26 October 2011

App Note 49

"Illumination circuitry for liquid crystal displays: Tripping the light fantastic..." 16 pages.

This app note is Part 2 in the grand saga of cold-cathode fluorescent lamps (CCFL). Part 1 was a single schematic (Figure 36 back in App Note 45, remember the 48 baby bottles?), reproduced here with minor changes as Figure 2. This problem is an important one in portable computers, since "The CCFL and its power supply are responsible for almost 50% of the battery drain."

The converters here use the Royer switching topology. Back in Appendix A of App Note 29, Jim discussed the disadvantages of the Royer topology at length. Mostly, he mocked it for being noisy. Here, he uses it as a high-voltage step-up circuit in a current-control loop. Footnote 3 explains, "Controlling a non-linear load's current, instead of its voltage, permits applying this circuit technique to a wide variety of nominally evil loads." Evil loads.

Of course, instrumentation plays a large role in this effort. High-voltage oscilloscope probes are recommended and discussed ("Don't say we didn't warn you!"). The dual-beam dual-time-base Tektronix 556 get considerable praise: Figures 3 and 6 would be nearly impossible without it ("Most oscilloscopes, whether analog or digital, will have trouble reproducing this display."). True RMS voltmeters ("the meter must employ a thermal type RMS converter") are also discussed on page 4 and in Appendix C. Of course, proper instrumentation is critical, as the footnote on page 11 explains, "It is worth considering that various constructors of Figure 2 have reported efficiency ranging from 8% to 115%."

Other design considerations are mentioned. Parasitic capacitance from mechanical layout is discussed. Transformer leakage due to silkscreen ink is considered. System designs employing two tubes are discussed on page 4 (this advice will be retracted in App Note 65). LCD-contrast-bias sources are shown in Figures 7 and 8.

Best quote (footnote 3 on page AN49-12): "The translation of this statement is to hide the probe when you are not using it. If anyone wants to borrow it, look straight at them, shrug your shoulders and say you don't know where it is. This is decidedly dishonest, but eminently practical. Those finding this morally questionable may wish to re-examine their attitude after producing a day's worth of worthless data with a probe that was unknowingly readjusted."

The next app note significantly expands on this topic.

1 comment:

Anonymous said...

Also noteworthy is it appears that figure 1 was taken with another rare Tektronix scope - the 536 X-Y scope. It's the only scope I know of with 10 divisions on both axises, and he's using it for an XY plot.