off-line flyback converters. I would be very happy if any EETimes reader with flyback converter design expertise could offer some hints on what may be happening here.
The set, a 5 year old Dell w2607C LCD that I received for free, exhibited the amber flashing LED symptom commonly complained about on various websites. After finding the full service manual with schematics on the web along with availability of third-party capacitor replacment kits for the PSU board, I figured the set could be restored with little more than $10 and a couple hours work. I did find an open fuse on the motherboard, caused by a shorted upper MOSFET in a local 1.2V DC converter. Although this hinted at potentially serious damage, the 1.2V rail itself did not measure short, so I took the gamble since the PSU caps, blown fuse and MOSFET replacement amounted to under $20.
After repairs, the rails of the PSU came up steady, all logic power on the motherboard was stable, backlight was bright, no smoke or smell but nothing on screen. Motherboard obviously didn't survive the MOSFET failure. I found and ordered a replacement motherboard on the web for $60, since the PSU was now apparently working with the new caps. Weeks later with the new motherboard in, I was please to see a steady blue power LED and the Dell logo up on screen. However, after 5 seconds, the set blanked the backlight and became unresponsive, making an audible "...ZZT ZZT ZZT..." noise. Power cycling the set several times, it powered up exactly like this each time. After about the 10th power-up, the noise ceased and the power LED started flashing amber again. Rats!! The same fuse on the new motherboard had blown and the same MOSFET was damage, measuring about 8-ohms D-S.
At this point, it's a lost cause unless the new motherboard somehow survived major damage. I'm pretty sure that the main flyback converter has an aging issue where it eventually loses stability under light load. During the brief time when the set powered up and displayed the Logo, all the main rails measured steady at nominal values (24V and 12V from the flyback secondary, plus the 16V rail generated from the 24V rail). When the backlight blanked, removing about 75Watts of load from the 24V rail, the supply went into rapid hiccupping where I could measure the rails bouncing all over the place. The converter uses the ON semi NCP1377 current feedback critical conduction mode controller. This stage, runs from around 350-400VDC, fed from a PFC circuit based on the Fuji FA5500. From the ON datasheet, the converter should be stable from 0 to full load. The schematic revealed reasonable conformance to the manufacturers' suggested designs, except for the rather elaborate SCR based OC/OV protection latch circuit. It's a different story on the motherboard, where I'm quite sure inadequate input decoupling to the local converters largely contributed to its destruction by the bouncing main rail.