Page 172 - The Art and Science of Analog Circuit Design

Tripping the Light Fantastic

Figure 11 -11.
Current sink per-
mits controlling
Royer power, but is
inefficient.

i = ~- (DELETE BASE CURRENT)
R

Any discussion of CCFL power supplies must consider lamp characteris-
tics. These lamps are complex transducers, with many variables affecting
their ability to convert electrical current to light. Factors influencing con-
version efficiency include the lamp's current, temperature, drive wave-
form characteristics, length, width, gas constituents, and the proximity to
nearby conductors.
These and other factors are interdependent, resulting in a complex
overall response. Figures 11-13 through 11-16 show some typical char-
acteristics. A review of these curves hints at the difficulty in predicting
lamp behavior as operating conditions vary. The lamp's current and tem-
perature are clearly critical to emission, although electrical efficiency
may not necessarily correspond to the best optical efficiency point.
Because of this, both electrical and photometric evaluation of a circuit is
often required. It is possible, for example, to construct a CCFL circuit
with 94% electrical efficiency which produces less light output than an
approach with 80% electrical efficiency (see Appendix C, "A Lot of Cut-
off Ears and No Van Goghs—Some Not-So-Great Ideas"). Similarly, the
performance of a very well matched lamp-circuit combination can be

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