Page 163 - The Art and Science of Analog Circuit Design

Jim Williams

call it "strategic partnering." My grandfather would have called it "such a
deal."
Goals for the backlight began to emerge. For best overall efficiency,
the display enclosure, optical design, lamp, and electronics had to be
simultaneously considered. My job was the electronics, although I met
regularly with Paul Donovan, who was working on the other issues. In
particular, I was actively involved in setting lamp specifications and eval-
uating lamp vendors.
The electronics should obviously be as efficient as possible. The cir-
cuit should be physically compact, have a low parts count, and assemble
easily. It should have a wide, continuous dimming range with no hystere-
sis or "pop-on," and should meet all RF and system emission require-
ments. Finally, it must regulate lamp intensity against wide power supply
shifts, such as when the computer's AC adapter is plugged in.

Help from Dusty Circuits

Where, I wondered, had I seen circuitry which contained any or all of
these characteristics? Nowhere. But, one place to start looking was oscil-
loscopes. Although oscilloscope circuits do not accomplish what I needed
to do, oscilloscope designers use high frequency sine wave conversion to
generate the high voltage CRT supply. This technique minimizes noise
and reduces transformer and capacitor size. Additionally, by doing the
conversion at the CRT, long high voltage runs from the main power sup-
ply are eliminated.
I looked at the schematic of the high voltage converter in a Tektronix
547 (Figure 11-4). The manual's explanation (Figure 11-5) says the
capacitor (C808) and transformer primary form a resonant tank circuit.
More subtly, the "transformer primary" also includes the complex imped-
ance reflected back from the secondary and its load. But that's a detail for
this circuit and for now. A CRT is a relatively linear and benign load.
The backlight's loading characteristics would have to be evaluated and
matched to the circuit.
This CRT circuit could not be used to drive a fluorescent backlight
tube in a laptop computer. For one reason, this circuit is not very efficient.
It does not have to be. A 547 pulls over 500 watts, so efficiency in this
circuit was not a big priority. Latter versions of this configuration were
transistorized (Figure 11-6, Tektronix 453), but used basically the same
architecture. In both circuits the resonating technique is employed, and a
feedback loop enforces voltage regulation. For another reason, the CRT
requires the high voltage to be rectified to DC. The backlight requires AC,
eliminating the rectifier and filter. And, the CRT circuit had no feedback.
Some form of feedback for the fluorescent lamp seemed desirable.
The jewel in the CRT circuit, however, was the resonating technique
used to create the sine wave. The transformer does double duty. It helps
create the sine wave while simultaneously generating the high voltage.

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