Page 180 - The Art and Science of Analog Circuit Design

Tripping the Light Fantastic

lamps appear to emit equal amounts of light. Layout and lamp matching
can influence C2's value. Some techniques for dealing with these issues
appear in the section "Layout Issues."

General Measurement and Optimization
Considerations

Several points should be kept in mind when observing operation of these
circuits. Li's high voltage secondary can only be monitored with a wide-
band, high voltage probe fully specified for this type of measurement, The
vast majority of oscilloscope probes will break down and fail if used for
this measurement. Tektronix probe types P6007 and P6Q09 (acceptable) or
types P6013A and P6015 (preferred) must be used to read Li's output.
Another consideration involves observing waveforms. The LT1172's
switching frequency is completely asynchronous from the Q1-Q2 Royer
converter's switching. As such, most oscilloscopes cannot simultaneously
trigger and display all the circuit's waveforms. Figure 11-19 was obtained
using a dual beam oscilloscope (Tektronix 556). LT1172 related Traces A
and B are triggered on one beam, while the remaining traces are triggered
on the other beam. Single beam instruments with alternate sweep and
trigger switching (e.g., Tektronix 547) can also be used, but are less ver-
satile and restricted to four traces.
3
Obtaining and verifying high efficiency requires some amount of dili-
gence. The optimum efficiency values given for Cl and C2 are typical, and
will vary for specific types of lamps. An important realization is that the
term "lamp" includes the total load seen by the transformer's secondary.
This load, reflected back to the primary, sets transformer input impedance.
The transformer's input impedance forms an integral part of the LC tank
that produces the high voltage drive. Because of this, circuit efficiency
must be optimized with the wiring, display housing and physical layout
arranged exactly the same way they will be built in production. Deviations
from this procedure will result in lower efficiency than might otherwise be
possible. In practice, a "first cut" efficiency optimization with "best guess"
lead lengths and the intended lamp in its display housing usually produces
results within 5% of the achievable figure. Final values for Cl and 02 may
be established when the physical layout to be used in production has been
decided on. Cl sets the circuit's resonance point, which varies to some

The terra "efficiency" as used here applies to electrical efficiency. In fact, the ultimate concern
centers around the efficient conversion of power supply energy into light. Unfortunately, lamp
types show considerable deviation in their current-to-light conversion efficiency. Similarly, the
emitted light for a given current varies over the life and history of any particular lamp. As such,
this publication treats "efficiency" on an electrical basis; the ratio of power removed from the
primary supply to the power delivered to the lamp. When a lamp has been selected, the ratio
of primary supply power to lamp-emitted light energy may be measured with the aid of a pho-
tometer. This is covered in Appendix B, "Photometric Measurements." See also Appendix D,
"Perspectives on Efficiency."
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