CCFL inverter circuits - Page 1
Forum section: electronics / projects

author: I_may_be_drunk_right_now
author_rank: Contributor
timestamp: February 24, 2014, 07:31:20 am
content: I picked up a 40 inch LCD TV from the trash as I am wont to do.I am baffled by the circuitry I see driving the CCFLs. There is one large transformer outputting some high AC voltage (I don't know what voltage, I have nothing here to measure high frequency high voltage) and this goes to a board with 17 small ferrite transformers.These appear to be all strung in series with some sort of sensing coil on the primary side that generates a go/no go level for each tube/xformer combo. (This I found out from watching youtube repair videos)The board number is SSB400WA16V.http://tvpartsfixit.co.uk/Images/Parts/2048.jpgWhat are the white devices with J refdeses that look like jumpers? Why are they there? Are there 16 CCFL tubes or 8 in there? I'm guessing 8 since I don't see anything on the other side of the panel to connect the tubes.What's the 17th transformer for? Feedback to the CPU board (HV present?) What's the other unused HV connector for?What's going on here? I can't find anything on Google about this topology. What's it called?The panel is cracked beyond repair so the TV is going to Silicon Heaven anyways, but I won't sleep until I find out what's going on here.(And also I need to repair my P6009 100X scope probe. Remember how I said I have nothing to measure high frequency high voltage signals with? I meant "anymore". I let the magic smoke out.... so that shows the voltage must be a bit higher than 1.5KV. Oh well.)

author: mariush
author_rank: Super Contributor
timestamp: February 24, 2014, 08:08:11 am
content: Those white things are probably fuses.The small transformers boost low voltage (usually 15-24v but can be higher) to 700-1500v (low current) depending if there's two ccfl tubes connected to one or not.At the bottom you see a bunch of diodes - if a lamp or transformer goes bad and the transformer overloads, the diodes lets voltage go through from that transformer to the controller or some other protection mechanism - it basically a sort of muxing of signals from several transformers, and the diodes block voltage going back to other transformers.Not sure how the transformers are used to power each lamp - it's possible two or three such small transformers are used to power 2 or more ccfl tubes and they resorted to 2 or 3 transformers to reduce the height of the inverter and make the tv more appealing.

author: amyk
author_rank: Super Contributor
timestamp: February 24, 2014, 12:23:00 pm
content: CCFLs have 2 terminals each, so you can count how many there are by seeing how many pairs of HV wires connect to the inverter board. The "CT-27" model on the lower transformer suggests it may be a current transformer for current-mode feedback. If you want to investigate it more you will need to reverse-engineer a schematic.

author: I_may_be_drunk_right_now
author_rank: Contributor
timestamp: February 24, 2014, 10:46:01 pm
content: Quote from: mariush on February 24, 2014, 08:08:11 amThat's the problem, all those ferrite cores are strung in series across the high voltage supply. The high voltage supply looks like thishttp://ww2.justanswer.com/uploads/Steve5281/2010-03-12_061414_BN44-00165A_Small.jpgOn the lower left you can see the high voltage transformer, with its two high voltage leads further covered in plastic tubes. These two wires go to the HV connector on the backlight PCB.This is the high voltage supply, so let's say there's 2kVp on one side of its secondary. It has to be pretty high to smoke my 1.5kV probe.So it's ~4kV p-p across 17 ferrites, so 235V each if the ferrites are all the same.I also assume the running voltage of a CCFL depends on many things, including its length.So you have one big inverter driving 17 floating step up transformers in a chain? And what's the second HV connector for?I'm still hoping to sell the boards (hey, I don't go around picking the garbage in full view of my neighbors for free.) so I'm not going to take them apart yet...